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Title: The Steam Engine Explained and Illustrated (Seventh Edition)

Author: Dionysius Lardner

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The Steam Engine
Explained and Illustrated

WATT.
Engraved by H. Adlard, from a Drawing by H. Corbould,
taken with the permission of James Watt, Esq.
FROM THE STATUE BY CHANTREY.

London: Taylor & Walton, Upper Gower Street.

THE

STEAM ENGINE

EXPLAINED AND ILLUSTRATED;

WITH
AN ACCOUNT OF ITS INVENTION AND PROGRESSIVE
IMPROVEMENT,
AND ITS APPLICATION TO

NAVIGATION AND RAILWAYS;
INCLUDING ALSO
A Memoir of Watt.

BY
DIONYSIUS LARDNER, D.C.L. F.R.S.
&c. &c.

SEVENTH EDITION,
ILLUSTRATED BY ENGRAVINGS ON WOOD.

LONDON:
PRINTED FOR TAYLOR AND WALTON,
28. UPPER GOWER STREET.

MDCCCXL.

London:
Printed by A. Spottiswoode,
New-Street-Square.

TO
THE RIGHT HONOURABLE
HENRY LORD BROUGHAM AND VAUX,
FELLOW OF THE ROYAL SOCIETY,
AND
MEMBER OF THE NATIONAL INSTITUTE OF FRANCE,

AS A MARK OF PUBLIC RESPECT
AND
A TESTIMONY OF PRIVATE REGARD,

THIS WORK
INSCRIBED, BY HIS ATTACHED FRIEND,
THE AUTHOR.

ADVERTISEMENT.

The Drawings for several of the Cuts in this Volume
have been taken, by the permission of Mr. Weale, from
the admirable Plates annexed to the last edition of
Tredgold on the Steam Engine and on Steam
Navigation. This acknowledgment is especially due for
the Illustrations which abound in this Volume.

London, June, 1840.

LONDON ENTRANCE TO THE BIRMINGHAM RAIL-ROAD.

CONTENTS.
CHAPTER I.
PRELIMINARY MATTER.
The Steam Engine, a Subject of popular Interest 4
Effects of Steam 5
Great Power of Steam 7
Object of this Work 9
Disputes respecting the Invention 11
Hero of Alexandria's Machine 13
Blasco De Garay's Proposition to propel Vessels by a Machine 16
Solomon De Caus 17
Giovanni Branca proposes to work Mills by Steam 22
Marquis of Worcester 23
Mechanical Properties of Fluids 25
Elastic and Inelastic Fluids 25
Elasticity of Gases 28
Effects of Heat 29
Application of these Principles to the Engines of Hero, De Caus, and
Lord Worcester 30
Sir Samuel Morland 34
Denis Papin 36
Atmospheric Pressure 38
Weight of Air 39
Pressure of Air 41
Barometer 41
Elastic Force of Air and Gases 42

Force obtained by a Vacuum 43
Common Pump 43
Rarefaction by Heat 44
Process of filling Thermometers 44
Papin's Method of producing a Vacuum 44
His Discovery of the Condensation of Steam 45
Thomas Savery 47

CHAP. II.
ENGINES OF SAVERY AND NEWCOMEN.
Savery's Engine 49
Boilers and their Appendages 50
Working Apparatus described 51
Mode of Operation 52
Defects of Savery's Engine 58
Newcomen's Engine described by Papin 62
Newcomen and Cawley obtain a Patent for Atmospheric Engine
65
Accidental Discovery of Condensation by Injection 69
Potter's Discovery of the Method of working the Valves 71
His Contrivance improved by the Substitution of a Plug Frame 72
Advantages of the Atmospheric Engine over that of Savery 72
The Power of Savery's Engine restricted 73
It contained no new Principle 73
Its practical Superiority 73

CHAP. III.
EARLY CAREER AND DISCOVERIES OF JAMES WATT.
Atmospheric Engine improved by Beighton 75

Smeaton's Improvements in the Atmospheric Engine 76
Brindley obtains a Patent for Improvement in 76
Invents the Self-regulating Feeder 76
Infancy of James Watt 77
His Descent and Parentage 77
Anecdotes of his Boyhood 78
His early Acquirements 79
Goes to London 80
Returns to Glasgow 80
Appointed Instrument-maker to the University 81
Opens a Shop in Glasgow 81
His Friends and Patrons 81
Professor Robison's Remarks on Watt's personal Character 82
His industrious and studious Habits 82
His Attention first directed to Steam 83
Experiments on High-pressure Engine 83
Repairs an Atmospheric Model 84
Experimental Inquiry consequent on this 84
Its Results 84
Observes great Defects in the Atmospheric Engine 85
His first Attempt to improve it 85
His early Experiments on Steam 87
Discovery by Experiment of the Expansion which Water undergoes
in Evaporation 90
Discovers the latent Heat of Steam 91
Informed by Dr. Black of the Theory of latent Heat 93
His Improvement not due to Black 93

CHAP. IV.
EXPOSITION OF PHYSICAL PRINCIPLES.

Construction of Thermometer 98
Method of graduating it 99
Freezing and boiling Points 99
Latent Heat of Water 101
Quantity of Heat necessary to convert Ice into Water, first noticed by
Dr. Black 101
Examination of the analogous Effects produced by the continued
Application of Heat to Water in the liquid State 102
Process of Boiling 104
Reconversion of Steam into Water 104
Conversion of Water into Steam 105
Latent Heat of Steam 107
Boiling Point varies 108
Different in different Places 109
Inquiry whether a Diminution of Pressure will produce a
corresponding Effect on the boiling Point 112
Table showing the Temperature at which Water will boil under
different Pressures of the Atmosphere 113
Mechanical Force of Steam 115
Facts to be observed in 117

CHAP. V.
FURTHER DISCOVERIES OF WATT.
Watt finds that Condensation in the Cylinder is incompatible with a
due Economy of Fuel 119
Conceives the Notion of condensing out of the Cylinder 120
Discovers separate Condensation 121
Invents the Air Pump 123
Substitutes Steam Pressure for Atmospheric Pressure 123
Invents the Steam Case, or Jacket 124

His first Experiments to realise these Inventions 125
His experimental Apparatus 125
His experimental Models fitted up at Delft House, in Glasgow
128
Difficulties of bringing the improved Engines into Use 129
Watt first employed by Roebuck as a Civil Engineer 130
His Partnership with Roebuck 130
His first Patent 130
Experimental Engine at Kinneal 131
Abstract of the Act of Parliament for the Extension of his Patent
132
Description of his single-acting Steam Engine 133

CHAP. VI.
WATT'S ENGINES.
Correspondence of Watt with Smeaton 145
Failure of Condensation by Surface 146
Improvements in Construction of Piston 147
Method of Packing 148
Improvements in boring the Cylinder 149
Disadvantages of the new compared with the old Engines 150
Greatly increased Economy of Fuel 150
Economy of the Engine 151
Expedients to force the new Engines into Use 151
Correspondence of Boulton 153
Correspondence with Smeaton 155
Efficiency of Fuel in the new Engines 156
Discovery of the expansive Action of Steam 157
Watt states it in a Letter to Dr. Small 157
Its Principle explained 158

Mechanical Effects resulting from it 162
Computed Effect of cutting off Steam at different Portions of the
Stroke 163
Produces a variable Power 163
Expedients for equalising the Power 164
Expansive Principle in Watt's Engines limited 165
Its more extensive Application in the Cornish Engines 165

CHAP. VII.
DOUBLE-ACTING ENGINE.
Common Steam 168
Superheated Steam 170
Laws of Dalton and Gay Lussac 171
Law of Mariotte 171
Relation between Temperature and Pressure of common Steam
171
Effects of the Expansion of common Steam 173
Mechanical Effects of Steam 173
Method of equalising the expansive Force 174
Hornblower's Engine 175
Woolf's Engine 176
Watt's Attempts to extend the Steam Engine to Manufactures 178
Papin's projected Applications of the Steam Engine 178
Savery's Application of the Engine to move Machinery 180
Jonathan Hull's Application to Water Wheels 180
Champion of Bristol applies the Atmospheric Engine to raise Water
181
Stewart's Application of the Engine to Mill-work 182
Wasbrough's Application of the Fly-wheel and Crank 183

Reasons why Watt's single-acting Steam Engine was not adapted to
produce continuous uniform Motion of Rotation 184
Watt's Second Patent 186
Sun-and-Planet Wheels 187
Valves of double-acting Engine 189

CHAP. VIII.
DOUBLE-ACTING ENGINE.
Methods of connecting the Piston-rod and Beam in the double-
acting Engine 193
Rack and Sector 194
Parallel Motion 195
Connection of Piston-rod and Beam 195
Connecting Rod and Crank 203
Fly-wheel 205
Throttle-valve 207
Governor 209
Construction and Operation of the double-acting Engine 216
Eccentric 225
Cocks and Valves 227
Single-clack Valves 227
Double-clack Valves 228
Conical Valves 228
Slide Valves 229
Murray's Slides 229
D Valves 230
Seaward's Slides 235
Single Cock 238
Two-way Cock 239
Four-way Cock 239

Pistons 242
Common hemp-packed Piston 242
Woolf's Piston 243
Metallic Pistons 244
Cartwright's Engine 245
Cartwright's Piston 247
Barton's Piston 248

CHAP. IX.
BOILERS AND FURNACES.
Analysis of Coal 252
Process of Combustion 253
Heat evolved in it 254
Form and Structure of Boiler 255
Waggon Boiler 255
Furnace 256
Method of feeding it 257
Combustion of Gas in Flues 260
Williams's Patent for Method of consuming unburned Gases 260
Construction of Grate and Ash-pit 261
Magnitude of heating Surface of Boiler 262
Steam-space and Water-space in Boiler 263
Position of Flues 264
Method of feeding Boiler 265
Method of indicating the Level of Water in Boiler 266
Level Gauges 266
Self-regulating Feeders 267
Steam Gauge 270
Barometer Gauge 272

Watt's Invention of the Indicator 274
Counter 278
Safety-valve 279
Fusible Plugs 280
Self-regulating Damper 281
Brunton's Self-regulating Furnace 283
Gross and useful Effect of an Engine 285
Power and Duty of Engines 287
Horse-power of Steam Engines 289
Evaporation proportional to Horse-power 290
Sources of Loss of Power 292
Absence of good practical Rules for Power 292
Common Rules followed by Engine-makers 292
Duty distinguished from Power 294
Duty of Boilers 294
Proportion of Stroke to Diameter of Cylinder 295
Duty of Engines 296
Cornish System of Inspection 297
Table showing the Improvement of Cornish Engines 298
Beneficial Effects of Cornish Inspection 299
Successive Improvements on which the increased Duty of Engines
depends, traced by John Taylor in his "Records of Mining" 299

CHAP. X.
LIFE OF WATT.
Watt's Friends and Associates at Birmingham 302
His Invention of the Copying Press 302
Heating Apartments and Buildings by Steam 303
Watt's Machine for drying Linen 303

His Share in the Discovery of the Composition of Water 303
The Merit of this Discovery shared between Cavendish, Lavoisier,
and Watt 305
Anecdote of Watt's Activity of Mind 309
His Introduction of the Use of Chlorine in Bleaching 310
One of the Founders of the Pneumatic Institution at Clifton 310
His first Marriage 311
Death of his first Wife 311
His second Marriage 311
Death of his younger Son 311
Extracts from his Letters 312
Character of Watt by Lord Brougham 313
Extract from Sir Walter Scott's "Monastery" on the Character of
Watt 314
Sketch of the Life of Watt from the "Scotsman" by Lord Jeffrey
315
Occupation of his old Age 318
His Invention of Machine for copying Sculpture 318
His last Days 318
Monuments to his Memory 319
Inscription by Lord Brougham on the Pedestal of the Monument in
Westminster Abbey 320

CHAP. XI.
LOCOMOTIVE ENGINES ON RAILWAYS.
High-pressure Engines 322
One of the earliest Forms of the Steam Engine 322
Description of Leupold's Engine 323
Non-condensing Engine of Messrs. Trevethick and Vivian 324

Construction of a Machine for moving Carriages on Railroads
328
Effects of Railway Transport 329
Moral and political Consequences of 334
History of the Locomotive Engine 337
Construction of Locomotive Engine by Blinkensop 337
Messrs. Chapman's Contrivance 337
Walking Engine 337
Mr. Stephenson's Engines at Killingworth 339
Liverpool and Manchester Railway 342
The Directors offer a Prize for the best Locomotive Engine 344
Experimental Trial 344
The "Rocket," "Sanspareil," and "Novelty" 344
Admirable Arrangement in the Rocket 345
Description of the "Sanspareil" 347
Description of the "Novelty" 349
The Superiority of the "Rocket" 350
Method of subdividing the Flue into Tubes 353
Progressive Improvement of Locomotive Engines 354
Dr. Lardner's Experiments in 1832 357
Adoption of Brass Tubes 360
Great Expense of Locomotive Power 361
Mr. Booth's Report 362
Detailed Description of the most improved Locomotive Engines
365
Substitution of Brass for Copper Tubes ascribed to Mr. Dixon 370
Power of Locomotive Engines 379
Position of the Eccentrics 379
Pressure of Steam in the Boiler 401
Dr. Lardner's Experiments in 1838 406

Resistance to Railway Trains 407
Dr. Lardner's Experiments on the Great Western Railway 408
Experiments on Resistance 409
Restrictions on Gradients 410
Compensating Effect of Gradients 412
Experiment with the "Hecla" 412
Disposition of Gradients should be uniform 415
Methods of surmounting steep Inclinations 415

CHAP. XII.
LOCOMOTIVE ENGINES ON TURNPIKE ROADS.
Railways and Stone Roads compared 420
Gurney's Steam Carriage 423
The Boiler of Gurney's Engine 423
His Method of cleansing Boilers 428
Convenience and Safety of Steam Carriages 432
Two Methods of applying Locomotive Engines upon common
Roads 434
Horse Carriages compared with Steam 435
Extract from Mr. Farey's Evidence before the House of Commons
435
Hancock's Steam Carriage 436
How it differs from that of Mr. Gurney 437
Ogle's Locomotive Carriage 438
Dr. Church's Steam Engine 439

CHAP. XIII.
STEAM NAVIGATION.
Form and Arrangement of Marine Engines 441

Arrangement of the Engine-room 446
Marine Boilers 448
Effects of Sea Water in Boilers 450
Remedies for them 451
Blow-off Cocks 452
Indicators of Saltness 453
Seaward's Indicator 454
His Method of blowing out 454
Field's Brine Pumps 456
Tubular Condensers applied by Mr. Watt 457
Hall's Condensers 458
Substitution of Copper for Iron Boilers 460
Process of Stoking 462
Watt's Expedient of attaching Felt to the Boiler Surface 463
Means of economising Fuel 463
Number and Arrangement of Furnaces and Flues 463
Howard's Marine Engine 464
Application of the expansive Principle in Marine Engines 466
Recent Improvements of Messrs. Maudslay and Field 467
Humphrey's Marine Engine 470
Common Paddle-wheel 472
Feathering Paddles 474
Galloway's Patent for a Paddle-wheel with movable Paddles 476
Split Paddle 478
Proportion of Power to Tonnage 480
Improved Efficiency of Marine Engines 482
Iron Steam Vessels 483
Steam Navigation to India 484

CHAP. XIV.

AMERICAN STEAM NAVIGATION.
Steam Navigation first established in America 487
Circumstances which led to it 488
Attempts of Fitch and Rumsey to apply the single-acting Engine to
the Propulsion of Vessels 489
Stevens of Hoboken commences Experiments in Steam Navigation
489
Experiments of Livingstone and Fulton 489
Fulton's first Boat 490
The Hudson navigated by Steam 491
Extension and Improvement of River Navigation 492
American Steamers 494
Difference between them and European Steamers 494
Steamers on the Hudson 494
American Paddle-wheels 495
Sea-going American Steamers 496
Speed attained by American Steamers 497
Lake Steamers 499
The Mississippi and its Tributaries 499
Steam-boats navigating it 500
Their Structure and Machinery 500
New Orleans Harbour 503
Steam Tugs 503

APPENDIX.
On the Relation between the Temperature, Pressure, and Density of
Common Steam.
Empirical Formula of Biot, showing the Relation between the
Pressure and Temperature 505
Empirical formula of

Southern 506
Tredgold 506
Mellet 506
De Pambour 506
MM. Dulong and Arago 506
Law of the Expansion of elastic Fluids, discovered by Dalton and
Gay Lussac 506
Formula for the Relation between the Volumes and Temperatures
507
Law of Mariotte 507
Table of Pressures, Temperatures, Volumes, and Mechanical Effects
of Steam 509
Empirical Formulæ for the Relation between the Volume of Water
and that of the Steam produced by its Evaporation under given
Pressures 511
Formula of Navier 511
Modified by De Pambour 511
On the Expansive Action of Steam.
Mechanical Effect produced during a given Extent of Expansion
511
Mechanical Effect produced during Evaporation and subsequent
Expansion 512
Application to double-acting Engines 513
Formula for Pressure of Steam in Cylinders 514
Formula for total Mechanical Effect per Minute of Steam when cut
off at any proposed Part of the Stroke 514
Formulæ exhibiting the Relation between the Resistance of the
Load, the Resistances of the Engine, the Evaporation, the Speed
of the Piston, and the Magnitude of the Cylinder 515
Formulæ showing the Relation between the Power of the Engine,
the Evaporation, and the useful Load 516
Formulæ for the useful Effect and the Duty 517

Estimates of the several Sources of Resistances 518
Tables to facilitate the Computation of the Effects of Expansive
Engines 519
Table of the Areas of Pistons 520
Examples of the Application of these Formulæ 521

INDEX.

VIADUCT, NEAR WATFORD, BIRMINGHAM RAIL-ROAD.

THE
STEAM ENGINE.

HERO OF ALEXANDRIA.

CHAPTER I.
[Pg003]

TOC INX

THE STEAM ENGINE, A SUBJECT OF POPULAR INTEREST.—THE OBJECT OF THIS
WORK.—DISPUTES RESPECTING THE INVENTION.—HERO.—DE GARAY.—DE
CAUS.—BRANCA.—MARQUIS OF WORCESTER.—PHYSICAL PRINCIPLES.—
ELASTIC AND INELASTIC FLUIDS.—THEIR PROPERTIES.—APPLICATION OF
THESE PRINCIPLES TO THE ENGINES OF HERO, DE CAUS, AND LORD
WORCESTER.—SIR SAMUEL MORLAND.—PAPIN.—ATMOSPHERIC PRESSURE.—
THE WEIGHT OF AIR.—LESS AT GREATER HEIGHTS.—BAROMETER.—PRESSURE
OF AIR.—ELASTIC FORCE OF AIR AND GASES.—FORCE PRODUCED BY A
VACUUM.—COMMON PUMP.—RAREFACTION BY HEAT.—PAPIN'S METHODS OF
PRODUCING A VACUUM.—HIS DISCOVERY OF THE CONDENSATION OF STEAM.
—SAVERY.

(1.)

That the history of the invention of a piece of mechanism, and the
description of its structure, operation, and [Pg004] uses, should be capable of
being rendered the subject matter of a volume, destined not alone for the
instruction of engineers or machinists, but for the information and
amusement of the public in general, is a statement which at no very remote
period would have been deemed extravagant and incredible.
Advanced as we are in the art of rendering knowledge popular, and
cultivated as the public taste is in the appreciation of the expedients by
which science ministers to the uses of life, there is still perhaps but one
machine of which such a proposition can be truly predicated: it is needless
to say that that machine is the STEAM ENGINE. There are many circumstances
attending this extraordinary piece of mechanism which impart to it an
interest so universally felt. Whether we regard the details of its structure
and operation, the physical principles which it calls into play, and the
beautiful contrivances by which these physical principles are rendered
available;—or, passing over these means, we direct our attention to the ends
which they attain, we are equally filled with astonishment and admiration.

The history of the steam engine offers to our notice a series of contrivances
which, for exquisite and refined ingenuity, stand without any parallel in the
annals of mechanical science. These admirable inventions, unlike other
results of scientific inquiry, have also this peculiarity, that, to understand
their excellence and to perceive their beauty, no previous or subsidiary
knowledge is necessary, save what may be imparted with facility and
clearness in the progress of the explanation and development of the
machine itself. A simple and clear exposition, divested of needless
technicalities and aided by well-selected diagrams, is all that is necessary to
render the construction and operation of the steam engine, in all its forms,
intelligible to persons of plain understanding and moderate information.
But if the contrivances by which this vast power is brought to bear on the
arts and manufactures, be rendered attractive by their great mechanical
beauty, how much more imposing will the subject become when the effects
which the steam engine has produced upon the well-being of the human
race are considered. It has penetrated the crust of the earth, and drawn from
beneath it boundless treasures [Pg005] of mineral wealth, which, without its aid,
would have been rendered inaccessible; it has drawn up, in measureless
quantity, the fuel on which its own life and activity depend; it has relieved
men from their most slavish toils, and reduced labour in a great degree to
light and easy superintendence. To enumerate its present effects, would be
to count almost every comfort and every luxury of life. It has increased the
sum of human happiness, not only by calling new pleasures into existence,
but by so cheapening former enjoyments as to render them attainable by
those who before could never have hoped to share them: the surface of the
land, and the face of the waters, are traversed with equal facility by its
power; and by thus stimulating and facilitating the intercourse of nation
with nation, and the commerce of people with people, it has knit together
remote countries by bonds of amity not likely to be broken. Streams of
knowledge and information are kept flowing between distant centres of
population, those more advanced diffusing civilisation and improvement
among those that are more backward. The press itself, to which mankind
owes in so large a degree the rapidity of their improvement in modern
times, has had its power and influence increased in a manifold ratio by its
union with the steam engine. It is thus that literature is cheapened, and, by

being cheapened, diffused; it is thus that Reason has taken the place of
Force, and the pen has superseded the sword; it is thus that war has almost
ceased upon the earth, and that the differences which inevitably arise
between people and people are for the most part adjusted by peaceful
negotiation.
Deep as the interest must be with which the steam engine will be
regarded in every civilised country, it presents peculiar claims upon the
attention of the people of Great Britain. Its invention and progressive
improvement are the work of our own time and our own country; it has
been produced and matured almost within the last century, and is the
exclusive offspring of British genius, fostered and sustained by British
enterprise and British capital.
The steam engine is a mechanical contrivance, by which coal, wood, or
other fuel is rendered capable of executing any [Pg006] kind of labour. Coals
are by it made to spin, weave, dye, print and dress silks, cottons, woollens,
and other cloths; to make paper, and print books upon it when made; to
convert corn into flour; to express oil from the olive, and wine from the
grape; to draw up metal from the bowels of the earth; to pound and smelt it,
to melt and mould it; to forge it; to roll it, and to fashion it into every
desirable form; to transport these manifold products of its own labour to the
doors of those for whose convenience they are produced; to carry persons
and goods over the waters of rivers, lakes, seas, and oceans, in opposition
alike to the natural difficulties of wind and water; to carry the wind-bound
ship out of port; to place her on the open deep ready to commence her
voyage; to throw its arms around the ship of war, and place her side by side
with the enemy; to transport over the surface of the deep persons and
information, from town to town, and from country to country, with a speed
as much exceeding that of the ordinary wind, as the ordinary wind exceeds
that of a common pedestrian.
Such are the virtues, such the powers, which the steam engine has
conferred upon COALS. The means of calling these powers into activity are
supplied by a substance which nature has happily provided in unbounded
quantity in every part of the earth; and though it has no price, it has
inestimable value: this substance is WATER.

A pint of water may be evaporated by two ounces of coals. In its
evaporation it swells into two hundred and sixteen gallons of steam, with a
mechanical force sufficient to raise a weight of thirty-seven tons a foot
high. The steam thus produced has a pressure equal to that of common
atmospheric air; and by allowing it to expand, by virtue of its elasticity, a
further mechanical force may be obtained, at least equal in amount to the
former. A pint of water, therefore, and two ounces of common coal, are thus
rendered capable of doing as much work as is equivalent to seventy-four
tons raised a foot high.
The circumstances under which the steam engine is worked on a railway
are not favourable to the economy of fuel. Nevertheless a pound of coke
burned in a locomotive engine [Pg007] will evaporate about five pints of water.
In their evaporation they will exert a mechanical force sufficient to draw
two tons weight on the railway a distance of one mile in two minutes. Four
horses working in a stage-coach on a common road are necessary to draw
the same weight the same distance in six minutes.
A train of coaches weighing about eighty tons, and transporting two
hundred and forty passengers with their luggage, has been taken from
Liverpool to Birmingham, and back from Birmingham to Liverpool, the trip
each way taking about four hours and a quarter, stoppages included. The
distance between these places by the railway is ninety-five miles. This
double journey of one hundred and ninety miles is effected by the
mechanical force produced in the combustion of four tons of coke, the value
of which is about five pounds. To carry the same number of passengers
daily between the same places by stage-coaches on a common road, would
require twenty coaches and an establishment of three thousand eight
hundred horses, with which the journey in each direction would be
performed in about twelve hours, stoppages included.
The circumference of the earth measures twenty-five thousand miles; and
if it were begirt with an iron railway, such a train as above described,
carrying two hundred and forty passengers, would be drawn round it by the
combustion of about thirty tons of coke, and the circuit would be
accomplished in five weeks.

In the drainage of the Cornish mines the economy of fuel is much
attended to, and coals are there made to do more work than elsewhere. A
bushel of coals usually raises forty thousand tons of water a foot high; but it
has on some occasions raised sixty thousand tons the same height. Let us
take its labour at fifty thousand tons raised one foot high. A horse worked in
a fast stage-coach pulls against an average resistance of about a quarter of a
hundred weight. Against this he is able to work at the usual speed through
about eight miles daily: his work is therefore equivalent to one thousand
tons raised one foot. A bushel of coals consequently, as used in Cornwall,
performs as much labour as a day's work of one hundred such horses. [Pg008]
The great pyramid of Egypt stands upon a base measuring seven hundred
feet each way, and is five hundred feet high, its weight being twelve
thousand seven hundred and sixty millions of pounds. Herodotus states, that
in constructing it one hundred thousand men were constantly employed for
twenty years. The materials of this pyramid would be raised from the
ground to their present position by the combustion of about four hundred
and eighty tons of coals.
The Menai Bridge consists of about two thousand tons of iron, and its
height above the level of the water is one hundred and twenty feet. Its mass
might be lifted from the level of the water to its present position by the
combustion of four bushels of coal.
The enormous consumption of coals produced by the application of the
steam engine in the arts and manufactures, as well as to railways and
navigation, has of late years excited the fears of many as to the possibility
of the exhaustion of our coal-mines. Such apprehensions are, however,
altogether groundless. If the present consumption of coal be estimated at
sixteen millions of tons annually, it is demonstrable that the coal-fields of
this country would not be exhausted for many centuries.
But in speculations like these, the probable, if not certain progress of
improvement and discovery ought not be overlooked; and we may safely
pronounce that, long before such a period of time shall have rolled away,
other and more powerful mechanical agents will supersede the use of coal.
Philosophy already directs her finger at sources of inexhaustible power in
the phenomena of electricity and magnetism. The alternate decomposition

and recomposition of water, by magnetism and electricity, has too close an
analogy to the alternate processes of vaporisation and condensation, not to
occur at once to every mind: the development of the gases from solid matter
by the operation of the chemical affinities, and their subsequent
condensation into the liquid form, has already been essayed as a source of
power. In a word, the general state of physical science at the present
moment, the vigour, activity, and sagacity with which researches in it are
prosecuted in every civilised [Pg009] country, the increasing consideration in
which scientific men are held, and the personal honours and rewards which
begin to be conferred upon them, all justify the expectation that we are on
the eve of mechanical discoveries still greater than any which have yet
appeared; and that the steam engine itself, with the gigantic powers
conferred upon it by the immortal Watt, will dwindle into insignificance in
comparison with the energies of nature which are still to be revealed; and
that the day will come when that machine, which is now extending the
blessings of civilisation to the most remote skirts of the globe, will cease to
have existence except in the page of history.

(2.)

The object of the present volume will be to deliver, in an easy and familiar
style, an historical view of the invention of the steam engine, and an
exposition of its structure and operation in the various forms in which it is
now used, and of its most important applications in the arts of life,
especially in transport by land and water. It is hoped that the details of these
subjects may be rendered easily intelligible to all persons of ordinary
information, whether urged by that natural and laudable spirit of inquiry
awakened by contemplating effects on the material and social condition of
our species, so rapid and so memorable as those which have followed the
invention of the steam engine, and by the pleasure which results from the
perception of the numerous instances of successful contrivances and
beautiful applications of science to art which it unfolds,—or impelled by
the exigencies of trade or profession to acquire an acquaintance with a
machine on which, more than any other, the prosperity of our commercial
and manufacturing interests depends. It will be our aim to afford to the
former class all the information which they can require; and, if this work be

not as comprehensive in its scope, and as minute in its details, as some of
the latter may wish, it will at least serve as an easy and convenient
introduction to other works more voluminous, costly, and detailed, but less
elementary in their matter, and less familiar in their style.
In explaining the different forms of steam engine which have been
proposed in the course of the progressive improvement [Pg010] of that machine
from its early rude and imperfect state to its present comparatively perfect
form, it will be necessary to advert to various physical phenomena and
mechanical principles, which, however obvious to those who are conversant
with matters of science, must necessarily be at least imperfectly known by
the great majority of our readers. To refer for information on such topics to
other works on Mechanics and general Physics, would be with most readers
ineffectual, and with all unsatisfactory. In former editions of the present
work, we consigned these necessary general principles of physics and
mechanics to a preliminary chapter; but it appears, on the whole, more
convenient not to remove the exposition of the principle from the place
where its application is required. We shall therefore pause as we proceed,
where these difficulties occur, to give such explanation and illustration as
may seem best suited to render them intelligible and interesting to the
unscientific reader.
The history of the arts and manufactures affords no example of any
invention the credit for which has been claimed by so many different
nations and individuals as that of the steam engine. The advocates of the
competitors for this honour have urged their pretensions, and pressed their
claims, with a zeal which has occasionally outstripped the bounds of
discretion, and the contest has not unfrequently been tinged with prejudices,
national and personal, and characterised by a degree of asperity altogether
unworthy of so noble a cause, and beneath the dignity of science.
"When a question is clearly proposed, it is already half resolved." Let us
see whether a careful attention to this maxim will aid us in the investigation
of the origin of the steam engine. The source of the power of that machine
is found in the following natural phenomena.

First. When fire is applied to water, the liquid swells into vapour, and in
undergoing this change exerts, as has been already stated, a considerable
amount of mechanical force. This force may, by proper means, be
rendered applicable to any purpose for which labour or power is needful.
Second. The vapour so produced is endowed with the property of
elasticity, in virtue of which it is capable of [Pg011] swelling or expanding
into increased dimensions, exerting, as it expands, a force, the energy of
which is gradually diminished as the dimensions of the vapour are
increased. This mechanical force is likewise capable of being applied to
any useful purpose for which labour or power is necessary.
Third. This vapour is capable, by proper means, of being reconverted into
water; and when so reconverted, it shrinks into its original dimensions,
deserting the large space which it occupied as vapour, and leaving that
space a vacuum. It is known in physics, that when a vacuum is produced,
surrounding bodies have a tendency to rush into it with a definite amount
of force. Consequently any agent which produces a vacuum, becomes a
source of a considerable amount of mechanical power. By its
reconversion into water, therefore, steam again becomes a mechanical
agent.

Such are the natural phenomena in which are found the original sources
of all steam power. In some forms of steam engine one of these is used, and
in some another, and in some the application of all of them is combined; but
in no existing form of steam engine whatever is there any other source of
mechanical power.
Neither these nor any other natural forces can be applied immediately to
any useful purpose. The interposition of mechanism is indispensable; on the
invention and contrivance of that mechanism depends altogether the useful
application of these natural forces.
The world owes the steam engine then partly to discovery, and partly to
invention.
He that discovered the fact, that mechanical force was produced in the
conversion of water into steam, must be justly held to be a sharer in the
merit of the steam engine, even though he should never have practically

applied his discovery. The like may be said of him who first discovered the
source of the mechanical power arising from the expansion of steam.
The discoverer of the fact, that steam being reconverted into water
greatly contracted its dimensions, and thereby produced a vacuum, is
likewise entitled to a share of the credit. [Pg012]
The mechanism by which these natural forces have been rendered so
universally available as a moving power, is very various and complicated,
and cannot be traced to one inventor. "If a watchmaker," says M. Arago,
"well instructed in the history of his art, were required to give a categorical
answer to the question, Who has invented watches? he would remain mute;
but the question would be divested of much of its difficulty if he were
required separately to declare who discovered the use of the main spring,
the different forms of escapement, or the balance wheel." So it is with the
steam engine. It is a combination of a great variety of contrivances, distinct
from each other, which are the production of several inventors. If, however,
one name more than the rest be entitled to special notice; if he is entitled to
the chief credit of the invention who by the powers of his mechanical
genius has imparted to the steam engine that form, and conferred upon it
those qualities, on which mainly depends its present extensive utility, and
by which it has become an agent of transcendant power, spreading its
beneficial effects throughout every part of the civilised globe, then the
universal voice will, as it were by acclamation, award the honour to one
individual, whose pre-eminent genius places him far above all other
competitors, and from the application of whose mental energies to this
machine may be dated those grand effects which render it a topic of interest
to all for whom the progress of civilisation has any attractions. Before the
era rendered memorable by the discoveries of James Watt, the steam
engine, which has since become an object of such universal interest, was a
machine of extremely limited power, inferior in importance and usefulness
to most other mechanical agents used as prime movers; but, from that
epoch, it is scarcely necessary here to state, that it became a subject not of
British interest only, but one having an important connection with the
progress of the human race.
Hero of Alexandria, 120 B. C.

(3.)

The discovery of the fact, that a
mechanical force is produced when
water is evaporated by the
application of heat, [Pg013] must be
considered as the first capital step
in the invention of the steam
engine. It is recorded in a work
entitled Spiritalia seu Pneumatica,
that Hero of Alexandria contrived a
machine, 120 years before the Fig. 1.
Christian era, which was moved by
the mechanical force of the vapour of water. The principle of this machine
admits of easy explanation: When a fluid issues from any vessel in which it
is confined, that vessel suffers a force equal to that with which the fluid
escapes from it, and in the opposite direction. If water issues from an
orifice, a pressure is produced behind the orifice corresponding to the force
with which the water escapes. If a man discharge a gun, the gases produced
by the explosion of the powder issue with a certain force from the muzzle,
and his shoulder is driven backwards by the recoil with a corresponding
force. If the muzzle, instead of being presented forwards, were turned at
right angles to the length of the gun, then, as the gases of explosion would
escape sideways, the recoil would likewise take place sideways, and the
shooter, instead of being driven backward, would be made to spin round as
a dancer pirouettes. This was the principle of Hero's steam engine. A small
globe or ball was placed on pivots at A and B (fig. 1.), on which it was
capable of revolving: steam was supplied through one of these pivots from
one of the tubes D C E F, which communicated with the boiler. This steam
filled the globe A I B K, and also the arms I H and K G. A lateral orifice,
represented at G, near the end of these arms, allowed the steam to escape in
a jet, and the reaction, producing a recoil, had a tendency to drive the arm
round. A small orifice at H, on the other side of the tube, produced a like
effect. In the same manner, any convenient number of arms might be
provided, surrounding the globe and communicating with its interior like

the spokes of a wheel. Thus these arms, having lateral orifices for the
escape of the steam, all placed so that the recoil may [Pg014] tend to turn the
globe in the same direction, a rotatory motion might be communicated to
any machinery which it was desired to move.
After having been allowed to slumber for nearly two thousand years, this
machine has recently been revived, and engines constructed similar to it are
now working in these countries. In the proper place we shall describe
Avery's Rotatory Engine, which it will be seen is, not only in its principle,
but almost in its details, the machine of Hero of Alexandria.
Although the elastic force of steam was not reduced to numerical
measure by the ancients, nor brought under control, nor applied to any
useful purpose, yet it appears to have been recognised in vague and general
terms. Aristotle, Seneca, and other ancient writers, accounted for
earthquakes by the sudden conversion of water into steam within the earth.
This change, according to them, was effected by subterranean heat. Such
tremendous effects being ascribed to steam, it can scarcely be doubted that
the Greeks and Romans were acquainted with the fact, that water in passing
into vapour exercises considerable mechanical power. They were aware that
the earthquakes, which they ascribed to this cause, exerted forces
sufficiently powerful to extend the natural limits of the ocean; to overturn
from their foundations the most massive monuments of human labour; to
raise islands in the midst of seas; and to heave up the surface of the land of
level continents so as to form lofty mountains.
Such notions, however, resulted not as consequences of any exact or
scientific principles, but from vague analogies derived from effects which
could not fail to have been manifested in the arts, such as those which
commonly occurred in the process of casting in metal the splendid statues
which adorned the temples, gardens, and public places of Rome and Athens.
The artisan was liable to the same accidents to which modern founders are
exposed, produced by the casual presence of a little water in the mould into
which the molten metal is poured. Under such circumstances, the sudden
formation of steam of an extreme pressure produces, as is well known,
explosions attended with destructive effects. The Grecian [Pg015] and Roman
artisans were subject to such accidents; and the philosopher, generalising

such a fact, would arrive at a solution of the grander class of phenomena of
earthquakes and volcanoes.
Before natural phenomena are rendered subservient to purposes of utility,
they are often made to minister to the objects of superstition. The power of
steam is not an exception to this rule. It is recorded in the Chronicles, that
upon the banks of the Weser the ancient Teutonic gods sometimes marked
their displeasure by a sort of thunderbolt, which was immediately
succeeded by a cloud that filled the temple. An image of the god Busterich,
which was found in some excavations, clearly explains the manner in which
this prodigy was accomplished by the priests. The head of the metal god
was hollow, and contained within it a pot of water: the mouth, and another
hole, above the forehead, were stopped by wooden plugs; a small stove,
adroitly placed in a cavity of the head under the pot, contained charcoal,
which, being lighted, gradually heated the liquid contained in the head. The
vapour produced from the water, having acquired sufficient pressure, forced
out the wooden plugs with a loud report, and they were immediately
followed by two jets of steam, which formed a dense cloud round the god,
and concealed him from his astonished worshippers.[1]
Among other amusing anecdotes showing the knowledge which the
ancients had of the mechanical force of steam, it is related that Anthemius,
the architect of Saint Sophia, occupied a house next door to that of Zeno,
between whom and Anthemius there existed a feud. To annoy his
neighbour, Anthemius placed on the ground floor of his own house several
close digesters, or boilers, containing water. A flexible tube proceeded from
the top of each of these, which was conducted through a hole made in the
wall between the houses, and which communicated with the space under the
floors of the rooms in the house of Zeno. When Anthemius desired to annoy
his neighbour, he lighted fires under his boilers, and the steam produced by
them rushed in such quantity and with [Pg016] such force under Zeno's floors,
that they were made to heave with all the usual symptoms of an earthquake.
[2]

Blasco de Garay, A. D. 1543.

(4.)

In the year 1826, M. de Navarrete published, in Zach's Astronomical
Correspondence, a communication from Thomas Gonzales, Director of the
royal archives of Simancas, giving an account of an experiment reported to
have been made in the year 1543, in which a vessel was propelled by a
machine having the appearance of a steam engine.
Blasco de Garay, a sea captain, proposed in that year to the Emperor
Charles V. to propel vessels by a machine which he had invented, even in
time of calm, without oars or sails. Notwithstanding the apparent
improbability attending this project, the Emperor ordered the experiment to
be made in the port of Barcelona, and the 17th of June, 1543, was the day
appointed for its trial. The commissioners appointed by Charles V. to attend
and witness the experiment were Don Henry of Toledo, Don Pedro of
Cardona, the treasurer Ravago, the vice chancellor and intendant of
Catalonia, and others. The vessel on which the experiment was made was
the Trinity, 200 tons burthen, which had just discharged a cargo of corn at
Barcelona. Garay concealed the nature of his machinery, even from the
commissioners. All that could be discovered during the trial was, that it
consisted of a large boiler containing water, and that wheels were attached
to each side of the vessel, by the revolution of which it was propelled. The
commissioners having witnessed the experiment, made a report to the king,
approving generally of the invention, particularly on account of the ease
and promptitude with which the vessel could be put about by it.
The treasurer Ravago, who was himself hostile to the project, reported
that the machine was capable of propelling a vessel at the rate of two
leagues in three hours; but the other commissioners stated that it made a
league an hour at the least, and that it put the vessel about as speedily as
would be accomplished with a galley worked according to the common [Pg017]
method. Ravago reported that the machinery was too complicated and
expensive, and that it was subject to the danger of the boiler bursting.
After the experiment was made, Garay took away all the machinery,
leaving nothing but the framing of wood in the arsenals of Barcelona.
Notwithstanding the opposition of Ravago, the invention was approved,
and the inventor was promoted and received a pecuniary reward, besides
having all his expenses paid.

From the circumstance of the nature of the machinery having been
concealed, it is impossible to say in what this machine consisted; but as a
boiler was used, it is probable, though not certain, that steam was the agent.
There have been various machines proposed, of which a furnace and boiler
form a part, and in which the agency of steam is not used. The machine of
Amontons furnishes an example of this. It is most probable that the
contrivance of Garay was identical with that of Hero. The low state of the
arts in Spain in the sixteenth century would be incompatible with the
construction of any machine requiring great precision of execution. But the
simplicity of Hero's contrivance would have rendered its construction and
operation quite practicable. As to the claims to the invention of the steam
engine advanced by the advocates of De Garay, founded on the above
document, a refutation is supplied by the admission, that though he was
rewarded and promoted by the government of the day, in consequence of
the experiment, and although the great usefulness of the contrivance in
towing ships out of port, &c., was admitted, yet it does not appear that a
second experiment was ever tried, much less that the machine was ever
brought into practical use.
Solomon de Caus, 1615.

(5.)

Solomon De Caus was engineer and architect to Louis XIII., king of France,
before the year 1612. In that year he entered the service of the Elector
Palatine, who married the daughter of King James I., with whom he came to
England. He was there employed by the Prince of Wales in ornamenting the
gardens of his house at Richmond. [Pg018] During his sojourn in England he
composed and published at London, in the same year, a Treatise on
Perspective. This person was the author of a work entitled, "Les Raisons des
Forces Mouvantes, avec diverses Machines tant utiles que plaisantes,"
which was apparently composed at Heidelburg, but published at Franckfort,
in 1615. The same work was subsequently republished in Paris in 1623.
The treatise commences with definitions of what were then considered
the four elements: earth, air, fire, and water. Air is defined to be a cold, dry,
and light element, capable of compression, by which it may be rendered

very violent. He says, "The violence will be great when water exhales in air
by means of fire, and that the said air is enclosed: as, for example, take a
ball of copper of one or two feet diameter, and one inch thick, which being
filled with water by a small hole, which shall be strongly stopped with a
peg, so that neither air nor water can escape, it is certain that if we put the
said ball upon a great fire, so that it will become very hot, that it will cause
a compression so violent, that the ball will burst in pieces, with a noise like
a petard."
The effect which is here described is due to the combined pressure of the
heated air contained in the ball and the high pressure steam raised from the
water, but much more to the latter than to the former. It is evident, however,
from the language of De Caus, that he ascribes the force entirely to the air,
and seems to consider that the force of the air proceeded from the water
which exhaled in it.
The first theorem is, "that the parts of the elements mix together for a
time, and then each returns to its place" (the elements here referred to being
apparently air and water). Upon this subject the following is an example:
"Take a round vessel of copper, soldered close on every side, and with a
tube, whereof one end approaches nearly to the bottom of the vessel, and
the other end, which projects on the outside of the vessel, has a stop-cock;
there is also a hole in the top of the vessel, with a plug to stop it. If this
vessel will contain three pots of water, then pour in one pot of water, and
place the vessel on the fire about three or four minutes, leaving the hole
open; then take the vessel off the [Pg019] fire, and a little after pour out the
water at the hole, and it will be found that a part of the said water has been
evaporated by the heat of the fire. Then pour in one pot of water as before,
and stop up the hole and the cock, and put the vessel on the fire for the
same time as before; then take it off, and let it cool of itself, without
opening the plug, and after it is quite cold pour out the water, and it will be
found exactly the same quantity as was put in. Thus we see that the water
which was evaporated (the first time that the vessel was put on the fire) is
returned into water the second time when that vapour has been shut up in
the vessel, and cooled of itself."

In the description of these experiments, the processes of evaporation and
condensation are obscurely indicated; but there is no intimation that the
author possessed any knowledge of the elastic force of steam. His theorem
is, that the parts of the element water mix for a time with the parts of the
element air; that fire causes this mixture, and that on removing the fire, and
dissipating the heat, then the parts of the water mixed with air return to their
proper place, forming again part of the water. There is no indication of a
change of property of the water in passing into vapour. It is difficult to
conceive, if De Caus had been aware that the vapour of water possessed the
same violent force which he distinctly and in terms ascribes to air, or if he
had been aware that in effect the vapour of the water produced by the fire
was a fluid, possessing exactly the same mechanical qualities, and
producing the same mechanical effects as air, that he would not have
expressed himself clearly on the subject.
He proceeds to give another demonstration that heat will cause the
particles of water to mix with those of air.
"After having put the measure of water into the vessel, and shut the vent-
hole, and opened the cock, put the vessel on the fire, and put the pot under
the cock; then the water of the vessel, raising itself by the heat of the fire,
will run out through the cock; but about one sixth or one eighth part of the
water will not run out, because the violence of the vapour which causes the
water to rise proceeds from the [Pg020] said water; which vapour goes out
through the cock after the water with great violence. There is also another
example in quicksilver, or mercury, which is a fluid mineral, but being
heated by fire, exhales in vapour, and mixes with the air for a time; but after
the said vapour is cooled, it returns to its first nature of quicksilver. The
vapour of water is much lighter, and therefore it rises higher," &c. &c.
In this second demonstration there appears to be some obscure indication
of the force of steam in the words "because of the violence of the vapour
which causes the water to rise," &c.
The fifth theorem is the following:—
"Water will mount by the help of fire higher than its level," which is
explained and proved in the following terms:—

"The third method of raising water is by
the aid of fire. On this principle may be
constructed various machines: I shall here
describe one. Let a ball of copper marked A;
well soldered in every part, to which is
attached a tube and stop-cock marked D, by
which water may be introduced; and also
another tube marked B C, which will be
soldered into the top of the ball, and the
lower end C of which shall descend nearly to
the bottom of the ball without touching it.
Let the said ball be filled with water through
the tube D, then shutting the stop-cock D, and
Fig. 2. opening the stop-cock in the vertical tube B C,
let the ball be placed upon a fire the heat
acting upon the said ball will cause the water to rise in the tube B C."
In the apparatus as here described, the space enclosed in the boiler above
the surface of the water is filled with air. By the action of the fire, two
effects are produced: first, the air enclosed above the water, being heated,
acquires increased elasticity, and presses with a corresponding force on the
surface of the water. By this means a column of water will be driven up the
tube A B at such a height as will balance the elasticity of the heated air
confined in the boiler; but besides [Pg021] this the water contained in the boiler
being heated, will produce steam, which being mixed with air contained in
the boiler, will likewise press with its proper elasticity on the surface of the
water, and will combine with the air in raising a column of water in the tube
A B. In the above description of the machine, the force which raises the
water in the tube A B is ascribed to the fire, no mention being made of the
water, or of the vapour or steam produced from it having any agency in
raising the water in the tube A B.
Antecedently to the date of this invention, the effect of heat in increasing
the elastic force of air was known, and so far as the above description goes,
the whole operation might be ascribed to the air by a person having no
knowledge whatever of the elasticity of steam. M. Arago, however, who, on

the grounds of this passage in the work of De Caus, claims for him a share
of the honour of the invention of the steam engine, contends that the agency
of steam in this apparatus was perfectly known to De Caus, although no
mention is made of steam in the above description, because in the second
demonstration above quoted he uses the words, "the violence of the vapour
which causes the water to rise proceeds from the said water; which vapour
goes out from the cock after the water with great violence." By these words
M. Arago considers that De Caus expresses the quality of elasticity proper
to the vapour, and that the context justifies the inference, that to this
elasticity he ascribed the elevation of the water in the tube C B.
There appears to be some uncertainty attending the birthplace of De
Caus. In the Biographie Universelle he is said to have been born and to
have died in Normandy. M. Arago assigns Dieppe, or its neighbourhood, as
his birthplace.
There was another engineer and architect, Isaac De Caus, a native of
Dieppe, who published a work in folio, entitled "Nouvelle Invention de
Lever l'Eau plus haut que sa Source, avec quelque Machines mouvantes,
par le Moyen de l'Eau, et un Discours de la Conduite d'Icelle." This volume
is without a date, but from the nature of its contents it would appear to have
been published before the work of Solomon De Caus already cited. The
drawings and machines described in both [Pg022] are exactly the same; but the
definitions and theorems quoted above on raising water by fire are not
given in the work of Isaac. It seems, therefore, that Solomon De Caus re-
published, with additions, the work of Isaac De Caus. From the same
birthplace being assigned to both these authors, as well as from the
similarity of their pursuits, it is likely they were members of the same
family, and from their christian names they were probably Jews.
The work cited above, was dedicated to Louis XIII., and in the dedication
Solomon De Caus calls himself the subject of that monarch; and in the
privilege prefixed to the work he is designated, "Our well-beloved Solomon
De Caus, master engineer, being at present in the service of our dear and
well-beloved cousin, the Prince Elector Palatine, has made known to us,"
&c.—"we, desiring to gratify the said De Caus, he being our subject," &c.

It is therefore certain, whatever may have been the birthplace of De Caus,
that he was at least a subject of France. The circumstance of his work being
written in French, though published beyond the Rhine, is also an argument
in favour of his being a native of that country.
Giovanni Branca, 1629.

(6.)

Giovanni Branca of Loretto in Italy, an engineer and architect, proposed to
work mills of different kinds by steam issuing from a large æolopile, and
blowing against the vanes of a wheel. Branca was the author of many
ingenious mechanical inventions, a collection of which he dedicated to M.
Cenci, the governor of Loretto. These were published in a work printed at
Rome in 1629. It is a thin quarto, entitled "Le Machine volume nuovo, et di
molto artificio da fare effetti maravigliosi tanto Spiritali quanto di Animale
Operatione, arichito di bellissime figure. Del Sig. Giovanni Branca,
Cittadino Romano. In Roma, 1629." The work contains sixty-three
engravings, accompanied by descriptions in Italian and Latin. Branca's
steam engine, represented in the twenty-fifth plate, consists of a wheel
furnished with flat vanes upon its rim, like the boards of a paddle wheel.
The steam is produced in a close vessel, and made to issue with violence
from the extremity [Pg023] of a pipe directed against the vanes, and causes the
wheel to revolve. This motion being imparted by the usual mechanical
contrivances, any machinery may be impelled by it. Different useful
applications of this power are contained in the work, viz. pestles and
mortars for pounding materials to make gunpowder, and rolling stones for
grinding the same; machines for raising water by buckets, for sawing
timbers, for driving piles, &c. &c.
This method of applying the force of steam has no analogy to any
application of steam in modern engines.
Edward Somerset, Marquis of Worcester, 1663.

(7.)

Of all the names which figure in the early annals of steam, by far the most
remarkable is that of the Marquis of Worcester, who has left a description of
a machine in a work, entitled "The Scantling of One Hundred Inventions,"
which has been generally in this country considered as giving him a right to
the honour of having been the inventor of the steam engine.
Lord Worcester having been engaged on the side of the Royalists in the
civil wars of the revolution, lost his fortune, and went to Ireland, where he
was imprisoned. He escaped from thence, and reached France; from that
country he ventured to London, as a secret agent of Charles II., but was
detected, and imprisoned in the Tower, where he remained until the
restoration, when he was set at liberty. Tradition has connected the
invention of the steam engine with the following anecdote:—One day,
during his imprisonment, Lord Worcester observed the lid of the pot in
which his dinner was being cooked, suddenly forced upwards by the vapour
of the water which was boiling in it. Reflecting on this, it occurred to him
that the same force which raised the cover of the pot might be rendered,
when properly applied, a useful and convenient moving power. After he
recovered his liberty, he accordingly proceeded to carry into effect this
conception. The contrivance to which he was ultimately led is described in
the following terms in the sixty-eighth invention, in the work above named:
—
"I have invented an admirable and forcible way to drive [Pg024] up water by
fire; not by drawing or sucking it upwards, for that must be, as the
philosopher terms it, infra sphœrum activitatis, which is but at such a
distance. But this way hath no bounder if the vessels be strong enough. For
I have taken a piece of whole cannon whereof the end was burst, and filled
it three quarters full of water, stopping and screwing up the broken end, as
also the touch-hole, and making a constant fire under it; within twenty-four
hours, it burst and made a great crack. So that, having a way to make my
vessels so that they are strengthened by the force within them, and the one
to fill after the other, I have seen the water run like a constant fountain
stream forty feet high. One vessel of water rarefied by fire driveth up forty
of cold water, and a man that tends the work has but to turn two cocks; that
one vessel of water being consumed, another begins to force and refill with

cold water, and so successively; the fire being tended and kept constant,
which the self-same person may likewise abundantly perform in the interim
between the necessity of turning the said cocks."
Since the date of the publication of the "Century of Inventions" was the
year 1663, the experiments here mentioned must have been made before
that year. The description of the machine here given, as well as others in the
same work, was intended by the author, not to convey a knowledge of the
nature of the mechanism which he used, but only to express the effects
produced, and to indicate the physical principle on which they depended. It
should also be observed, that an air of mystery was thrown by Worcester
over the accounts of all the machines which he described; and therefore any
obscurity in the above description ought not to be regarded as an evidence
against his claim to the discovery of the mechanical agency of steam, so far
as that agency is indicated by the effects said by him to be produced. The
above account is, however, sufficiently distinct and explicit to enable any
one possessing a knowledge of the mechanical qualities of steam to
perceive the general nature of the machine described. To render this
machine, and that of De Caus, previously described, intelligible to those
who are not familiar with physical science, we must here explain some
general principles on which their agency depends. [Pg025]

(8.)

Fluid bodies are of two kinds, inelastic fluids, or liquids, and elastic fluids,
or gases. Of the former of these classes, water is the most familiar example,
and of the latter, air.
These two species of fluids are each distinguished by peculiar
mechanical properties.

(9.)

The constituent particles of a liquid are distinguished from those of solids
by having little or no coherence; so that unless the mass be confined by the
sides of the vessel which contains it, the particles will fall asunder by their
gravity. A mass of liquid, therefore, unlike a solid, can never retain any

particular form, but will accommodate
itself to the form of the vessel in which it is
placed. It will press against the bottom of
the vessel which contains it with the whole
force of its weight, and it will press against
the sides with a force proportional to the
depth of the particles in contact with the
sides measured from the surface of the
liquid above. This lateral pressure also
distinguishes liquids from solids. Let us Fig. 3.
take for illustration the case of a square or a
cubical vessel, A B C D, fig. 3. If a solid body, such as a piece of lead, be cut
to the shape of this vessel, so as to fit in it without pressing with any force
against its sides, the mechanical effect which would be produced by it when
placed in the vessel, would be merely a pressure upon the bottom, B C, the
amount of which would be equal to the weight of the metallic mass. No
pressure would be exerted against the sides; for the coherence of the
particles of the solid maintaining them in their position, the removal of the
sides would not subject the solid body contained in the vessel to any
change.
Now let us suppose this solid mass of lead to be rendered liquid by being
melted. The constituent particles will then be deprived of that cohesion by
which they were held together; they will accordingly have a tendency to
separate, and fall asunder by their gravity, and will only be prevented from
actually doing so by the support afforded to them by the sides, [Pg026] A B, D C,
of the vessel. They will therefore produce a pressure against the sides,
which was not produced by the lead in its solid state. This pressure will
vary at different depths: thus a part of the side of the vessel at P will receive
a pressure proportional to the depth of the point P below the surface of the
lead. If, for example, we take a square inch of the inner surface of the side
of the vessel at P, it will sustain an outward pressure equal to the weight of a
column of lead having a square inch for its base, and a height equal to P A.
And, in like manner, every square inch of the sides of the vessel will sustain
an outward pressure equal to the weight of a column of lead having a square

inch for its base, and a height equal to the depth of the point below the
surface of the lead.

(10.)

We have here proceeded upon the supposition that no force acts on the
upper surface A D of the lead. If any force presses A D downwards, that force
would be transferred to the bottom by the lead, and would produce a
pressure on the bottom B C equal to its own amount in addition to the weight
of the lead; and if the lead were solid, this would be the only additional
mechanical effect which such a force acting on the surface A D of the lead
would produce. But if, on the other hand, the lead were liquified, then the
force now adverted to, acting on the surface A D, would not only produce a
pressure on the bottom B C, equal to its own amount in addition to the
weight of the lead, but it would also produce a pressure against every part
of the sides of the vessel, equal to that which it would produce upon an
equal magnitude of the surface A D.
Thus if we suppose any mechanical cause producing a pressure on the
surface A D amounting to ten pounds on each square inch, the effect which
would be produced, if the lead were solid, would be an additional pressure
on the base B C amounting to ten pounds per square inch. But if the lead
were liquid, besides this pressure on each square inch of the base B C, there
would likewise be a pressure of ten pounds on every square inch of the
sides of the vessel.
All that has been here stated with respect to a square or a cubical vessel
will be equally applicable to a vessel of any other form. [Pg027]

(11.)

The second class of fluids are distinguished from liquids by the particles not
merely being destitute of cohesion, but having a tendency directly the
reverse, to repel each other, and fly asunder with more or less force. Thus if
a vessel, such as that represented in fig. 3., were filled with a fluid of this
kind, being open at the top, and not being restrained by any pressure

incumbent upon it, the particles of the fluid would not rest in the vessel by
their gravity, as those of the liquid would do; but they would, by their
mutual repulsion, fly asunder, and rise out of the vessel, as smoke is seen to
rise from a chimney, or steam from the spout of a kettle. Let us suppose,
then, that the vessel in which an elastic fluid is contained is closed on every
side by solid surfaces. In fact, let us imagine that the square or cubical
vessel represented in fig. 3. is closed by a square lid at the top A D, having
contained in it an elastic fluid, such as atmospheric air.
If such a cover, or lid, had been placed upon a liquid, the cover would
sustain no pressure from the fluid, nor would any mechanical effect be
produced, save those already described in the case of the open vessel; but
when the fluid contained in the vessel is elastic, as is the case with air, then
the elasticity (by which name is expressed the tendency of the particles of
the fluid to fly asunder) will produce peculiar mechanical effects, which
have no existence whatever in the case of a liquid.
It is true that, supposing the fluid to be air or any other gas or vapour, a
pressure will be produced upon the bottom B C of the vessel equivalent to
the weight of such fluid, and lateral pressures will be produced on the
different points of the sides by the weight of that part of the fluid which is
above these points; but gases and vapours are bodies of such extreme levity,
that these effects due to their weight are neglected in practice.
Putting, then, the weight of the air contained in the vessel out of the
question, let us consider the effect of its elasticity. If the vessel, as already
described, be supposed to contain atmospheric air in its ordinary state, the
tendency of the constituent particles to fly asunder will be such as to
produce on every square inch of the inner surface of the vessel [Pg028] a
pressure amounting to fifteen pounds; this pressure being, as already stated,
quite independent of the weight of the air. In fact, this pressure would
continue to exist if the air contained in the vessel actually ceased to have
weight by being removed from the neighbourhood of the earth, which is the
cause of its gravity.

(12.)

Different gases are endowed with different degrees of elasticity, and the
same gas may have its elasticity increased or diminished, either by varying
the space within which it is confined, or by altering the temperature to
which it is exposed.
If the space within which an elastic fluid is enclosed be enlarged, its
elasticity is found to diminish in the same proportion. Thus if the air
contained in the vessel A B C D (fig. 3.) be allowed to pass into a vessel of
twice the magnitude, the elasticity of the particles will cause them to repel
each other, so that the same quantity of air shall diffuse itself throughout the
larger vessel, assuming double its former bulk. Under such circumstances,
the pressure which it would exert upon the sides of the larger vessel would
be only half that which it had exerted on the sides of the smaller vessel. If,
on the other hand, it were forced into a vessel of half the magnitude of A B C
D, as it might be, then its elasticity would be double, and it would press on
the inner surface of that vessel with twice the force with which it pressed on
that of the vessel A B C D.
This power of swelling and contracting its dimensions according to the
dimensions of the vessel in which it is confined, or to the force compressing
it, is a quality which results immediately from elasticity, and is
consequently one which is peculiar to the gases or elastic fluids, and does
not at all appertain to liquids. If the liquid contained in the vessel A B C D
were transferred to a vessel of twice the magnitude, it would only occupy
half the capacity of that vessel, and it could not by any means be
transferred, as we have supposed the air or gas to be, to a vessel of half the
dimensions, since it is inelastic and incompressible.

(13.)

The elasticity of gases is likewise varied by varying the temperature to
which they are exposed; thus, in general, [Pg029] if air or any other gas be
augmented in temperature, it will likewise be increased in elasticity; and if,
on the other hand, it be diminished in temperature, it will be likewise
diminished in its elastic force. The more heated, therefore, any air or gas

confined in a vessel becomes, the greater will be the force with which it will
press on the inner surface of that vessel, and tend to burst it.

(14.)

The same body may, by the agency of heat, be made to pass successively
through the different states of solid, liquid, and gas, or vapour. The most
familiar and obvious example of these successive transitions is presented by
water. Exposed to a certain temperature, water can only exist as a solid; as
the temperature is increased, the ice, or solid water, is liquefied; and by the
continued application of heat, this water again undergoes a change, and
assumes the form, and acquires the mechanical qualities, of air or gas: in
such a state it is called STEAM.
This is a common property of all liquids. If they be exposed for a
sufficient length of time to a sufficient degree of heat, they will always be
converted into elastic fluids. These are usually distinguished from air and
other permanent gases, which never are known to exist in the liquid form,
by the term vapour, by which, therefore, must be understood an elastic fluid
which at common temperatures exists in the liquid or solid state; by steam
is expressed the vapour of water; and by gases, those elastic fluids which
like air are never known—at least, under ordinary circumstances—to exist
in any other but the elastic form.

(15.)

When a liquid is caused, by the application of heat, to take the form of an
elastic fluid, or is evaporated, besides acquiring the property of elasticity, it
always undergoes a considerable change of bulk. The amount of this change
is different with different liquids, and even with the same liquid it varies
with the circumstances under which the change is produced.

(16.)

When water is evaporated under ordinary circumstances,—that is, when
exposed to no other external pressure than that of the atmosphere,—it
increases its volume about seventeen-hundred-fold. Thus a cubic inch of

liquid [Pg030] water would form about seventeen hundred cubic inches of
common steam. If, however, the water be confined by a greater pressure
than that produced by the common atmosphere, then the increase of volume
which takes place in its evaporation would be less in proportion.
These important physical circumstances are now only indicated in a
general way. As we proceed with our account of the invention and
improvement of the steam engine, they will be developed more fully and
accurately.

(17.)

After duly considering what has been just explained, no difficulty will be
found in comprehending the principles on which the first rude attempts at
the mechanical application of steam already stated depend. In the apparatus
ascribed to Hero of Alexandria, the elasticity of the vapour contained in the
arms of the revolving ball causes that vapour to issue from the lateral
orifices in the arms, such as that of G, fig. 1. As these orifices, however, are
exposed to the common atmosphere pressing inwards with a force, the
mean amount of which has been stated to be about fifteen pounds per
square inch, it follows that the steam cannot escape from these orifices until
its pressure or elasticity exceeds this amount, and that when it does, the
force with which it will so escape will be the excess of its elasticity above
that of the atmosphere; and it is the reaction produced by this difference of
pressure, causing the arms to recoil, which will give motion to the machine.
In the case of the apparatus of De Caus (5.), the heat of the fire acting on
the vessel D C (fig. 2.) will raise the temperature of the water contained in it,
and also of the air confined within it above the surface of that water. This
air, as it is increased in temperature, will also increase in elasticity; it will
therefore press on the surface of the water with increased force, and will
gradually force the water upwards in the tube; and this effect would
continue until all the water in the vessel would be forced up the tube.
But at the same time that the heat acting on the vessel increases the
temperature of the air above the water, it also produces a partial evaporation
of the water, so that more or less steam is mixed with the air in the vessel

above the surface [Pg031] of the water; and this steam possessing elasticity,
unites with the air in pressing on the surface of the water, and in raising it in
the tube.
Let us now revert to
the brief account of the
engine of the Marquis of
Worcester, described in
"The Century of
Inventions." We collect
from that description that
the vessel in which the
water was evaporated
was separate from those
which contained the
water to be elevated; also
that there were two
vessels of the like
description, the contents
of which were alternately
elevated by the pressure
of the "water rarefied by
the fire;" in other words
Figs. 4, 5, and 6. by steam; and that the
water was raised in an
uninterrupted stream, by
the management of two cocks communicating with these vessels and with
the boiler. The following is such an apparatus as would answer this
description. Let E (fig. 4.) be the vessel containing the water to be
evaporated, placed over a proper furnace A; let S be a pipe to allow the
steam produced from the boiling water in E to pass into the vessels where its
mechanical action is required. Let R represent a cock or regulator, having in
it a curved passage, leading from S to the tube T, when the lever or handle L
is in the position represented by the cut; but leading to the tube T′, when the
lever L is turned one quarter of a revolution to the right, as represented in

fig. 5. By the shifting of this lever, therefore, the steam pipe S may be made
to communicate alternately with the tubes T and T′. The tubes T and T′ are
carried respectively to two vessels V and V′, which are filled with the water
required to be raised. In these [Pg032] vessels tubes enter at C and C′, descending
nearly to the bottom: these tubes have valves at B and B′, opening upwards,
by which water will be allowed to pass into the vertical tube F, but which
will not allow it to return downwards, the valves B and B′ being then closed
by the weight of the water above them.
Let G G′ be a pipe entering the sides of the vessels V and V′, for the
purpose of filling them with the water to be raised: let K be a cock having a
curved passage similar to the cock R, and leading to a tube by which water
is supplied from the reservoir or other source from which the water to be
raised is drawn. When the cock K is placed as represented in fig. 4., the
water from the reservoir will flow through the curved passage in the cock K
into the tube G′, and thence into the vessel V′; but when this cock is turned
one quarter round, by shifting the lever to the left, it will take the position
represented in fig. 6., and the water will flow through the curved passage
into the tube G, and thence into the vessel V. Let us now suppose the vessel
V already filled with water to be elevated, and the vessel V′ to have
discharged its contents. The cock R is turned, so as to allow the steam
generated in the boiler E to pass into the tube T, and thence into the upper
part of the vessel V, while the cock K is turned so as to allow the water from
the reservoir to pass into the tube G′, and thence into the vessel V′. The
steam collecting in the upper part of the vessel V′ presses with its elastic
force on the surface of the water therein, and forces the water upwards in
the tube C; it passes through the valve B, which it opens by the upward
pressure received from the action of the steam, and thence into the tube F,
its descent into the tube C′ being prevented by the valve V′, which can only
be opened upwards. As the steam is gradually supplied from the boiler E,
the water in the vessel V is forced up the tube C, through the valve B, and
into the tube F, until all the contents of the vessel V above the lower end of
the tube C have been raised. In the meanwhile, the vessel V′ has been filled
with water, through the cock K: when this has been accomplished, the man

who attends the machine shifts the cocks R and K, so as to give them the
position represented in fig. 5. and fig. 6. [Pg033] In this position, the steam from
the boiler, being excluded from the tube T, will be conducted to the tube T′,
and thence to the vessel V′, while the water from the reservoir will be
excluded from the tube G′, and conducted through the tube G to the vessel V.
The vessel V will thus be replenished and, by a process similar to that
already described, the contents of the vessel V′ will be forced up the tube C′,
through the valve B′, and into the tube F; its descent into the tube C being
prevented by the valve B, which will then be closed. After the contents of
the vessel V′ have thus been raised, and the vessel V replenished, the two
cocks R and K are once more shifted, and the contents of V raised while V′ is
replenished, and so on.

If, having
comprehended the
apparatus here described,
the reader refers to the
description of the
Marquis of Worcester's
machine, he will find that
all the conditions therein
laid down are fulfilled by
it. One vessel (E) of
"water rarefied by fire"
may by such means
"drive up forty (or more)
of cold water; and the
man that tends the work
has but to turn two cocks,
that one vessel (V) of
water being consumed,
another (V′) begins to
Fig. 4, 5, and 6. force and refill with cold
water, and so on
successively, the fire being tended and kept constant; which the self-same
person may likewise abundantly perform, in the interim between the
necessity of turning the said cocks."
On comparing this with the contrivance previously suggested by De
Caus, it will be observed, that even if De Caus [Pg034] knew the physical agent
by which the water was driven upwards in the apparatus described by him,
still it was only a method of causing a vessel of boiling water to empty
itself; and before a repetition of the process could be made, the vessel
should be refilled, and again boiled. In the contrivance of Lord Worcester,
on the other hand, the agency of the steam was employed in the same
manner as it is in the steam engines of the present day, being generated in
one vessel, and used for mechanical purposes in another. Nor must this
distinction be regarded as trifling or insignificant, because on it depends the

whole practicability of using steam as a mechanical agent. Had its action
been confined to the vessel in which it was produced, it never could have
been employed for any useful purpose.
Although many of the projects contained in Lord Worcester's work were
in the highest degree extravagant and absurd, yet the engine above
described is far from being the only practicable and useful invention
proposed in it. On the contrary, many of his inventions have been
reproduced, and some brought into general use since his time. Among these
may be mentioned, stenography, telegraphs, floating baths, speaking
statues, carriages from which horses can be disengaged if unruly,
combination locks, secret escutcheons for locks, candle moulds, the rasping
mill, the gravel engine, &c.
Sir Samuel Morland, 1683.

(18.)

Sir Samuel Morland was the son of a baronet of the same name, who had
received his title at the restoration for some services to the royalist party,
performed by him during the wars of the Commonwealth. He appears to
have devoted much attention to mechanics, in which he attained some
celebrity. He was the reputed inventor of several ingenious contrivances,
such as the drum capstan for ships, the plunger pump, &c. He also
investigated various questions in acoustics, and among others, the
determination of the best form for the speaking-trumpet.
In 1680, Sir Samuel Morland was appointed Master [Pg035] of the Works to
Charles II., and in the following year was sent to France, to execute some
waterworks for Louis XIV. In 1683, while in France, he wrote in the French
language, a work entitled "Elevation des Eaux par toute sorte de Machines,
reduite à la Mesure, au Poids et à la Balance. Presentée à sa Majesté très
Chrestienne, par le Chevalier Morland, Gentilhomme Ordinaire de la
Chambre Privée, et Maistre des Méchaniques du Roi de la Grande
Brétagne, 1683." This book is preserved in manuscript in the Harleian
Collection in the British Museum. It is written on vellum, and consists of
only thirty-eight pages. It contains tables of measures and weights,

theorems for the calculation of the volumes of cylinders, the weights of
columns of water, the thickness of lead for pipes, and is concluded by a
chapter on steam, consisting of four pages, of which the following is a
translation:—
"The principles of the new force of fire invented by Chevalier Morland in
1682, and presented to His Most Christian Majesty in 1683:—
"'Water being converted into vapour by the force of fire, these vapours
shortly require a greater space (about 2000 times) than the water before
occupied, and sooner than be constantly confined would split a piece of
cannon. But being duly regulated according to the rules of statics, and by
science reduced to measure, weight, and balance, then they bear their load
peaceably (like good horses), and thus become of great use to mankind,
particularly for raising water, according to the following table, which shows
the number of pounds that may be raised 1800 times per hour to a height of
six inches by cylinders half filled with water, as well as the different
diameters and depths of the said cylinders.'"
There is nothing in the description here given which can indicate the
form of the machine by which Morland proposed to render the force of
steam a useful mover. It is, however, remarkable, that at this early period,
before experiments had been made on the expansion which water undergoes
in evaporation, he should have given so near an approximation to [Pg036] the
actual amount of that expansion. It is scarcely supposable that such an
estimate could be obtained by him otherwise than by experiment.
The work containing the above description was not printed; but a work
bearing nearly the same title, containing, however, no mention of the force
of steam, was published by him in Paris in the year 1685. In this he
describes various experiments made by him at St. Germains on the weight
of the water of the Seine, and gives weights of the columns of water, the
contents of cylinders, &c.
Soon after the publication of this work, Morland returned to England, and
resided near the court till his death. The celebrated John Evelyn mentioned
having paid a visit to him at his house at Hammersmith, in 1695, when he
had become aged and blind, but was still remarkable for his mechanical

ingenuity. "On the 25th of October," says Evelyn, "the Archbishop and
myself went to Hammersmith to visit Sir Samuel Morland, who was
entirely blind; a very mortifying sight. He showed us his invention of
writing (short-hand), which was very ingenious; also his wooden kalendar,
which instructed him all by feeling; and other pretty and useful inventions
of mills, pumps, &c.; and the pump he had erected, that serves water to his
garden and to passengers, with an inscription, and brings from a filthy part
of the Thames near it a most perfect and pure water."[3]
He died at Hammersmith, in January 1696; and before his death, as a
penance for his past life, was guilty of the eccentricity of burying in the
ground six feet deep a great collection of music which he possessed.[4]
Denis Papin, 1688.

(19.)

Denis Papin, a native of Blois in France, and professor of mathematics at
Marbourg, is the name which stands next recorded in the progressive
invention of the steam engine. To this philosopher is due the discovery of
one of the qualities of steam, to the proper management of which is owing
much of the efficacy of the modern steam engine. [Pg037]
Papin was born at Blois in France. He devoted his youth to the study of
medicine, in which he took a degree at Paris. The revocation of the Edict of
Nantes having driven him into exile, he went to England, where the
celebrated Boyle associated him in several of his experiments with the air-
pump, and caused him to be elected a fellow of the Royal Society in 1681.
Having been invited to Germany by the Landgrave of Hesse, he discharged
during several years the duties of professor of mathematics at the university
of Marbourg, where he died in 1710. Notwithstanding his discoveries
respecting the agency of steam, he never received any mark of distinction in
his own country. The truth is, the importance and value of these
investigations were not apparent until long afterwards.
This philosopher conceived the idea of producing a moving power by
means of a piston working in a cylinder, in the manner which we shall now

briefly explain.
Let A B (fig. 7.) be a cylinder open at the top, and let a
piston P be fitted into it, so as to move in it air tight. At the
bottom of the cylinder suppose an opening provided,
which can be closed at pleasure, by a stop-cock, or
otherwise, so that the communication may be opened and
closed at will between the interior of the cylinder and the
external air. This stop-cock being opened, let the piston be
drawn upwards till it reach the top of the cylinder. Let the
stop-cock at the bottom be then removed, and imagine that
some means can be supplied by which the air within the
cylinder can be suddenly annihilated. The piston, now at
the top, will have above it the pressure of the atmosphere;
Fig. 7. and having no air below, it will be resisted in its descent
by no force save that arising from its friction with the
cylinder. If, then, the force of the air above the piston be greater than the
resistance arising from this friction, the piston will descend with the excess
of this force, and will continue so to descend until it reach the bottom of the
cylinder. Having attained that position, let us [Pg038] suppose the stop-cock in
the bottom opened, so as to allow the external air to pass freely below the
piston. The piston may now be drawn to the top of the cylinder again,
offering no resistance save that of its weight, and its friction with the
cylinder. Having reached the top of the cylinder once more, let the stop-
cock be closed, and the air included within the cylinder once more
annihilated. A second descent of the piston will take place, with the same
force as before, and in like manner the process may be continued
indefinitely.
Now, if it should appear that means could be provided suddenly and
repeatedly to annihilate the air within the cylinder, and that the pressure of
the atmosphere above the piston should exert a force compared with which
the weight of the piston and its friction are trifling, it is evident that a
moving power would be obtained which would be capable, by proper
mechanism, of being applied to any useful purpose, but which would more
especially be applicable to the working of pumps, the motion of which

corresponds with that which has been just ascribed to the piston in the
cylinder. Such were the first ideas of Papin. But in order to enable those
who are not conversant with physical science fully to appreciate their
importance, it will be necessary here to explain some of the mechanical
properties of atmospheric air.

(20.)

The atmosphere is the thin, transparent, colourless, and therefore invisible,
fluid in which we live and move, which by respiration sustains animal life,
and is otherwise connected with various important functions of organised
matter. This fluid is so light and attenuated, that it might at first be doubted
whether it be really a body at all; and, indeed, the name expressing
incorporeal beings, spirit, is a word in its origin signifying air.[5] The air,
however, is light only as compared with other material substances, which
exist in a more condensed state: it possesses the quality of weight as
absolutely as the most solid and massive bodies in nature, and to render this
quality manifest, it is only necessary to submit a sufficient quantity of air to
any of the usual tests of gravitation. [Pg039]
A direct demonstration of this may be given by the following experiment:
—On the mouth of a flask let a stop-cock be fastened so as to be air-tight.
The interior of the flask may then be put into free communication with the
external air, or that communication may be cut off at pleasure, by opening
or closing the stop-cock. If a syringe be applied to the mouth of the flask,
the stop-cock being open a part of the air contained in it may be drawn out.
After this, the stop-cock being closed, and the syringe detached, let the flask
be placed in the dish of a good balance, and accurately counterpoised by
weights in the other dish. This counterpoise will then represent the weight
of the flask, and of the air which has remained in it. If the stop-cock be now
opened, air will immediately rush in, and replace that which the syringe had
withdrawn from the flask; and immediately the dish of the balance
containing the flask will sink by the effect of the weight of the air thus
admitted into the flask.
If the weight of quantity of air so small as to be capable of being
withdrawn by a syringe from an ordinary flask be thus of sensible amount,

it may be easily imagined that the vast mass of atmosphere extending from
the surface of the earth upwards, to a height not ascertained with precision,
but certainly not being less than thirty miles, must be very considerable.
Such a force, pressing as it must constantly do, upon the surfaces of all
bodies, whether solid or fluid, and resisting and modifying their
movements, would play an important part in all mechanical phenomena;
and it is, therefore, not sufficient merely to have recognised its existence,
but it is most needful to measure its amount with that degree of certainty
and precision, which will enable us to estimate its effects on those
phenomena which we shall have to investigate.

(21.)

The amount of the pressure of the atmosphere on each square inch of
horizontal surface on which it rests, is obviously the weight of the column
of air extending from that square inch of surface upwards to the top of the
atmosphere. This force is measured by the following means:—
Take a glass tube, A B (fig. 8.), above 32 inches long, open at one end A,
and closed at the other end B, and let it [Pg040] be filled with mercury
(quicksilver). Let a glass vessel or cistern C, containing a quantity of
mercury, be also provided. Applying the finger at A, so as to prevent the
mercury in the tube from falling out, let the tube be inverted, and the end,
stopped by the finger, plunged into the mercury in C. When the end of the
tube is below the surface of the mercury in C (fig. 9.), let the finger be
removed. It will be found that the mercury in the tube will not, as might be
expected, fall to the level of the mercury in the cistern C, which it would do
were the end B open, so as to admit the air into the upper part of the tube.
On the other hand, the level D of the mercury in the tube will be nearly 30
inches above the level C of the mercury in the cistern.
The cause of this effect is, that the weight of the atmosphere rests on the
surface C of the mercury in the cistern, and tends thereby to press it up, or
rather to resist its fall in the tube; and as the fall is not assisted by the
weight of the atmosphere on the surface D (since B is closed), it follows,

that as much mercury remains suspended in
the tube above the level C, as the weight of the
atmosphere is able to support.
If the section of the tube were equal to the
magnitude of a square inch, the weight of the
column of mercury in the tube above the level
C would be exactly equal to the weight of the
atmosphere on each square inch of the surface
C.

(22.)

If such an apparatus be observed from time to
time, it will be found that the column of
mercury sustained in the tube will be subject
to variation between certain limits, never
falling below twenty-eight inches, and never
Figs. 8., 9. rising above thirty-one inches. This variation
of the mercurial column is produced by a
corresponding variation in the weight of the atmosphere.
If the apparatus be transported to any height above its ordinary position,
it will have a less quantity of atmosphere above it, and therefore the surface
of the mercury in the cistern will be pressed by a less weight, and
consequently the [Pg041] column of mercury will fall proportionally. In virtue
of this effect, such an instrument has been rendered a means of measuring
heights, such as the heights of mountains, the ascents of balloons, &c. &c.

(23.)

If a proper scale be attached to the tube containing the mercurial column,
showing the absolute height of the column sustained at any time, and
indicating its changes of height, the instrument becomes a Barometer.
Two cubic inches of mercury weigh very nearly one pound avoirdupois.
[6]
Hence, when the barometric column measures thirty inches, the weight

of the atmosphere resting on each square inch of surface is about fifteen
pounds.

(24.)

It is an established property of fluids, that they press equally in all
directions; and air, like every other fluid, participates in this quality. Hence,
it follows, that when the downward pressure or weight of the atmosphere is
fifteen pounds on the square inch, the lateral, upward, and oblique pressures
are of the same amount. But, independently of the general principle, it may
be satisfactory to give experimental proof of this.
Let four glass tubes, A, B, C, D
(fig. 10.), be constructed of
sufficient length, closed at one end,
A, B, C, D, and open at the other. Let
the open ends of three of them be
bent, as represented in the tubes B,
C, D. Being previously filled with
mercury, let them all be gently
inverted, so as to have their closed
ends up, as here represented. It will
be found that the mercury will be
sustained in all, and that the
difference of the levels in all will be
the same.[7] Thus, the mercury is
sustained in A by the upward
pressure of the atmosphere; in B, by
its horizontal or lateral pressure; in
C, by its downward pressure; [Pg042]
Fig. 10.
and in D, by its oblique pressure:
and, as the difference of the levels is the same in all, these pressures are
exactly equal.

(25.)

The same arrangement by which the pressure of the atmosphere is measured
by a mercurial column of equivalent weight, also supplies the means of
measuring the pressure or elasticity of atmospheric air, or any other gas or
vapour, whether in a more or less compressed or rarefied state; and as
instruments constructed on this principle are of considerable use in steam
engines, we shall take this occasion to explain their principle and
application.
In the experiments described in (21), the space D B in the top of the
barometer-tube, from which the mercury descended, is a vacuum. If,
however, it were occupied by a quantity of air in a rarefied state, or any
other gas or vapour, such gas or vapour would press on the surface of the
mercury at D, with a force determined by its elasticity. In that case, the
atmospheric pressure acting on the surface of the mercury C in the cistern,
would be balanced by the combined forces of the weight of the mercurial
column sustained in the tube, and the elasticity of the gas or vapour in the
upper part of it. Now if we know the actual amount of the atmospheric
pressure,—that is to say, the height of the column of mercury which it
would be capable of sustaining,—we should then be able to determine the
pressure of the rarefied air in the space C D.
For example, let us suppose that the barometric column, when B D (fig.
9.) is a vacuum, measures thirty inches: the atmospheric pressure, therefore,
would be equal to the weight of a column of mercury of that height. Let us
suppose that the elasticity of the gas or vapour occupying the upper part of
the tube D B causes the column to fall to the height of twenty-six inches: it is
evident, then, that the pressure of the air in the top of the tube would be
equal to the weight of a column of mercury of four inches. In fine, to
determine the pressure of the rarefied gas or vapour in the top of the tube, it
is only necessary to observe the difference between the height of the
column of mercury actually sustained in the tube, and the column sustained
at the same time and [Pg043] place in a common barometer: the difference of the
two will be the column of mercury whose weight will represent the pressure
of the vapour or gas in the top of the tube.

(26.)

Whenever the air contained in any vessel or other enclosed space has by
any means had its pressure reduced so as to be rendered less than that of the
external air, the external air will have a tendency to rush into such vessel or
enclosed space with a force proportionate to the excess of the pressure of
such external air over that of the air within; and if any communication be
opened between the interior of such vessel or enclosed space, and the
external air, the latter will rush in until an equilibrium be established
between the pressures within and without. It is evident that the force thus
obtained by diminishing the pressure of air within a vessel may be applied
to any mechanical purpose.
It is by such means that water is raised in an ordinary pump. A portion of
the air contained between the piston of the pump and the surface of the
water below, is withdrawn by the action of the piston, and the pressure of
the air remaining under the piston is thereby diminished. The superior
pressure of the atmosphere upon the external surface of the water in the
well then forces up a column of water in the pump-barrel, and this is
continued as the air is more and more rarefied by the action of the piston.
By whatever means, therefore, the air can be wholly or partially withdrawn
from any space, a mechanical power will be thereby developed,
proportional in its amount and efficacy to the quantity of air so withdrawn.
If, however, such air be withdrawn by any mechanical process, such as by a
syringe, by a common pump, or by an air-pump, the quantity of force
expended in withdrawing it is always equivalent to the amount of
mechanical power obtained by the vacuum or partial vacuum so produced.
Indeed the power expended is greater than the power so obtained, inasmuch
as the friction, leakage, &c. of the exhausting apparatus must be allowed
for.

(27.)

There are, however, various other means by which air may be partially
expelled from a vessel besides the direct application of mechanical force.
Thus if heat be applied to [Pg044] the vessel, the air, as has been already
explained, will acquire increased elasticity, and will rush from the vessel
with a force proportionate to the excess of its elasticity above that of the

external air, and this process may be continued by increasing the heat to
which the vessel is exposed, until a very considerable portion of the air has
been expelled. If the orifice by which the air has escaped be then closed,
and the vessel be allowed to cool, the air within, by having its temperature
reduced to that of the external air, will lose all the elasticity which it had
gained from the heat, and will be in the same condition as if an equivalent
quantity of air had been withdrawn by any mechanical agent. The external
air, therefore, will have a tendency to rush in with a force corresponding to
the difference of pressures.
The process of filling thermometers with mercury shows one use of
producing a high degree of rarefaction by heat. To construct the instrument
it is necessary to fill the bulb and a part of the tube with mercury; but the
bore of the tube is so small that the mercury cannot be introduced by any
ordinary means. It is therefore held over flame until heated to a high
temperature. The air within it gradually increasing in pressure as its
temperature is raised, is forced through the small bore of the tube, until the
pressure of the air within becomes no more than equal to the pressure of the
external atmosphere; this air being so rarefied that quantity in the bulb bears
a very small proportion to its contents at common temperatures. The mouth
of the tube is then plunged into mercury, and as the bulb cools, the air
within it loses its elasticity, and the superior pressure upon the external
surface forces the mercury into the tube. This continues until the air
remaining within the bulb has been so contracted, that its pressure
combined with the weight of the mercury, shall balance the atmospheric
pressure. The tube is then reversed, and the air which remained rises in a
bubble to the surface, and escapes.

(28.)

Let us now return to the proceedings of Papin. How great a power would
result from such a machine as he conceived, will be apparent, if it be
considered that the unresisted atmosphere exercises a pressure of about
fifteen pounds on [Pg045] each square inch of surface exposed to it, and that if
the piston in the cylinder imagined by Papin, had a diameter of only one
foot, its superficial magnitude would be about 114 square inches. The

pressure of the atmosphere upon it, therefore, would be 114 times fifteen
pounds, or 1710 pounds. Papin first proposed to produce the vacuum under
the piston by means of common air pumps, worked by a water-wheel; and
by such means he conceived that the power of a river, stream, or waterfall
might be conveyed by pipes to a distance. While he was in England, in
1687, he laid his contrivance before the Royal Society of London, but was
met by objections and difficulties, the nature of which he does not explain.
It is, however, apparent, from what has been already explained, that such
a method of proceeding would amount to a mere transfer of power, and
would not, properly speaking, be itself a moving force: the moving power
would, in reality, be the force of the water by which the water-wheel would
be driven; and the air-pumps, tubes, together with the piston and cylinder,
would be merely means of conveying the power of the water-wheel to the
objects to be moved, or the machinery to be driven. Papin states, that, long
before this, he had attempted to expel the air from his cylinder by means of
gunpowder; but, notwithstanding all the precautions which he could take,
there always remained a considerable quantity; so much, indeed, as to
deprive the vacuum of more than half its proper force. At length he adopted
an expedient for the production of a vacuum which forms a most important
step in the progressive invention of the steam engine, and which gives to
Papin's name a high place in the history of that machine. This method is
explained in the following paragraph of a work published by Papin in 1695,
at Cassel, entitled "Recueil de diverses Pièces touchant quelques nouvelles
Machines", p. 53.
"I have endeavoured," says he, "to attain this end (viz. the production of a
vacuum in the cylinder) in another way. As water has the property of
elasticity, when converted into steam by heat, and afterwards of being so
completely recondensed by cold, that there does not remain the least [Pg046]
appearance of this elasticity, I have thought that it would not be difficult to
work machines in which, by means of a moderate heat and at a small cost,
water might produce that perfect vacuum which has vainly been sought by
means of gunpowder."
This remarkable passage is given in the work just cited, as an extract
from the "Leipsic Acts," of August, 1690.

Let us pause here to explain more fully this important discovery.

(29.)

We have explained that, in its conversion into vapour, by the application of
heat, water, besides acquiring the property of elasticity, undergoes a vast
enlargement of bulk, filling, under ordinary circumstances, about 1700
times more space than it occupied in the liquid form. This fact was known
generally, though not with numerical accuracy, by Papin, having been the
foundation of the machines previously invented and published by De Caus
and Lord Worcester; the happy idea of reversing the process occurred to
him. If water in its conversion into steam swelled into many hundred times
its original bulk, it would necessarily follow, that steam, being reconverted
into water, would shrink into its primitive dimensions. Papin therefore saw,
that if he could by any means expel the air from his cylinder under the
piston, and replace it by the pure vapour of water, he could cause that
vapour to be reconverted into a comparatively minute quantity of water by
depriving it of the heat which sustained it in the state of steam, and that by
accomplishing this, the space in the cylinder under the piston would
become a vacuum; that by such means, the pressure of the atmosphere
above the piston would take full effect, and would urge the piston down;
that by introducing more steam under the piston, it might be again raised by
the elastic force of the steam, the destruction of which by cold water would
again produce the descent of the piston with the same mechanical force; and
that in this way the alternate ascent and descent of the piston might be
continued indefinitely.
In accordance with these ideas, Papin constructed a model consisting of a
small cylinder, in which was placed a solid piston; [Pg047] and in the bottom of
the cylinder under the piston was contained in a small quantity of water.
The piston being in immediate contact with this water, so as to exclude the
atmospheric air, on applying fire to the bottom of the cylinder, steam was
produced, the elastic force of which raised the piston to the top of the
cylinder; the fire being then removed, and the cylinder being cooled by the
surrounding air, the steam was condensed and reconverted into water,
leaving a vacuum in the cylinder into which the piston was pressed by the

force of the atmosphere. The fire being applied and subsequently removed,
another ascent and descent were accomplished; and in the same manner the
alternate motion of the piston might be continued. Papin described no other
form of machine by which this property could be rendered available in
practice; but he states generally, that the same end may be attained by
various forms of machines easy to be imagined.[8]
Thomas Savery, 1698.

(30.)

The discovery of the method of making a vacuum by the condensation of
steam was reproduced, before 1698, by Captain Thomas Savery, to whom a
patent was granted in that year for a steam engine to be applied to the
raising of water, &c. Savery proposed to combine the machine described by
the Marquis of Worcester with an apparatus for raising water by suction into
a vacuum produced by the condensation of steam.
Savery appears to have been ignorant of the publication of Papin, and
stated that his discovery of the condensing principle arose from the
following circumstance:—
Having drunk a flask of Florence at a tavern, and flung the empty flask
on the fire, he called for a basin of water to wash his hands. A small
quantity which remained in the flask began to boil, and steam issued from
its mouth. It occurred to him to try what effect would be produced by
inverting the flask and plunging its mouth in the cold water. Putting on a
thick glove to defend his hand from the heat, he seized the [Pg048] flask, and the
moment he plunged its mouth in the water the liquid immediately rushed up
into the flask and filled it.
Savery stated that this circumstance immediately suggested to him the
possibility of giving effect to the atmospheric pressure by creating a
vacuum in this manner. He thought that if, instead of exhausting the barrel
of a pump by the usual laborious method of a piston and sucker, it was
exhausted by first filling it with steam, and then condensing the same
steam, the atmospheric pressure would force the water from the well into
the pump-barrel, and into any vessel connected with it, provided that vessel

were not more than about thirty-four feet above the elevation of the water in
the well. He perceived also, that, having lifted the water to this height, he
might use the elastic force of steam in the manner described by the Marquis
of Worcester to raise the same water to a still greater elevation, and that the
same steam which accomplished this mechanical effect would serve, by its
subsequent condensation, to reproduce the vacuum, and draw up more
water. It was on this principle that Savery constructed the first engine in
which steam was ever brought into practical operation.

BRANCA'S ENGINE.

FOOTNOTES:
[1] Arago, Eloge historique de James Watt; p. 22.
[2] Ibid., p. 21. note.
[3] Farey, Treatise on the Steam Engine, p. 93.
[4] Arago, sur les Machines à Vapeur, Annuaire, 1829, p. 165
[5] Spiritus, breath or air.
[6] Exactly 15·68 oz. = 0·98 lb.
[7] This experiment with the tube A requires to be very carefully executed, and the tube
should be one of small bore.
[8] Recueil de diverses Pièces touchant quelques nouvelles Machines, p. 38.

SAVERY'S ENGINE.

CHAP. II.

ENGINES OF SAVERY AND NEWCOMEN.
[Pg049]

TOC INX

SAVERY'S ENGINE.—BOILERS AND THEIR APPENDAGES.—WORKING APPARATUS.—
MODE OF OPERATION.—DEFECTS OF THE ENGINE.—NEWCOMEN AND CAWLEY.
—ATMOSPHERIC ENGINE.—ACCIDENTAL DISCOVERY OF CONDENSATION BY
INJECTION.—HUMPHREY POTTER MAKES THE ENGINE WORK ITSELF.—
ADVANTAGES OF THE ATMOSPHERIC ENGINE OVER THAT OF SAVERY.—IT
CONTAINED NO NEW PRINCIPLE.—ITS PRACTICAL SUPERIORITY.

(31.)

The steam engine contrived by Savery, like every other which has since
been constructed, consists of two parts, essentially distinct. The first is that
which is employed to [Pg050] generate the steam, which is called the boiler; and
the second, that in which the steam is applied as a moving power.
The former apparatus in
Savery's engine consists of
two strong boilers, sections
of which are represented at
D and E in fig. 11.; D the
greater boiler, and E the
less. The tubes T and T′
communicate with the
working apparatus, which
we shall presently describe.
A thin plate of metal R, is
applied closely to the top of
Fig. 11. the great boiler D, turning
on a centre C, so that by

moving a lever applied to the axis C on the outside of the top, the sliding
plate R can be brought from the mouth of the one tube to the mouth of the
other alternately. This sliding valve is called the regulator, since it is by it
that the communications between the boiler and two steam vessels
(hereafter described) are alternately opened and closed, the lever which
effects this being moved at intervals by the hand of the attendant.
Two gauge cocks are represented at G, G′, the use of which is to determine
the depth of water in the boiler. One, G, has its lower aperture a little above
the proper depth; and the other, G′, a little below it. Cocks are attached to
the upper ends G, G′, which can be opened or closed at pleasure. The steam
collected in the top of the boiler pressing on the surface of the water, forces
it up in the tubes G, G′, if their lower ends be immersed. Upon opening the
cocks G, G′, if water be forced from both, there is too much water in the
boiler, since the mouth of G is below its level. If steam issue from both,
there is too little water in the boiler, since the mouth of G′ is above its level.
But if steam issue from G, and water from G′, the water in the boiler is at its
proper level. This ingenious contrivance for determining the level of the
water in the boiler is the invention of Savery, and is used in many instances
at the present day.
The mouth of the pipe G should be at a level of a little less [Pg051] than one
third of the whole depth, and the mouth of G′ at a level little lower than one
third; for it is requisite that about two thirds of the boiler should be kept
filled with water. The tube I forms a communication between the greater
boiler D and the lesser or feeding boiler E, descending nearly to the bottom
of it. This communication can be opened and closed at pleasure by the cock
K. A gauge pipe is inserted similar to G, G′, but extending nearly to the
bottom. From this boiler a tube F extends, which is continued to a cistern C
(fig. 12.), and a cock is placed at M, which, when opened, allows the water
from the cistern to flow into the feeding boiler E, and which is closed when
that boiler is filled. The manner in which this cistern is supplied will be
described hereafter.
Let us now suppose that the principal boiler is filled to the level between
the gauge pipes, and that the subsidiary boiler is nearly full of water, the

cock K and the gauge cocks G G′ being all closed. The fire being lighted
beneath D, and the water boiled, steam is produced, and is transmitted
through one or other of the tubes T, T′, to the working apparatus. When
evaporation has reduced the water in D below the level of G′, it will be
necessary to replenish the boiler D. This is effected thus:—A fire being
lighted beneath the feeding boiler E, steam is produced in it above the
surface of the water, which, having no escape, presses on the surface so as
to force it up in the pipe I. The cock K being then opened, the boiling water
is forced into the principal boiler D, into which it is allowed to flow until
water issues from the gauge cock G′. When this takes place, the cock K is
closed, and the fire removed from E until the great boiler again wants
replenishing. When the feeding boiler E has been exhausted, it is
replenished from the cistern C (fig. 12.), through the pipe F, by opening the
cock M.

(32.)

We shall now describe the working apparatus in which the steam is used as
a moving power.
Let V V′ (fig. 12.) be two steam vessels communicating by the tubes T T′
(marked by the same letters in fig. 11.) with the greater boiler D.
Let S be a pipe, called the suction pipe, descending into [Pg052] the well or
reservoir from which the water is to be raised, and communicating with
each of the steam vessels through tubes D D′, by valves A A′, which open
upwards. Let F be a pipe continued from the level of the engine to whatever
higher level it is intended to elevate the water. The steam vessels V V′
communicate with the force-pipe F by valves B B′, which open upwards,
through the tubes E E′. Over the steam vessels and on the force-pipe is
placed a small cistern C, already mentioned, which is kept filled with cold
water from the force-pipe, and from the bottom of which proceeds a pipe
terminated with a cock G. This is called the condensing pipe, and can be
brought alternately over each steam vessel. From this cistern another pipe
communicates with the feeding boiler (fig. 11.), by the cock M.[9]

The communication of the pipes
T T′ with the boiler can be opened
and closed alternately, by the
regulator R (fig. 11.), already
described.
Now suppose the steam vessels
and tubes to be all filled with
common atmospheric air, and that
the regulator be placed so that the
communication between the tube T
and the boiler be opened, the
communication between the other
tube T′ and the boiler being closed,
steam will flow into V through T. At
first, while the vessel V is cold, the
steam will be condensed, and will
fall in drops of water on the bottom
Fig. 12.
and sides of the vessel. The
continued supply of steam from the
boiler will at length impart such a degree of heat to the vessel V, that it will
cease to condense it. Mixed with the heated air [Pg053] contained in the vessel
V, it will have an elastic force greater than the atmospheric pressure, and
will therefore force open the valve B, through which a mixture of air and
steam will be driven until all the air in the vessel V will have passed out,
and it will contain nothing but the pure vapour of water.
When this has taken place, suppose the regulator be moved so as to close
the communication between the tube T and the boiler, and to stop the further
supply of steam to the vessel V; and at the same time let the condensing
pipe G be brought over the vessel V, and the cock opened so as to let a
stream of cold water flow upon it. This will cool the vessel V, and the steam
with which it is filled will be condensed and fall in a few drops of water,
leaving the interior of the vessel a vacuum. The valve B will be kept closed
by the atmospheric pressure. But the elastic force of the air between the

valve A and the surface of the water in the well, or reservoir, will open A, so
that a part of this air will rush in, and occupy the vessel V. The air in the
suction pipe S, being thus allowed an increased space, will be proportionally
diminished in its elastic force, and its pressure will no longer balance that of
the atmosphere acting on the external surface of the water in the reservoir.
This pressure will, therefore, force water up in the tube S until its weight,
together with the elastic force of the air above it, balances the atmospheric
pressure. When this has taken place, the water will cease to ascend.
Let us now suppose that, by shifting the regulator, the communication is
opened between T and the boiler, so that steam flows again into V. The
condensing cock G being removed, the vessel will be again heated as before,
the air expelled, and its place filled by the steam. The condensing pipe
being again allowed to play upon the vessel V, and the further supply of
steam being stopped, a vacuum will be produced in V, and the atmospheric
pressure will force the water through the valve A into the vessel V, which it
will nearly fill, a small quantity of air, however, remaining above it.
Thus far the mechanical agency employed in elevating the water is the
atmospheric pressure; and the power of steam is no further employed than
in the production of a vacuum. [Pg054] But, in order to continue the elevation of
the water through the force pipe F, above the level of the steam vessel, it
will be necessary to use the elastic pressure of the steam. The vessel V is
now nearly filled by the water which has been forced into it by the
atmosphere. Let us suppose that, the regulator being shifted again, the
communication between the tube T and the boiler is opened, the condensing
cock removed, and that steam flows into V. At first, coming in contact with
the cold surface of the water and that of the vessel, it is condensed; but the
vessel is soon heated, and the water formed by the condensed steam collects
in a sheet or film upon the surface of the water in V, so as to form a surface
as hot as boiling water.[10] The steam then being no longer condensed,
presses on the surface of the water with its elastic force; and when that
pressure becomes greater than the atmospheric pressure, the valve B is
forced open, and the water issuing through it, passes through E into the

force-pipe F; and this is continued until the steam has forced all the water
from V, and occupies its place.
The further admission of steam through T is once more stopped by
moving the regulator; and the condensing pipe being again allowed to play
on V, so as to condense the steam which fills it, produces a vacuum. Into
this vacuum, as before, the atmospheric pressure will force the water, and
fill the vessel V. The condensing pipe being then closed, and steam admitted
through T, the water in V will be forced by its pressure through the valve B
and tube E into F, and so the process is continued.
We have not yet noticed the other steam vessel V′, which, as far as we
have described, would have remained filled with common atmospheric air,
the pressure of which on the valve A′ would have prevented the water raised
in the suction pipe S from passing through it. However, this is not the case;
for, during the entire process which has been described in V, similar effects
have been produced in V′, which we have only omitted to notice to avoid the
confusion which the two processes might produce. It will be remembered,
that after the steam, in the first instance, having flowed from the boiler [Pg055]
through T, has blown the air out of V through B, the communication between
T and the boiler is closed. Now the same motion of the regulator which
closes this, opens the communication between T′ and the boiler; for the
sliding plate R (fig. 11.) is moved from the one tube to the other, and at the
same time, as we have already stated, the condensing pipe is brought to play
on V. While, therefore, a vacuum is being formed in V by condensation, the
steam, flowing through T′, blows out the air through B′, as already described
in the other vessel V; and while the air in S is rushing up through A into V,
followed by the water raised in S by the atmospheric pressure, the vessel V′
is being filled with steam, and the air is completely expelled from it.
The communication between T and the boiler is now again opened, and
the communication between T′ and the boiler closed by moving the
regulator R (fig. 11.) from the tube T to T′; at the same time the condensing
pipe is removed from over V, and brought to play upon V′. While the steam
once more expels the air from V through B, a vacuum is formed by

condensation in V′, into which the water in S rushes through the valve A′. In
the mean time V is again filled with steam. The communication between T
and the boiler is now closed, and that between T′ and the boiler is opened,
and the condensing pipe removed from V′, and brought to play on V. While
the steam from the boiler forces the water in V′ through B′ into the force-
pipe F, a vacuum is being produced in V, into which water is raised by the
atmospheric pressure.
Thus each of the vessels V V′ is alternately filled from S, and the water
thence forced into F. The same steam which forces the water from the
vessels into F, having done its duty, is condensed, and brings up the water
from S, by giving effect to the atmospheric pressure.
During this process, two alternate motions or adjustments must be
constantly made; the communication between T and the boiler must be
opened, and that between T′ and the boiler closed, which is done by one
motion of the regulator. The condensing pipe at the same time must be
brought from V to play on V′, which is done by the lever placed upon it.
Again [Pg056] the communication between T′ and the boiler is to be opened, and
that between T and the boiler closed; this is done by moving back the
regulator. The condensing pipe is brought from V′ to V by moving back the
other lever, and so on alternately.
For the clearness and convenience of description, some slight and
otherwise unimportant changes have been made in the position of the parts.
A perspective view of this engine is represented at the head of this chapter.
The different parts already described will easily be recognised.
The engine of Savery was very clearly described in a small work
published in London in 1702, entitled, The Miner's Friend, or an Engine to
raise Water by Fire described, and the Manner of Fixing it in Mines; with
an Account of the several Uses it is applicable unto, and an Answer to the
Objection made against it; by Thomas Savery, Gentleman. This volume was
dedicated to William III. (to whom the engine had been exhibited at
Hampton Court palace), to the Royal Society, and to the mining adventurers
of England. The following are the uses to which Savery proposed the

engine should be applied: First, to raise water for turning all sorts of mills;
second, supplying palaces and houses with water, and supplying means of
extinguishing fire therein by the water so raised; third, the supplying cities
and towns with water; fourth, draining fens or marshes; fifth, for ships;
sixth, the drainage of mines.
Dr. Harris, in his Lexicon Technicum, or Dictionary of Arts and Sciences,
mentions a machine of Savery's for propelling a vessel in a calm, by paddle-
wheels placed at the side; but it does not appear that Savery contemplated
the application of a steam engine to work these wheels.
It is only from scattered passages in publications of the day that it can be
ascertained to what extent the engines of Savery were practically applied. In
his address to the Royal Society, he speaks of the "difficulties and expense
which he encountered in instructing artisans to make engines according to
his wish; but that after much experience the workmen had become such
masters of the thing, that they bound themselves to deliver the engines
'exactly tight and fit for [Pg057] service, and such as he (Savery) dare warrant
them to every one that has occasion for them.'"
In his address to the miners of England he also says, "that the frequent
disorders and cumbersomeness of water engines then in use encouraged
him to invent engines to work by this new force; that though they were
obliged to encounter the oddest and almost insuperable difficulties, yet he
spared neither time, pains, nor money, till he had conquered them."
In Bradley's Improvements of Planting and Gardening, 1718, the author
thus speaks of an engine erected by Savery:—
"Supposing the situation of a house or garden to be a considerable height
above any pond, river, or spring, and that it has at present no other
conveniency of water than what is brought continually by men or horses to
it. In this case, the wonderful invention of the late Mr. Savery, F.R.S., for
raising water by fire, will not only supply the defect, by flinging up as much
water as may be desired, but may be maintained with very little trouble and
very small expense.
"It is now about six years since Mr. Savery set up one of them for that
curious gentleman Mr. Balle, at Cambden House, Kensington, near London,

which has succeeded so well that there has not been any want of water since
it has been built; and, with the improvements since made to it, I am apt to
believe will be less subject to be out of order than any engine whatever."
It is remarkable that, notwithstanding the high pressure steam necessary
for the operation of Savery's engine, he does not appear to have adopted the
obvious expedient of a safety valve. The safety valve had been previously
known, having been invented about the year 1681, by Papin, for his
digester, which was a close boiler, contrived by him for stewing meat and
digesting bones, by submitting them to a higher temperature than that of
water boiling in an open vessel.
The safety valve which has ever since been used for steam boilers of
every kind is a valve which opens outwards, and is fitted to an aperture in
the boiler, so as to be steam tight. It is pressed down by a weight, the
amount of which is regulated by the maximum pressure to which it is
intended the steam [Pg058] shall be limited. Thus, if the magnitude of the valve
be a square inch, and the pressure of the steam be limited to 10 lbs. per
square inch above the pressure of the atmosphere, then the valve would be
loaded with a weight of 10 lbs.; but as it was found necessary to vary from
time to time the limiting pressure of the steam, or the load of the safety
valve, these valves were usually constructed so as to be held down by the
pressure of a lever having a sliding weight upon it. By moving the weight
on the arm of the lever, the pressure on the valve could be increased or
diminished at the discretion of the engineer. This contrivance was first
applied to Savery's engines, by Desaguliers, about the year 1717, before
which year Savery died.
It is justly observed by Mr. Farey, in his treatise on the steam engine,
that, "when a comparison is made between Captain[11] Savery's engine and
those of his predecessors, the result will be in every respect favourable to
his character as an inventor, and as a practical engineer; all the details of his
invention are made out in a masterly style, and accidents and contingencies
are provided for, so as to render it a real working engine; whereas De Caus,
the Marquis of Worcester, Sir Samuel Morland, and Papin, though
ingenious philosophers, only produced mere outlines, which required great

labour and skill of subsequent inventors to fill up, and make them
sufficiently complete to be put in execution."
About the year 1718 further improvements were made in the construction
of Savery's engine, by Dr. Desaguliers; but it is probable that some of these
were suggested by the proceedings of the inventors of the atmospheric
engine, which shall presently describe.

(33.)

In order duly to appreciate the value of improvements, it is necessary first
to perceive the defects which these improvements are designed to remove.
Savery's steam engine, considering how little was known of the value and
properties of steam, and how low the general standard of mechanical
knowledge was in his day, is certainly highly [Pg059] creditable to his genius.
Nevertheless it had very considerable defects, and was finally found to be
inefficient for the most important purposes to which he proposed applying
it.
At the time of this invention, the mines in England had greatly increased
in depth, and the process of draining them had become both expensive and
difficult; so much so, that it was found in many instances that their produce
did not cover the cost of working them. The drainage of these mines was
the most important purpose to which Savery proposed to apply his steam
engine.
It has been already stated that the pressure of the atmosphere amounts to
about fifteen pounds on every square inch. Now, a column of water, whose
base is one square inch, and whose height is thirty-four feet, weighs about
fifteen pounds. If we suppose that a perfect vacuum were produced in the
steam vessels V V′ (fig. 12.) by condensation, the atmospheric pressure
would fail to force up the water, if the height of the top of these vessels
above the water to be raised exceeded thirty-four feet. It is plain, therefore,
that the engine cannot be more than thirty-four feet above the water which it
is intended to elevate. But in fact it cannot be so much; for the vacuum
produced in the steam vessels V V′ is never perfect. Water, when not
submitted to the pressure of the atmosphere, will vaporise at a very low

temperature, as we shall hereafter explain; and it was found that a vapour
possessing a considerable elasticity would, notwithstanding the
condensation, remain in the vessels V V′ and the pipe S, and would oppose
the ascent of the water. In consequence of this, the engine could never be
placed with practical advantage at a greater height than twenty-six feet
above the level of the water to be raised.

(34.)

When the water is elevated to the engine, and the steam vessels filled, if
steam be introduced above the water in V, it must first balance the
atmospheric pressure, before it can force the water through the valve B.
Here, then, is a mechanical pressure of fifteen pounds per square inch
expended, without any water being raised by it. If steam of twice that elastic
force be used, it will elevate a column in F of thirty-four feet in height; and
if steam of triple the force be used, it will raise a column of sixty-eight feet
high, [Pg060] which, added to twenty-six feet raised by the atmosphere, gives a
total lift of ninety-four feet.
In effecting this, steam of a pressure equal to three times that of the
atmosphere acts on the inner surface of the vessels V V′. One third of this
bursting pressure is balanced by the pressure of the atmosphere on the
external surface of the vessels; but an effective pressure of thirty pounds per
square inch still remains, tending to burst the vessels. It was found that the
apparatus could not be constructed to bear more than this with safety; and,
therefore, in practice, the lift of such an engine was limited to about ninety
perpendicular feet. In order to raise the water from the bottom of the mine
by these engines, therefore, it was necessary to place one at every ninety
feet of the depth; so that the water raised by one through the first ninety feet
should be received in a reservoir, from which it was to be elevated the next
ninety feet by another, and so on.
Besides this, it was found that sufficient strength could not be given to
those engines, if constructed upon a large scale.
They were, therefore, necessarily very limited in their dimensions, and
were incapable of raising the water with sufficient speed. Hence arose a

necessity for several engines at each level, which greatly increased the
expense.

(35.)

These, however, were not the only defects of Savery's engines. The
consumption of fuel was enormous, the proportion of heat wasted being
much more than what was used in either forcing up the water, or producing
a vacuum. This will be very easily understood by attending to the process of
working the engine already described.
When the steam is first introduced from the boiler into the steam vessels
V V′, preparatory to the formation of a vacuum, it is necessary that it should
heat these vessels up to the temperature of the steam itself; for until then the
steam will be condensed the moment it enters the vessel by the cold surface.
All this heat, therefore, spent in raising the temperature of the steam vessels
is wasted. Again, when the water has ascended and filled the vessels V V′,
and steam is introduced to force this water through B B′ into F, it is
immediately condensed by the cold surface in V V′, and does not [Pg061] begin
to act until a quantity of hot water, formed by condensed steam, is collected
on the surface of the cold water which fills these vessels. Hence another
source of the waste of heat arises.
When the steam begins to act upon the surface of the water in V V′, and to
force it down, the cold surface of the vessels is gradually exposed to the
steam, and must be heated while the steam continues its action; and when
the water has been forced out of the vessel, the vessel itself has been heated
to the temperature of the steam which fills it, all which heat is dissipated by
the subsequent process of condensation. It must thus be evident that the
steam used in forcing up the water in F, and in producing a vacuum, bears a
very small proportion indeed to what is consumed in heating the apparatus
after condensation.

(36.)

There is also another circumstance which increases the consumption of fuel.
The water must be forced through B, not only against the atmospheric

pressure, but also against a column of sixty-eight feet of water. Steam is
therefore required of a pressure of forty-five pounds on the square inch.
Consequently the water in the boiler must be boiled under this pressure.
That this should take place, it is necessary that the water should be raised to
a temperature considerably above 212°, even so high as 275°; and thus an
increased heat must be given to the boiler. Independently of the other
defects, this intense heat weakened and gradually destroyed the apparatus.
Savery was the first who suggested the method of expressing the power
of an engine with reference to that of horses. In this comparison, however,
he supposed each horse to work but eight hours a day, while the engine
works for twenty-four hours. This method of expressing the power of steam
engines will be explained hereafter.

(37.)

The failure of the engines proposed by Captain Savery in the work of
drainage, from the causes which have been just mentioned, and the
increasing necessity for effecting this object, arising from the large property
in mines which became every year unproductive by being flooded,
stimulated the ingenuity [Pg062] of mechanics to contrive some means of
rendering those powers of steam exhibited in Savery's engine available.
Thomas Newcomen, the reputed inventor of the atmospheric engine, was
an ironmonger, or, according to some, a blacksmith, in the town of
Dartmouth in Devonshire. From his personal acquaintance and intercourse
with Dr. Hooke, the celebrated natural philosopher, it is probable that he
was a person of some education, and therefore likely to be above the
position of a blacksmith. Being in the habit of visiting the tin mines in
Cornwall, Newcomen became acquainted with the engine invented by
Savery, and with the causes which led to its inefficiency for the purposes of
drainage.
It has been stated that Papin, about the year 1690, proposed the
construction of an engine working by the atmospheric pressure acting on
one side of a piston against a vacuum produced by the condensation of
steam on the other side. Papin was not conscious of the importance of this

principle; for, so far from ever having attempted to apply it to practical
purposes, he probably never constructed, even on a small scale, any
machine illustrating it. On the contrary, he abandoned the project the
moment he was informed of the principle and structure of the steam engine
of Savery; and he then proposed an engine for raising water, acting by the
expansive force of steam similar to Savery's, but abandoning the method of
working by a vacuum.
This engine is described by Papin in a work published in 1707.

Fig. 13.

A (fig. 13.) is an oval boiler, having a safety-valve B, which limits the
pressure of the steam. It is connected with a cylinder C, by a curved pipe
having a stop-cock at D. A pipe with a stop-cock G opens from the top of the
cylinder into the atmosphere, and a safety-valve F is placed upon the

cylinder. A hollow copper piston H moves freely in the cylinder, and floats
upon the water. O is a funnel with a valve L in the bottom, opening
downwards, through which the cylinder C may be filled with water to the
level of the top of the funnel. A close air-vessel communicates with the
cylinder C by the curved tube, and has a valve K opening upwards. The
force-pipe through which the water is raised communicates [Pg063] with the air-
vessel I. If the cock D be shut, and the cock G opened, water poured into the
funnel O will rise into the cylinder C, the air which fills the cylinder
escaping through the open pipe G. When the cylinder is thus filled with
water, let the cock G be closed, and the cock D opened. The steam from the
boiler, after heating the metal of the cylinder, will force the piston
downwards, and drive the water through the curved tube into the vessel I,
from which its return is prevented by the valve K, which is closed by its
weight. The air which filled the vessel I will then be compressed, and by its
elasticity will drive a column of water up the pipe N. After the contents of
the cylinder have been thus discharged it may be refilled in the same
manner, and the process repeated.
It will be perceived that this project is nothing more than a reproduction
of the engine of the Marquis of Worcester. In the preface to the work
containing this description, Papin gives an extract from a letter addressed
by him to Leibnitz in 1698, from which it appears that he had abandoned
his idea of working the piston by the atmospheric pressure acting against a
vacuum, considering it to be a contrivance inferior [Pg064] to the engine now
described. "We now raise water," he says, "by the force of fire, in a more
advantageous manner than that which I had published some years before;
for besides the suction, we now also use the pressure which the water exerts
upon other bodies in dilating itself by heat; instead of which I before
employed the suction only, the effects of which are more limited."
From documents which have been preserved in the Royal Society, it
appears that Newcomen was acquainted with Papin's writings, and therefore
probably first derived from them the suggestion which he subsequently
realised in the atmospheric engine. Among some papers of Dr. Hooke's have
been found notes for the use of Newcomen, on Papin's method of

transmitting the force of a stream or fall of water to a distance by pipes.
Hooke dissuaded Newcomen from attempting any machine on this
principle, which, as first proposed by Papin, was impracticable. He exposed
the fallacy of Papin's first project in several discourses before the Royal
Society, and considered his improved edition of it, though free from fallacy,
as impracticable.
Papin's project for producing a vacuum under a piston by condensing the
steam having been published in the Actæ Eruditorum, in Latin, in 1690, and
in French, at Cassel, in 1695, and subsequently, in the Philosophical
Transactions, in England in 1697, cannot be supposed to be unknown to Dr.
Hooke; and if known to him, would probably have been communicated to
Newcomen. Dr. Hooke died in 1703, some years before the date of
Newcomen's invention.
John Cawley, who was the associate of Newcomen in his experiments
and inquiries, was a plumber and glazier of the same town. Newcomen and
Cawley obtained a patent for the atmospheric engine in 1705, in which
Savery was associated, he having previously obtained a patent for the
method of producing a vacuum by the condensation of steam, which was
essential to Newcomen's contrivance. It was not, however, until about the
year 1711 that any engine had been constructed under this patent.
In the latter end of that year, according to Desaguliers, the patentees
"made proposals to drain a colliery at Griff, in [Pg065] Warwickshire, in which
work five hundred horses were constantly employed. This proposal not
being accepted, they contracted, in the following March, to drain water for
Mr. Back of Wolverhampton, where, after many laborious attempts, they
succeeded in making their engine work; but not being either philosophers to
understand the reason, or mathematicians enough to calculate the power and
proportions of the parts, they very luckily, by accident, found what they
sought for."

Fig. 14.

Newcomen resumed the old method of raising the water from the mines
by ordinary pumps, but conceived the idea of working these pumps by some
moving power less expensive than that of horses. The means whereby he
proposed effecting this, was by connecting the end of the pump-rod D (fig.
14.) by a chain with the arch head A of a [Pg066] working-beam A B, playing on
an axis C. The other arch head B of this beam was connected by a chain with

the rod E of a solid piston P, which moved air-tight in a cylinder F. If a
vacuum be created beneath the piston P, the atmospheric pressure acting
upon it will press it down with a force of fifteen pounds per square inch;
and the end A of the beam being thus raised, the pump-rod D will be drawn
up. If a pressure equivalent to the atmosphere be then introduced below the
piston, so as to neutralise the downward pressure, the piston will be in a
state of indifference as to the rising or falling; and if in this case the rod D
be made heavier than the piston and its rod, so as to overcome the friction,
it will descend, and elevate the piston again to the top of the cylinder. The
vacuum being again produced, another descent of the piston, and
consequent elevation of the pump-rod, will take place; and so the process
may be continued.
Such was Newcomen's first conception of the atmospheric engine; and
the contrivance had much, even at the first view, to recommend it. The
power of such a machine would depend entirely on the magnitude of the
piston; and being independent of highly elastic steam, would not expose the
materials to the destructive heat which was necessary for working Savery's
engine. Supposing a perfect vacuum to be produced under the piston in the
cylinder, an effective downward pressure would be obtained, amounting to
fifteen times as many pounds as there are square inches in the section of the
piston.[12] Thus, if the base of the piston were 100 square inches, a pressure
equal to 1500 pounds would be obtained.

(38.)

In order to accomplish this, two things were necessary: 1. To make a speedy
and effectual vacuum below the [Pg067] piston in the descent; and, 2. To
contrive a counterpoise for the atmosphere in the ascent.
The condensation of steam immediately presented itself as the most
effectual means of accomplishing the former; and the elastic force of the
same steam previous to condensation an obvious method of effecting the
latter. Nothing now remained to carry the design into execution, but the
contrivance of means for the alternate introduction and condensation of the
steam; and Newcomen and Cawley were accordingly granted a patent in

1707, in which Savery was united, in consequence of the principle of
condensation for which he had previously received a patent being necessary
to the projected machine. We shall now describe the atmospheric engine, as
first constructed by Newcomen:—
The boiler K (fig. 14.) is placed over a furnace I, the flue of which winds
round it, so as to communicate heat to every part of the bottom of it. In the
top, which is hemispherical, two gauge-cocks G G′ are placed, as in Savery's
engine, and a puppet valve V, which opens upward, and is loaded at one
pound per square inch; so that when the steam produced in the boiler
exceeds the pressure of the atmosphere by more than one pound on the
square inch, the valve V is lifted, and the steam escapes through it, and
continues to escape until its pressure is sufficiently diminished, when the
valve V again falls into its seat. This valve performs the office of the safety-
valve in modern engines.
The great steam-tube is represented at S, which conducts steam from the
boiler to the cylinder; and a feeding pipe T, furnished with a cock, which is
opened and closed at pleasure, proceeds from a cistern L to the boiler. By
this pipe the boiler may be replenished from the cistern, when the gauge
cock G′ indicates that the level has fallen below it. The cistern L is supplied
with hot water, by means which we shall presently explain.

(39.)

To understand the mechanism necessary to work the piston, let us consider
how the supply and condensation of steam must be regulated. When the
piston has been forced to the bottom of the cylinder by the atmospheric
pressure acting against a vacuum, in order to balance that pressure, [Pg068] and
enable it to be drawn up by the weight of the pump-rod, it is necessary to
introduce steam from the boiler. This is accomplished by opening the cock
R in the steam pipe S. The steam being thus introduced from the boiler, its
pressure balances the action of the atmosphere upon the piston, which is
immediately drawn to the top of the cylinder by the weight of the pump-rod
D. It then becomes necessary to condense this steam, in order to produce a
vacuum. To accomplish this, the further supply of steam must be cut off,

which is done by closing the cock R. The supply of steam from the boiler
being thus suspended, the application of cold water on the external surface
of the cylinder becomes necessary to condense the steam within it. This was
done by enclosing the cylinder within another, leaving a space between
them.[13] Into this space cold water was allowed to flow from a cock M
placed over it, supplied by a pipe from the cistern N. This cistern is supplied
with water by a pump O, which is worked by the engine.
The cold water supplied from M, having filled the space between the two
cylinders, abstracts the heat from the inner one; and condensing the steam,
produces a vacuum, into which the piston is forced by the atmospheric
pressure. Preparatory to the next descent, the water which thus fills the
space between the cylinders, and which is warmed by the heat abstracted
from the steam, must be discharged, in order to give room for a fresh supply
of cold water from M. An aperture, furnished with a cock, is accordingly
provided in the bottom of the cylinder, through which the water is
discharged into the cistern L; and being warm, is adapted for the supply of
the boiler through T, as already mentioned.
The cock R being now again opened, steam is admitted below the piston,
which, as before, ascends, and the descent is again accomplished by closing
the cock R, and opening the cock M, admitting cold water between the
cylinders, and thereby condensing the steam below the piston.
The condensed steam, thus reduced to water, will collect [Pg069] in the
bottom of the cylinder, and resist the descent of the piston. It is therefore
necessary to provide an exit for it, which is done by a valve opening
outwards into a tube which leads to the feeding cistern L, into which the
condensed steam is driven.
That the piston should continue to be air-tight, it was necessary to keep a
constant supply of water over it; this was done by a cock similar to M,
which allowed water to flow from the pipe M on the piston.

(40.)

Soon after the first construction of these engines, an accidental
circumstance suggested to Newcomen a much better method of
condensation than the application of cold water on the external surface of
the cylinder. An engine was observed to work several strokes with unusual
rapidity, and without the regular supply of the condensing water. Upon
examining the piston, a hole was found in it, through which the water,
which was poured on to keep it air-tight, flowed, and instantly condensed
the steam under it.
On this suggestion Newcomen abandoned the external cylinder, and
introduced a pipe H, furnished with a cock Q, into the bottom of the
cylinder, so that, on turning the cock, the pressure of the water in the pipe H,
from the level of the water in the cistern N, would force the water to rise as
a jet into the cylinder, and would instantly condense the steam. This method
of condensing by injection formed a very important improvement in the
engine, and is still used.

(41.)

Having taken a general view of the parts of the atmospheric engine, let us
now consider more particularly its operation.
When the engine is not working, the weight of the pump-rod D (fig. 14.)
draws down the beam A, and draws the piston to the top of the cylinder,
where it rests. Let us suppose all the cocks and valves closed, and the boiler
filled to the proper depth. The fire being lighted beneath it, the water is
boiled until the steam acquires sufficient force to lift the valve V. When this
takes place, the engine may be started. For this purpose the regulating valve
R is opened. The steam rushes in, and is first condensed by the cold
cylinder. After a short time the cylinder acquires the temperature of the
steam, which then [Pg070] ceases to be condensed, and mixes with the air which
filled the cylinder. The steam and heated air, having a greater force than the
atmospheric pressure, will open a valve placed at the end X of a small tube
in the bottom of the cylinder, and which opens outwards. From this (which
is called the blowing valve[14]) the steam and air rush in a constant stream,
until all the air has been expelled, and the cylinder is filled with the pure

vapour of water. This process is called blowing the engine preparatory to
starting it.
When it is about to be started, the engine-man closes the regulator R, and
thereby suspends the supply of steam from the boiler. At the same time he
opens the condensing valve H[15]; and thereby throws up a jet of cold water
into the cylinder. This immediately condenses the steam contained in the
cylinder, and produces the vacuum. (The atmosphere cannot enter the
blowing valve, because it opens outwards, so that no air can enter to vitiate
the vacuum.) The atmospheric pressure above the piston now takes effect,
and forces it down in the cylinder. The descent being completed, the engine-
man closes the condensing valve H, and opens the regulator, R. By this
means he stops the play of the jet within the cylinder, and admits the steam
from the boiler. The first effect of the steam is to expel the condensing
water and condensed steam which are collected in the bottom of the
cylinder, through the tube Y, containing a valve which opens outwards
(called the eduction valve), which leads to the hot cistern L, into which this
water is therefore discharged.
When the steam admitted through R ceases to be condensed, it balances
the atmospheric pressure above the piston, and thus permits it to be drawn
to the top of the cylinder by the weight of the rod D. This ascent of the
piston is also assisted by the circumstance of the steam being somewhat
stronger than the atmosphere.
When the piston has reached the top, the regulating valve R is closed, and
the condensing valve H opened, and another descent produced, as before,
and so the process is continued. [Pg071]
The manipulation necessary in working this engine was, therefore, the
alternate opening and closing of two valves; the regulating and condensing
valves. When the piston reached the top of the cylinder, the former was to
be closed, and the latter opened; and, on reaching the bottom, the former
was to be opened, and the latter closed.

(42.)

The duty of working the engine requiring no great amount of labour, or
skill, was usually entrusted to boys, called, cock boys. It happened that one
of the most important improvements which has ever been made in the
working of steam engines was due to the ingenuity of one of these boys. It
is said that a lad, named Humphrey Potter, was employed to work the cocks
of an atmospheric engine, and being tempted to escape from the
monotonous drudgery to which his duty confined him, his ingenuity was
sharpened so as to prompt him to devise some means by which he might
indulge his disposition to play without exposing himself to the
consequences of suspending the performance of the engine. On observing
the alternate ascending and descending motion of the beam above him, and
considering it in reference to the labour of his own hands, in alternately
raising and lowering the levers which governed the cocks, he perceived a
relation which served as a clue to a simple contrivance, by which the steam
engine, for the first time, became an automaton. When the beam arrived at
the top of its play, it was necessary to open the steam valve by raising a
lever, and to close the injection valve by raising another. This he saw could
be accomplished by attaching strings of proper length to these levers, and
tying them to some part of the beam. These levers required to be moved in
the opposite direction when the beam attained the lowest point of its play.
This he saw could be accomplished by strings, either connected with the
outer arm of the beam, or conducted over rods or pulleys. In short, he
contrived means of so connecting the levers which governed the two cocks
by strings with the beam, that the beam opened and closed these cocks with
the most perfect regularity and certainty as it moved upwards and
downwards.
Besides rendering the machine independent of manual [Pg072]
superintendence, this process conferred upon it much greater regularity of
performance than any manual superintendence could ensure.
This contrivance of Potter was very soon improved by the substitution of
a bar, called a plug frame, which was suspended from the arm of the beam,
and which carried upon it pins, by which the arms of the levers governing
the cocks were struck as the plug-frame ascended and descended, so as to
be opened and closed at the proper times.

The engine thus improved required no other attendance except to feed the
boiler occasionally by the cock T, and to attend the furnace.

(43.)

However the merit of the discovery of the physical principles on which the
mechanical application of steam depends may be awarded, it must be
admitted that the engine contrived by Newcomen and his associates,
considered as a practical machine, was immeasurably superior to that which
preceded it; superior, indeed, to such a degree, that while the one was
incapable of any permanently useful application, the other soon became a
machine of extensive utility in the drainage of mines; and, even at the
present time, the atmospheric engine is not unfrequently used in preference
to the modern steam engine, in districts where fuel is abundant and cheap;
the expense of constructing and maintaining it being considerably less than
that of an improved steam engine. The low pressure of the steam used in
working it, rendered it perfectly safe. While Savery's engine, to work with
effect, required that the steam confined in the vessels should have a bursting
pressure amounting to about thirty pounds per square inch, the pressure of
steam in the boiler and cylinder of the atmospheric engine required only a
pressure about one pound per square inch. The high pressure also of the
steam used in Savery's engine, was necessarily accompanied, as we shall
presently explain, by a greatly increased temperature. The effect of this was,
to weaken and gradually destroy the vessels, especially those which, like
the steam vessels V and V′ (fig. 12.), were alternately heated and cooled.
Besides these defects, the power of Savery's engines was [Pg073] also very
restricted, both as to the quantity of water raised and as to the height to
which it was elevated. On the other hand, the atmospheric engine was
limited in its power only by the dimensions of its piston. Another
considerable advantage which the atmospheric engine possessed over that
of Savery, was the facility with which it was capable of driving machinery
by means of the working-beam. The merit, however, of Newcomen's
engine, regarded as an invention, and apart from merely practical
considerations, must be ascribed principally to its mechanism and
combinations. We find in it no new principle, and scarcely even a novel

application of a principle. The agency of the atmospheric pressure acting
against a vacuum, or partial vacuum, had been long known: the method of
producing a vacuum by the condensation of steam had been suggested by
Papin, and carried into practical effect by Savery. The mechanical power
obtained from the direct pressure of the elastic force of steam, used in the
atmospheric engine to balance the atmosphere during the ascent of the
piston, was suggested by De Caus and Lord Worcester. The boiler, gauge
pipes, and the regulator, were all borrowed from the engine of Savery. The
idea of using the atmospheric pressure against a vacuum or partial vacuum,
to work a piston in a cylinder, had been suggested by Otto Guericke, an
ingenious German philosopher, who invented the air-pump; and this,
combined with the production of a vacuum by the condensation of steam,
was subsequently suggested by Papin. The use of a working-beam could not
have been unknown. Nevertheless, the judicious combination of these
scattered principles must be acknowledged to deserve considerable credit.
In fact, the mechanism contrived by Newcomen rendered a machine which
was before altogether inefficient, highly efficient: and, as observed by
Tredgold, such a result, considered in a practical sense, should be more
highly valued than the fortuitous discovery of a physical principle. The
method of condensing the steam by the sudden injection of water, and of
expelling the air and water from the cylinder by the injection of steam, are
two contrivances not before in use, which are quite essential to the [Pg074]
effective operation of the engine. These processes, which are still necessary
to the operation of the improved steam engine, appear to be wholly due to
the inventors of the atmospheric engine.

ATMOSPHERIC ENGINE.

FOOTNOTES:
[9] This pipe is represented as proceeding from the force-pipe above the cistern C, in the
perspective view of Savery's engine at the head of this chapter.
[10] Hot water being lighter than cold, floats on the surface.
[11] "Captain" is a title given in Cornwall to the superintendent of the works connected
with a mine.
[12] As the calculation of the power of an engine depends on the number of square inches
in the section of the piston, it may be useful to give a rule for computing the number
of square inches in a circle. The following rule will always give the dimensions with
sufficient accuracy:—Multiply the number of inches in the diameter by itself; divide
the product by 14, and multiply the quotient thus obtained by 11, and the result will
be the number of square inches in the circle. Thus, if there be 12 inches in the
diameter, this multiplied by itself gives 144, which divided by 14 gives 104⁄44,
which multiplied by 11 gives 115, neglecting fractions. There are, therefore, 115
square inches in a circle whose diameter is 12 inches.
[13] The external cylinder is not represented in the diagram.
[14] Also called the snifting valve, from the peculiar noise made by the air and steam
escaping from it.
[15] Also called the injection valve.

GREENOCK, IN 1824.

CHAP. III.
[Pg075]

TOC INX

PROGRESS OF THE ATMOSPHERIC ENGINE.—SMEATON'S IMPROVEMENTS.—
BRINDLEY, ENGINEER OF THE BRIDGEWATER CANAL.—INVENTS THE SELF-
REGULATING FEEDER.—JAMES WATT.—HIS DESCENT AND PARENTAGE.—
ANECDOTES OF HIS BOYHOOD.—HIS EARLY ACQUIREMENTS.—GOES TO
LONDON.—RETURNS TO GLASGOW.—IS APPOINTED INSTRUMENT-MAKER TO
THE UNIVERSITY.—OPENS A SHOP IN GLASGOW.—HIS FRIENDS AND PATRONS.
—ADAM SMITH.—DR. BLACK.—ROBERT SIMSON.—PROFESSOR ROBISON.—
WATT'S PERSONAL CHARACTER.—INDUSTRIOUS AND STUDIOUS HABITS.—HIS
ATTENTION FIRST DIRECTED TO STEAM.—EXPERIMENTS ON HIGH-PRESSURE
STEAM.—REPAIRS AN ATMOSPHERIC MODEL.—EXPERIMENTAL INQUIRY
CONSEQUENT ON THIS.—ITS RESULTS.—DISCOVERS THE GREAT DEFECTS OF
THE ATMOSPHERIC ENGINE.—DISCOVERY BY EXPERIMENT OF THE EXPANSION
WHICH WATER UNDERGOES IN EVAPORATION.—DISCOVERS THE LATENT HEAT
OF STEAM.—IS INFORMED BY DR. BLACK OF THE THEORY OF LATENT HEAT.

(44.)

The atmospheric engine was brought to a state of considerable efficiency
and improvement by Mr. Beighton, in 1718. From that time it continued in
use without any change in its [Pg076] principle, and with little improvement in
its structure, for half a century. Although engines of this kind continued to
be extensively constructed, they were usually executed by ordinary
mechanics, incapable of applying to them the just principles of practical
science; and, consequently, little attention was paid to their proportions. It
was not until about the year 1772, that Mr. John Smeaton, the celebrated
engineer, applied the powers of his mind to the investigation of this
machine, as he had previously done with such success to wind and water
mills. Although he did not introduce any new principle into the atmospheric
engine, yet it derived greatly augmented power from the proportions which
he established for engines of different magnitudes.
In 1759, Mr. James Brindley, whose name is so celebrated as the engineer
of the Duke of Bridgewater's canal, obtained a patent for some

improvements in the atmospheric engine. He proposed that the boiler
should be made of wood and stone, with a stove or fire-place of cast iron
within it, so that the fire should be surrounded on every side by water. The
chimney was to be an iron pipe or tube, conducted through the boiler; so
that the heated air, in passing from the fire, should impart a portion of its
heat to the water. He also proposed a method of feeding the boiler, which,
by self-acting machinery, would keep the water in the boiler at a fixed level,
independently of any attention on the part of the engine-man. This was to be
accomplished by a buoy or float upon the surface of the water in the boiler,
which should communicate with a valve in the feed-pipe, so that when the
level of the water in the boiler fell, the float or buoy, falling with it, would
open the valve and supply the feed. It is stated, in the Biographia
Britannica, that Mr. Brindley, in 1756, undertook to erect an engine at
Newcastle-under-Lyne; but he is said to have been discouraged by the
obstacles which were thrown in his way, and to have abandoned the steam
engine.
The interval between the invention of the atmospheric engine, and the
amelioration it received at the hands of Smeaton, has been rendered
memorable by the advent of one who was destined to work a mighty change
in the condition [Pg077] of the human race by the application of his vast genius
to the adaptation of steam power to the uses of life.

(45.)

James Watt was born at Greenock, in Scotland, on the nineteenth day of
January, in the year 1736.[16]
The great-grandfather of Watt, a farmer in Aberdeenshire, was killed in
one of the battles of Montrose. The victorious party, not thinking death a
sufficient expiation for the political opinions in support of which he had
fought and bled, punished him in the person of his son, by confiscating his
little property. Thomas Watt, the son, thus deprived of support, was received
by distant relations, and, for a time, applied himself to study, by which he
was enabled, after the restoration of tranquillity, to establish himself at
Greenock as a teacher of practical mathematics and navigation. He resided
in the burgh or barony of Crawford's Dyke, and attained a position of

sufficient respectability to be elected to the office of baron-baillie, or chief
magistrate, and died in 1734, at the advanced age of ninety-two years.
Thomas Watt had two sons. The elder, John, adopted the profession of his
father, and was a teacher of mathematics and navigation at Glasgow: he
died in 1737, at the age of fifty years. The second son, James, the father of
the celebrated engineer, was, during a quarter of a century, treasurer of the
town council of Greenock, and a local magistrate. He was remarked for the
ardent zeal and enlightened spirit with which he discharged his public
duties. His business was that of a ship-chandler, builder, and general
merchant; but, unhappily, notwithstanding his active industry, he lost, in the
decline of his life, by unsuccessful commercial speculations, a part of the
property which he had so honourably acquired. He died in 1782, at the age
of eighty-four years.
James Watt, to whom the world is so largely indebted for the extension
and improvement of steam power, had from his birth an extremely delicate
constitution. From his mother, [Pg078] whose family name was Muirhead, he
received his first lessons in reading, and he learned from his father writing
and arithmetic. Although he was entered as a pupil in the grammar school
of Greenock, yet such was his delicate state of health, that his attendance
there was so interrupted by constant indisposition that he could derive but
little benefit from the opportunities of instruction which it afforded. For a
great period of the year he was confined to his room, where he devoted
himself to study without the aid of instruction. It was in the retirement of
the sick chamber that the high intellectual faculties of Watt, which were
destined to produce such precious fruits, began to unfold themselves. He
was too sickly to be subjected to the restraints which the business of
education usually imposes on children. His parents, therefore, found it
necessary to leave him at liberty to choose his occupations and
amusements. The following anecdotes will show the use he made of this
freedom.
A friend of his father found the boy one day stretched upon the hearth
tracing with chalk various lines and angles. "Why do you permit this child,"
said he, "to waste his time so; why not send him to school?" Mr. Watt
replied, "You judge him hastily; before you condemn us, ascertain how he is

employed." On examining the boy, then six years of age, it was found that
he was engaged in the solution of a problem of Euclid!
Having observed the tendency of his son's mind, Mr. Watt placed at his
disposal a collection of tools. These he soon learned to use with the greatest
skill. He took to pieces and put together, again and again, all the children's
toys which he could procure; and he was constantly employed in making
new ones. Subsequently he used his tools in constructing a little electrical
machine, the sparks proceeding from which became a great subject of
amusement to all the playfellows of the poor invalid.
Though endowed with great retentive powers, Watt would probably never
have figured among the prodigies of a common school: he would have been
slow to commit his lessons to memory, from the repugnance which he
would feel to repeat like a parrot anything which he did not perfectly [Pg079]
understand. The natural tendency of his mind to meditate on whatever came
before it, would give him, to superficial observers, the appearance of
dullness. Happily, however, he had a parent who was sufficiently clear-
sighted, and who entertained high hopes of the growing faculties of his son.
More distant and less sagacious relations were not so sanguine. One day
Mrs. Muirhead, the aunt of the boy, reproaching him for what she conceived
to be listless idleness, desired him to take a book and occupy himself
usefully. "More than an hour has now passed away," said she, "and you
have not uttered a single word. Do you know what you have been doing all
this time? You have taken off, and put on, repeatedly, the lid of the tea-pot;
you have been holding the saucers and the spoons over the steam, and you
have been endeavouring to catch the drops of water formed on them by the
vapour. Is it not a shame for you to waste your time so?"
Mrs. Muirhead was little aware that this was the first experiment in the
splendid career of discovery which was subsequently to immortalise her
little nephew. She did not see, as we now can, in the little boy playing with
the tea-pot, the great engineer preluding to those discoveries which were
destined to confer on mankind benefits so inestimable.
One of the social qualities of mind which was remarkable throughout his
life, was the singular felicity and grace with which he related anecdotes.
This power was manifested even in his earliest childhood. The following is

an extract from a letter written by Mrs. Marion Campbell, his cousin, and
the playfellow of his childhood:—
"He was not fourteen when his mother brought him to Glasgow to visit a
friend of hers; his brother John accompanied him. On Mrs. Watt's return to
Glasgow, some weeks after, her friend said, 'You must take your son James
home; I cannot stand the degree of excitement he keeps me in; I am worn
out for want of sleep. Every evening before ten o'clock, our usual hour of
retiring to rest, he contrives to engage me in conversation, then begins some
striking tale, and, whether humorous or pathetic, the interest is so
overpowering that the family all listen to him with breathless attention, and
hour after hour strikes unheeded.'" [Pg080]
Watt had a younger brother, John, who was subsequently lost by
shipwreck, in a voyage from Scotland to the United States. This lad, having
determined on following the business of his father, left James more
completely at liberty to choose his own occupation. But such a choice was
difficult for a student who commanded equal success in every thing to
which he directed his attention.
The excursions which he was in the habit of making on the Scottish
mountains surrounding Loch Lomond, naturally directed his attention to
botany and mineralogy, in each of which he attained considerable
knowledge. His love of anecdote and romance was likewise gratified by the
scenery which he enjoyed in these walks; and the traditions and popular
songs with which they made him acquainted. When from ill-health, as
constantly happened, he was confined to the house, he devoted himself to
chemistry, natural philosophy, and even to medicine and surgery. In
chemistry he acquired some experimental skill, and studied with eager zeal
the elements of natural philosophy by S'. Gravesande. His own unhappy
maladies prompted him to read works on surgery and medicine; and to such
an extent did the activity of his mind impel him on these subjects, that he
was found one day dissecting, in his room, the head of a child, who had
died of some unknown disease, with a view to ascertain the cause of its
death.
In 1775, at the age of nineteen, at the recommendation of Dr. Dick,
professor of natural philosophy in the university of Glasgow, he went to

London, where he employed himself in the house of Mr. John Morgan, a
mathematical instrument maker, in Finch Lane, Cornhill, to whom he
apprenticed himself for three years. He remained, however, only a year, at
the expiration of which (probably owing to his delicate state of health) he
was released from his apprenticeship, and returned to Glasgow, with the
intention of establishing himself in business as an optician and
mathematical instrument maker. In the fulfilment of this intention, however,
he was obstructed by the interposition of the Corporation of Trades in that
town, who regarded him as an intruder, not qualified by the necessary
apprenticeship to carry on business. All means of conciliation being [Pg081]
exhausted, the Professors of the University interfered, and gave him the use
of three apartments within the college, for carrying on his business, and
likewise appointed him mathematical instrument maker to the University.
Soon afterwards the opposition of the local trades seems to have given way,
and he opened a shop in Glasgow for the sale of mathematical instruments.
After the celebrity at which he has arrived, it will be easily believed that
every trace of his earlier connection with Glasgow college is carefully
cherished. There are accordingly preserved at that place little instruments
and pieces of apparatus of exquisite workmanship, which were executed
entirely by the hand of Watt, at a time when he was not in a condition to
command the aid of workmen under him.
At the time of obtaining this appointment in the University, Watt was in
his twenty-first year. His natural talents and winning manners were speedily
the means of gaining for him the esteem and friendship of all those eminent
persons connected at the time with that university whose regard was most
valued. Among these the earliest of his friends and patrons were—Adam
Smith, the author of "The Wealth of Nations;" Black, afterwards celebrated
for his chemical discoveries, and more especially for his theory of latent
heat; and Robert Simson, rendered illustrious by his works on ancient
geometry. In releasing Watt from the persecution of the Glasgow
corporation, these distinguished persons first imagined that they were
conferring a benefit merely on an industrious and clever artisan, whose
engaging manners won their regard; but a short acquaintance with him was
sufficient to convince them how superior his mind was to his position, and

they conceived towards him the most lively friendship. His shop became
the common rendezvous, the afternoon lounge, of all who were most
distinguished for literary and scientific attainments among the professors
and students. There they met to discuss the topics of the day in art, science,
and literature. Among these students, the name which afterwards attained
the highest distinctions, and among these distinctions, not the least, the
lasting personal friendship and esteem of Watt himself, was Robison, [Pg082]
the author of a well known work on Mechanics, and one of the contributors
to the Encyclopœdia Britannica.
The following extract from an unpublished manuscript by Robison
himself will show at once the estimation in which Watt was held, and will
illustrate one of the most interesting traits of his personal character:—
"I had always, from my earliest youth, a great relish for the natural
sciences, and particularly for mathematical and mechanical philosophy,
when I was introduced by Drs. Simson, Dick, and Moor, gentlemen eminent
for their mathematical abilities, to Mr. Watt. I saw a workman, and expected
no more; but was surprised to find a philosopher as young as myself, and
always ready to instruct me. I had the vanity to think myself a pretty good
proficient in my favourite study, and was rather mortified at finding Mr.
Watt so much my superior.. .. Whenever any puzzle came in the way of any
of the young students, we went to Mr. Watt. He needed only to be
prompted, for every thing became to him the beginning of a new and
serious study, and we knew that he would not quit it till he had either
discovered its insignificancy, or had made something of it. He learnt the
German language in order to peruse Leupold's 'Theatrum Machinarum;' so
did I, to know what he was about. Similar reasons made us both learn the
Italian language. * * * When to his superiority of knowledge is added
the naïve simplicity and candour of Mr. Watt's character, it is no wonder
that the attachment of his acquaintances was strong. I have seen something
of the world, and am obliged to say I never saw such another instance of
general and cordial attachment to a person whom all acknowledged to be
their superior. But that superiority was concealed under the most amiable
candour, and a liberal allowance of merit to every man. Mr. Watt was the
first to ascribe to the ingenuity of a friend things which were nothing but his

own surmises, followed out and embodied by another. I am the more
entitled to say this, as I have often experienced it in my own case."
Watt never permitted the inquiries which arose out of these reunions to
interfere with the discharge of the duties of his workshop. There he passed
the day, devoting the [Pg083] night to study. Every inquiry appeared to him to
be attractive in proportion to its difficulty, and to have charms in proportion
as it was removed from the common routine of his business. As an example
of this may be mentioned the fact, that, being himself so insensible to the
charms of music that he could not distinguish one note from another, he was
actually induced to undertake the construction of an organ, in which he was
nevertheless completely successful. The instrument he constructed, as might
have been expected, contained many improvements in its mechanism; but
what is much more remarkable, its tone and its musical qualities
commanded the admiration of all the professional musicians who heard it.
In the construction of this instrument Watt showed that vigorous spirit of
investigation which characterised all the subsequent labours of his life. He
made out the scale of temperament by the aid of the phenomena of beats, of
which he could only obtain a knowledge by a profound but obscure work
published by Dr. Robert Smith of Cambridge.
The earliest occasion on which the attention of Watt is said to have been
called to the agency of steam, was in the year 1759, when his friend
Robison entertained some speculations for applying that agent as a means
of propelling wheel carriages; and he consulted Watt on the subject. No
record, however, has been preserved of any experiments which were tried
on this occasion; nor does it appear that the inquiry was carried farther than
a verbal discussion, such as habitually took place on other subjects of
science between Watt and his friends.

(46.)

In 1762, Watt tried some experiments on the force of steam at a high
pressure, confined in a close digester; and he then constructed a small
model to show how motion could be obtained from that power. The
practicability of what has since been called the High Pressure Engine, was
demonstrated by him on this occasion; but he did not pursue the inquiry, on

account of the supposed danger of working with such compressed steam as
was required.
It is usual to provide, in the cabinets of experimental apparatus for the
instruction of the students of universities, [Pg084] small working models of the
most useful machines. In the collection for the illustration of the lectures
delivered to the Natural Philosophy class in the University of Glasgow was
a working model of Newcomen's atmospheric engine, applied to a pump for
raising water; which, however, had never been found to work satisfactorily.
The Professor of Experimental Philosophy of that day, Dr. John Anderson
(the founder of the celebrated Andersonian Institution), sent this model in
1763 to Watt's workshop, to be repaired. Its defects soon disappeared, and it
was made to work to the satisfaction of the professor and students.
This simple discharge of his duty, however, did not satisfy the artisan;
and his wonted activity of mind rendered this model a subject of profound
meditation, and led him into a course of practical inquiry respecting it,
which formed the commencement of a most brilliant career of mechanical
discovery. The improvement—we might almost say the creation—of the
steam engine, by this great man, must not therefore be regarded, as so often
happens with mechanical discoveries, as the result of fortuitous observation,
or even of a felicitous momentary inspiration. Watt, on the other hand,
conducted his investigation by a course of deep thought, and of experiments
marked by the last refinement of delicacy and address. If he had received a
more extended and liberal education, one would have thought that he had
adopted for his guide the celebrated maxim of Bacon:—
"To write, speak, meditate, or act, when we are not provided with facts to
direct our thoughts, is to navigate a coast full of dangers without a pilot, and
to launch into the immensity of the ocean without either rudder or
compass."
The model which he had repaired, had a cylinder of only two inches
diameter, and six inches stroke. After he had put it in complete order, he
found, that although the boiler was much larger in proportion to the cylinder
than those of real engines, yet, that it was incapable of supplying the
cylinder with steam in sufficient quantity to keep it at work. To enable it to
continue to move, he found it necessary to lessen the quantity of water

raised by its pump, so as to [Pg085] reduce the load on its piston very much
below the proper standard according to the common rules for large engines.
He ascribed the great inferiority in the performance of the model,
compared with the performance of the large engines, to the small size of the
cylinder, and to its material. The cylinder of the model was brass, while
those of large engines were of cast iron; and brass being a better conductor
of heat than iron, he concluded that more heat in proportion was lost from
this cause in the model, than in the larger engines. He observed that the
small cylinder was so heated when the steam was admitted into it, that it
could not be touched by the hand; but, nevertheless, that this heat
contributed nothing to its performance, inasmuch as before the piston
descended, the cylinder required to be cooled.

(47.)

His first attempt to improve the engine, was by using a wooden cylinder
instead of an iron one. He accordingly made a model with a cylinder of
wood, soaked in linseed oil, and baked to dryness. With this he made
numerous experiments, and found that it required a less quantity of water to
be thrown into the cylinder to condense the steam, and that it was worked
with a less supply of steam from the boiler than was necessary with the
metallic cylinder.
Still he found that the force with which the piston descended was
considerably less than that which the atmospheric pressure ought to supply,
supposing a tolerably perfect vacuum to be produced under the piston. This
led him to suspect that the water injected into the cylinder was not perfectly
effectual in condensing the steam. The experiments which he had
previously made on the increased temperature at which water boils under
pressures greater than that of the atmosphere, led him by analogy to the
conclusion that it would boil at lower temperatures if it were submitted to a
pressure less than the atmosphere, and he was aware that Dr. Cullen and
others had then recently discovered that in vacuo, water would boil at so
low a temperature as 100°. These notions suggested the probability that the
water injected into the cylinder being heated by the condensed steam, might
produce vapour of a low temperature [Pg086] and reduced pressure under the

piston, which would account for the deficiency he observed in the power of
the engine.
No means occurred to him by which he could ascertain, by direct
experiment, the temperatures at which water would boil under pressures
less than that of the atmosphere. He sought, however, to determine it by the
following method. Having ascertained, by repeating and multiplying the
experiments which he had tried in 1762, on high-pressure steam, he
obtained a table of the temperatures at which water boils at various
pressures greater than that of the atmosphere. These results he laid down in
a series forming a curve, of which the abscissa represented the
temperatures, and the ordinates the pressures. He then continued this curve,
backwards as it were, and obtained, by analogy, an approximation to the
boiling temperatures, corresponding to pressures less than that of the
atmosphere. In other words, having obtained by his experiments a notion,
however imperfect, of the law or rule observed by the temperatures at
which water boils at different pressures greater than that of the atmosphere,
he calculated by the same law or rule what the pressures would be at
different pressures less than that of the atmosphere.
Applying these results to the interior of the cylinder of the atmospheric
engine, he obtained an approximation to the pressure of the vapour which
would be produced from the warm water formed by the cold water injected
into the cylinder, and the steam condensed by it; and he accordingly found
that vapour, having a pressure seriously injurious to the power of the engine
would be produced in the cylinder, unless considerably more water of
injection was thrown in than was customary.
It was apparent that the actual quantity of steam usefully employed in the
cylinder at each stroke, was only the quantity which filled the cylinder; and
therefore, in order to ascertain the quantity of steam lost by the
imperfections of the machine, it was necessary to compare the actual
quantity of steam transmitted by the boiler to the cylinder at each stroke,
with the quantity which would just fill the cylinder. The difference would of
course be wasted. But to determine [Pg087] the actual quantity of steam
supplied by the boiler to the cylinder, there was no other means than by
observing the quantity of water evaporated in the boiler. That being

observed, it was necessary to know the quantity of steam which that water
formed; and it was therefore necessary to determine the quantity or volume
of steam which a given volume of water produced.

(48.)

On considering more attentively the operation of the machine, the following
circumstances gradually unfolded themselves to him.
Let us suppose the piston at the top of the cylinder, and the space in the
cylinder below it, filled with steam so as to balance the pressure of the
atmosphere above the piston. Under such circumstances the steam, as will
presently be explained, must have the temperature of boiling water. But that
the steam should have, and should maintain, this temperature, it was
evidently necessary that the inner surface of the cylinder in contact with it
should have the same temperature: for if it had a lower temperature, it
would take heat from the steam, and reduce the temperature of the latter.
Now the cylinder being a mass of metal, has a quality in virtue of which
heat passes freely through its dimensions, so that its inner surface could not
be maintained at a temperature more elevated than that of its dimensions
extending from the inner surface to the outer surface. Therefore, to maintain
the steam contained in the cylinder at the proper temperature, it was
essential that the whole of the solid metal composing the cylinder should be
itself at that temperature.
Things being in this state, it was required that a vacuum should be
produced under the piston to give effect to the atmospheric pressure above
it, by relieving it from the pressure below. This, indeed, would appear to
have been attained by introducing as much cold water within the cylinder as
would be sufficient to reconvert the steam contained in it into water; but
Watt found, in his experiments on the atmospheric model, that the piston
would not descend with the proper force, unless a vastly greater quantity of
water were introduced into the cylinder than the quantity which he had
ascertained to be [Pg088] necessary for the reconversion of the steam into water.
The cause of this he perceived and fully explained.

If we suppose as much, and no more, cold water introduced into the
cylinder as would reconvert the steam contained in it into water, then we
should have in the bottom of the cylinder a quantity of warm water with a
vacuum above it: but the entire mass of metal composing the cylinder itself,
which was previously at the temperature of boiling water, would still be at
the same temperature. The warm water, resting in contact with this metal in
the bottom of the cylinder, would be immediately heated by it, and would
rise in its temperature, while the metal of the cylinder itself would be
somewhat lowered in temperature by the heat which it would thus impart to
the warm water contained in it. Under these circumstances, as we shall
presently explain, steam would be produced from the water, which would
fill the cylinder; and although such steam would not have a mechanical
pressure equal in amount to the atmosphere, and therefore would not
altogether prevent the piston from descending if it had no load to move, yet
it would deprive the engine of so great a portion of its legitimate power as
to render it altogether inefficient. But this defect would be removed by
throwing into the cylinder a sufficient quantity of cold water, not only to
destroy the steam contained in it, but also to cool the entire mass of metal
composing the cylinder itself, until it would be reduced to such a
temperature that the vapour proceeding from the water contained in it
would have so small a pressure that it would not seriously or injuriously
obstruct the descent of the piston.
The piston being made to descend with such force as to render the
machine practically efficient, it would then be necessary again to make it
ascend; and to accomplish this, Watt found that the boiler should supply a
quantity of steam many times greater than was necessary to fill the cylinder.
Mature reflection on the circumstances which have been just explained,
enabled him to discover how this undue quantity of steam was rendered
necessary.
Let it be recollected, that when the piston has reached the bottom of the
cylinder, the whole mass of the cylinder, and [Pg089] the piston itself, are
reduced to so low a temperature that the vapour of water, having the same
temperature, has no pressure sufficiently great to obstruct the action of the
machine. When, in order to make the piston ascend, steam is introduced

from the boiler into the cylinder under the piston, this steam encounters, in
the first instance, the cold surfaces of the metal forming the bottom of the
cylinder and the bottom of the piston. The first effect of this is to convert
the steam which comes from the boiler into water, an effect which is
produced by that steam imparting its heat to the metal with which it comes
into contact. This destruction of steam continues until the metal exposed to
contact with it has been heated up to the temperature of boiling water. Then,
and not till then, the steam below the piston will have a pressure equal to
that of the atmosphere above it, and the piston will begin to ascend. As it
ascends, however, the sides of the cylinder which it exposes to the contact
of the steam are cold, and partially destroy the steam. Steam, therefore,
must be supplied from the boiler to replace the steam thus destroyed; nor
can the piston reach the top of the cylinder until such a quantity of steam
shall have flowed from the boiler into the cylinder, as shall be sufficient not
only to fill the cylinder under the piston, but likewise, by its condensation,
to raise the whole mass of the cylinder and piston to the temperature of
boiling water.
Such were the circumstances which forced themselves upon the attention
of Watt, in the course of repairing, and subsequently trying, the model of
the atmospheric engine, at Glasgow. Being informed generally of the uses
of the engine in the drainage of mines, and of the vast expense attending its
operation, by reason of the quantity of fuel which it consumed, he saw how
important any improvement would be by which the extensive sources of
waste which had thus presented themselves could be removed. He saw also,
that all that portion of steam which was expended, not in filling the cylinder
under the piston, but in heating the great mass of metal composing the
cylinder and piston, from a low temperature to that of boiling water, upon
each stroke of the piston, was so much heat lost, and that the proportion of
the fuel expended in evaporating the steam thus wasted would be saved, if
by any [Pg090] expedient he could make the piston descend without cooling the
cylinder. But in order to estimate the full amount of this waste, and to
discover the most effectual means of preventing it, it was necessary to
investigate the quantity of heat necessary for the evaporation of a given
quantity of water; also, the quantity of steam which a given quantity of
water would produce, as well as other circumstances connected with the

temperature and pressure of steam. He, therefore, applied himself to make
experiments with a view to elucidate these questions; and succeeded in
obtaining results which led to the discovery of some of the most important
of those physical phenomena, on the due application of which, the efficacy
of the steam engine, which he afterwards invented, depended, and which
also form striking facts in the general physics of heat.

(49.)

The first question to which he directed his experiments, was the
determination of the extent to which water enlarged its volume, or
magnitude, when it passed into steam. To ascertain this, he filled a thin
Florence flask with steam, of a pressure equal to the atmosphere, and
weighed it accurately. The same flask was then filled with water, and
weighed again. Finally, the weight of the flask itself was ascertained. It is
evident, that by such means, the exact weight of the steam which filled the
flask, and of the same bulk of water, would be obtained. He found that the
water weighed about eighteen hundred times more than the steam; from
whence he inferred that the steam which filled the flask contained about
eighteen hundred times less water than the flask would contain.[17]
Having once ascertained this point, he was able, by observing the
[Pg091]

quantity of water evaporated in the boiler of the atmospheric model, to
compute the volume of steam which was supplied to the cylinder. It was
evident, that for every cubic inch of water evaporated in the boiler, eighteen
hundred cubic inches of steam were supplied to the cylinder. Having
accurately observed the evaporation of the boiler for a short time, and the
number of strokes made by the piston in the same time, he found that the
quantity of water evaporated in the boiler would supply about four times as
much steam as the cylinder would require. He consequently inferred, that
about three-fourths of the steam produced was wasted.
The next question to which he directed his experiments, was to ascertain
the quantity of cold water necessary to be injected into the cylinder, in order
to condense the steam contained in it. To ascertain this, he attached a pipe to
a boiler, by which he was enabled to conduct the steam from the boiler into
a glass jar containing cold water at fifty-two degrees of temperature. The

steam, as it passed from the boiler through the pipe, was condensed by the
cold water, and continued to be so condensed, until, by the heat which it
imparted to the water, the latter began to boil, and would then condense no
more steam. On comparing the water in the glass jar, when boiling, with the
water originally contained in it at fifty-two [Pg092] degrees, the quantity was
found to be increased in the proportion of six to seven, very nearly; from
which he inferred, that to reduce one ounce of steam to water, it was
necessary to mix about six ounces of cold water with it.
He was further led to the conclusion, that steam contains a vast quantity
of heat, by the following experiment. He heated, in a close digester, a
quantity of water several degrees above the common boiling point. When
thus heated, by opening a stop-cock, he allowed the compressed steam to
escape into a cold vessel; in three or four seconds, he found that the heat of
the water in the digester was reduced from a very high temperature to the
common boiling point; yet, that all the steam which escaped from it, and
which carried off with it the superabundant heat, formed only a few drops
of water when condensed; from which he inferred, that this small quantity
of water, in the form of steam, contained as much heat as was sufficient to
raise all the water in the digester from the boiling point to the temperature
at which it was before the steam was allowed to escape.
Having thus ascertained the exact quantity of cold water which ought to
be injected into the cylinder in order to condense the steam which filled the
cylinder, he found, on comparing the quantity necessary to be injected in
order to enable the piston to descend, that this quantity was about four times
as great as that which was necessary to condense the steam. This led him to
the conclusion, that about four times as much heat was destroyed in the
cylinder as needed to be destroyed, if the object were the mere condensation
of the steam. This result fully corroborated the other conclusion, deduced,
from the proportion which he found between the quantity of steam supplied
by the boiler and the actual contents of the cylinder.

(50.)

Watt was forcibly struck with these circumstances, not only on account of
their importance in an economical point of view, when their relation to

steam power was considered, but still more so, as indicating phenomena in
the physics of heat altogether novel to him.
He, therefore, eagerly sought his friend Dr. Black, to whom he
communicated these results. Then, for the first time, he [Pg093] was informed,
by Black, of the theory of LATENT HEAT, which had recently been discovered
by him, and of which these very phenomena formed the basis.
Some passages in the works of Dr. Robison produced an erroneous
impression, that a large share of the merit of the discoveries of Watt which
have been just explained was due to Dr. Black, to whose instructions on the
subject of latent heat Watt was represented to have owed the knowledge of
those facts which led to his principal inventions and improvements. We
shall here give, in the words of Watt himself, his explanation of the
circumstances which led to this error. This explanation is given in a letter
addressed by Watt to Dr. Brewster, in May 1814, and prefixed to the third
volume of Brewster's edition of Robison's Mechanical Philosophy:—
"The representations of friends whose opinions I highly value
induce me to avail myself of this opportunity of noticing an error into
which not only Dr. Robison, but apparently also Dr. Black, has fallen,
in relation to the origin of my improvements upon the steam engine,
and which not having been publicly controverted by me, has, I am
informed, been adopted by almost every subsequent writer upon the
subject of latent heat.
"Dr. Robison, in the article Steam Engine, after passing an
encomium upon me, dictated by the partiality of friendship, qualifies
me as the 'pupil and intimate friend of Dr. Black,'—a description
which not being there accompanied with any inference, did not
particularly strike me at the time of its first perusal. He afterwards, in
the dedication to me of his edition of Dr. Black's lectures upon
chemistry, goes the length of supposing me to have professed to owe
my improvements upon the steam engine to the instructions and
information I had received from that gentleman, which certainly was a
misapprehension; as, though I have always felt and acknowledged my
obligations to him for the information I had received from his
conversation, and particularly for the knowledge of the doctrine of

latent heat, I never did nor could consider my improvements as
originating in those communications. He is also mistaken in his
assertion (p. 8. of the preface to the above work), that 'I had attended
two courses [Pg094] of the doctor's lectures;' for, unfortunately for me, the
necessary avocations of my business prevented me from attending his
or any other lectures at college; and as Dr. Robison was himself absent
from Scotland for four years at the period referred to, he must have
been misled by erroneous information. In p. 184. of the lectures, Dr.
Black says, 'I have the pleasure of thinking that the knowledge we
have acquired concerning the nature of elastic vapours, in consequence
of my fortunate observation of what happens in its formation and
condensation, has contributed in no inconsiderable degree to the public
good by suggesting to my friend Mr. Watt of Birmingham, then of
Glasgow, his improvement on this useful engine' (meaning the steam
engine of which he is then speaking). There can be no doubt from what
follows in his description of the engine, and from the very honourable
mention which he has made of me in various parts of his lectures, that
he did not mean to lessen any merit that might attach to me as an
inventor; but, on the contrary, he was always disposed to give me fully
as much praise as I deserved.
"And were that otherwise doubtful, it would, I think, be evident
from the following quotation from a letter of his to me, dated 13th
February 1783, where, speaking of an intended publication by a friend
of mine, on subjects connected with the history of steam, he says, 'I
think it is very proper for you to give him a short account of your
discoveries and speculations; and particularly to assert clearly and
fully your sole right to the honour of the improvements of the steam
engine.' And in a written testimonial which he very kindly gave me, on
the occasion of a trial at law against a piracy of my invention in 1796-
7, after giving a short account of the invention, he adds, 'Mr. Watt was
the sole inventor of the capital improvement and contrivance above
mentioned.'
"Under this conviction of his candour and friendship, it is very
painful to me to controvert any assertion or opinion of my revered

friend; yet, in the present case I find it necessary to say, that he appears
to me to have fallen into an error; and I hope, in addition to my
assertion, to make that appear by the short history I have given of my
invention, in my [Pg095] notes upon Dr. Robison's essay, as well as by the
following account of the state of my knowledge previous to my
receiving any explanation of the doctrine of latent heat; and also from
that of the facts which principally guided me in the invention.
"It was known very long before my time, that steam was condensed
by coming into contact with cold bodies, and that it communicated
heat to them; witness the common still, &c. &c.
"It was known, by some experiments of Dr. Cullen and others, that
water and other liquids boiled in vacuo at very low heats; water below
100°.
"It was known to some philosophers that the capacity or equilibrium
of heat, as we then called it, was much smaller in mercury and tin than
in water.
"It was also known that evaporation caused the cooling of the
evaporating liquid, and bodies in contact with it.
"I had myself made experiments to determine the following facts:—
"First, the capacities of heat for iron, copper, and some sorts of
wood, comparatively with water.
"Second, the bulk of steam compared with that of water.
"Third, the quantity of water evaporated in a certain boiler by a
pound of coals.
"Fourth, the elasticities of steam at various temperatures greater than
that of boiling water, and an approximation to the law which it
followed at other temperatures.
"Fifth, how much water in the form of steam was required every
stroke by a small Newcomen's engine, with a wooden cylinder six
inches diameter, and twelve inches stroke.

"Sixth, the quantity of cold water required in every stroke to
condense the steam in that cylinder, so as to give it a working power of
about 7 lb. on the inch.
"Here I was at a loss to understand how so much cold water could
be heated so much by so small a quantity of water in the form of
steam; and I accordingly applied to Dr. Black, and then first
understood what was called latent heat.
"But this theory, though useful in determining the quantity of
injection necessary where the quantity of water [Pg096] evaporated by the
boiler, and used by the cylinder, was known, and in determining, by
the quantity and heat of the hot water emitted by Newcomen's engines,
the quantity of steam required to work them did not lead to the
improvements I afterwards made in the engine. These improvements
proceeded upon the old established fact, that steam was condensed by
the contact of cold bodies; and the later known one, that water boiled
in vacuo at heats below 100°, and consequently that a vacuum could
not be obtained unless the cylinder and its contents were cooled every
stroke to below that heat."

LOCH LOMOND.

FOOTNOTES:
[16] We are indebted for many of the anecdotes of the life of Watt to the Eloge Historique,
recently published by M. Arago, who was furnished with all the documents and
circumstances relating to this celebrated person which were considered proper for
publication, by his son, the present James Watt, Esq., of Aston Hall, near
Birmingham, and to the notes added to this memoir by Mr. Muirhead, a relative of
Mr. Watt.
[17] The following is the account of these experiments given in Watt's own words:—
"It being evident that there was a great error in Dr. Desagulier's calculations of Mr.
Beighton's experiments on the bulk of steam, a Florence flask, capable of containing
about a pound of water, had about one ounce of distilled water put into it; a glass
tube was fitted into its mouth, and the joining made tight by lapping that part of the
tube with packthread covered with glazier's putty. When the flask was set upright, the
tube reached down near to the surface of the water, and in that position the whole
was placed in a tin reflecting oven before a fire until the water was wholly
evaporated, which happened in about an hour, and might have been done sooner, had
I not wished the heat not much to exceed that of boiling water. As the air in the flask
was heavier than the steam, the latter ascended to the top, and expelled the air
through the tube. When the water was all evaporated, the oven and flask were
removed from the fire, and a blast of cold air was directed against one side of the
flask, to collect the condensed steam in one place. When all was cold, the tube was
removed, the flask and its contents were weighed with care; and the flask being made
hot, it was dried by blowing into it by bellows, and when weighed again was found
to have lost rather more than four grains, estimated at 41⁄3 grains. When the flask
was filled with water, it was found to contain about 171⁄8 ounces avoirdupois of that
fluid which gave about 1800 for the expansion of water converted into steam of the
heat of boiling water.
"This experiment was repeated with nearly the same result, and in order to ascertain
whether the flask had been wholly filled with steam, a similar quantity of water was
for the third time evaporated; and, while the flask was still cold, it was placed
inverted with its mouth (contracted by the tube) immersed in a vessel of water, which
it sucked in as it cooled, until in the temperature of the atmosphere it was filled to
within half an ounce measure of water.
"In repetitions of this experiment at a later date, I simplified the apparatus by omitting the
tube, and laying the flask upon its side in the oven, partly closing its mouth by a
cork, having a notch on one side, and otherwise proceeding as has been mentioned.

GLASGOW.

CHAP. IV.
[Pg097]

TOC INX
EXPOSITION OF PHYSICAL PRINCIPLES.—THERMOMETER.—METHOD OF
GRADUATING IT.—FREEZING AND BOILING POINTS.—LATENT HEAT OF WATER.
—QUANTITY OF HEAT NECESSARY TO CONVERT ICE INTO WATER.—QUANTITY
OF HEAT GIVEN OUT BY WATER IN BEING CONVERTED INTO ICE.—PROCESS OF
BOILING.—OF RECONVERSION OF STEAM INTO WATER.—QUANTITY OF HEAT
NECESSARY TO CONVERT WATER INTO STEAM.—BOILING POINT OF WATER.—
DIFFERENT IN DIFFERENT PLACES.—DEPENDS ON THE BAROMETER.—VARIES
WITH THE PRESSURE.—EXPERIMENTAL PROOF OF THIS.—BOILS AT LOWER
TEMPERATURES THAN 212° UNDER PRESSURES LESS THAN THE ATMOSPHERE.
—SUM OF LATENT AND SENSIBLE HEAT OF STEAM ALWAYS THE SAME.—THE
FUEL NECESSARY TO EVAPORATE WATER THE SAME, WHATEVER BE THE
TEMPERATURE OR PRESSURE AT WHICH IT IS EVAPORATED.—MECHANICAL
FORCE OBTAINED BY EVAPORATION.—THIS FORCE NEARLY THE SAME UNDER
ALL CIRCUMSTANCES.

(51.)

We shall pause here to put the reader in possession of the physical and
mechanical principles connected with the evaporation of water and other
liquids, which are necessary to enable him to understand the full extent of
the value and the merit of the discoveries of Watt, and to comprehend the
[Pg098] structure and operation of the steam engine in its improved form, as it

has passed to us from his hands.
As we shall frequently have occasion to refer to the indications of a
thermometer, we shall first explain the principle of that instrument as it is
commonly used in this country.
The thermometer is an instrument used for the purpose of measuring and
indicating the temperature or sensible heat of material substances.
Heat, like all other physical agents, can only be measured by its effects.
One of these effects best suited for this purpose, is the change of dimension
which all bodies undergo in consequence of their change of temperature. In

general, when heat is applied to a material substance, that substance
undergoes an enlargement of bulk; and if heat be abstracted from it, it
suffers a diminution of bulk. This variation of magnitude is not always in
the same proportion as the increase or diminution of temperature; but it is
so when applied to certain substances and between certain limits. One of the
substances whose expansion and contraction through an extensive range of
temperature has been found to be nearly uniform, and which is attended
with other convenient qualities for a thermometer, is the liquid called
mercury or quicksilver. A mercurial thermometer is constructed in the
following way:—
A glass tube is made with a small and uniform bore: upon the end of this
tube, a bulb is blown, having a magnitude very great compared with the
bore of the tube. Let us suppose this bulb and a part of the tube to be filled
with mercury. If the mercury contained in the bulb be heated, it will expand,
and being more susceptible of expansion than the glass which contains it,
the bulb will be too small for its augmented volume: the mercury in the
bulb can only, therefore, obtain room for its increased bulk by pressing the
mercury in the tube upwards, which it will accordingly do. The increase of
volume which the mercury in the bulb therefore undergoes, will be
exhibited by the increased length of the column in the tube. Since the bore
of the tube is made so exceedingly minute compared with the magnitude of
the bulb, a very small quantity of mercury forced [Pg099] from the bulb into the
tube, will cause a considerable increase of the length of the column. Small
degrees of expansion will therefore be rendered very apparent, and may be
accurately measured. The following is the method by which the
thermometer called Fahrenheit's thermometer is graduated.
The tube and bulb being prepared and supplied with mercury, as already
explained, let the instrument be plunged in a vessel of melting ice. It will be
found that the mercury will stand in the tube at a certain point, from which
it will not vary so long as any ice remains not completely melted in the
vessel. Let a mark be made on the tube, or on a scale attached to the tube, at
the point corresponding to the top of the column: the point thus marked is
called the freezing point.

Now let the instrument be immersed in a vessel of boiling water, the
barometer at the time having the height of thirty inches. It will be found that
so long as the water is kept boiling, the column of mercury in the tube will
remain stationary. Let the point corresponding with the top of the column be
marked on the tube, or on the scale attached to it. This is called the boiling
point. Let the space on the scale between the freezing and boiling points be
now divided into 180 equal parts: each of these parts is called a degree. Let
the same divisions be continued upon the scale below the freezing point,
until thirty-two divisions be taken; let the lowest division be then marked 0,
and let the successive divisions upwards from that be numbered 1, 2, 3, &c.
In like manner, let the same divisions be continued above the boiling point,
as far as the tube will admit.
It is evident that, under these circumstances, the freezing point will be
marked by 32, and the boiling point by 212. It is usual to express the
degrees of a thermometer in the same manner as the degrees of a circle, by
placing a small ° above the number. Thus the freezing point is expressed by
32°, and the boiling point by 212°.
The reason the degrees were commenced at 32° below the freezing point
was, because, when the thermometer was invented, that temperature was
supposed to be the lowest degree of cold possible, being that of a certain
mixture of [Pg100] snow and salt. This, however, has since been found to be an
error, very much lower temperatures being obtained by various physical
expedients.
The temperature of a body is, then, that elevation to which the
thermometer would rise when immersed in that body. Thus, if in plunging
the thermometer in water we found the mercury to rise or fall to the division
marked 100, we should then say, the temperature of the water was 100°.
Let us suppose a spirit lamp, or other regular source of heat, applied to a
bath of mercury, so as to maintain the mercury at a fixed temperature of
200°, and let another vessel, containing a quantity of ice at a temperature of
20° be immersed in the mercury. Let a thermometer be placed in the
mercury, and another in the ice. The following effects will then ensue. The
thermometer immersed in the ice will be observed gradually to rise from
20° upwards, until it indicates the temperature of 32°. It will then become

stationary, and the ice which had hitherto remained in a solid state will
begin to melt and be converted into water. This process of liquefaction will
continue for a considerable time, during which the thermometer immersed
in the ice will constantly be maintained at 32°. At the moment, however,
when the last portion of ice is liquefied, the thermometer will begin again to
rise. The coincidence of this ascent of the thermometer with the completion
of the liquefaction of the ice, may be very easily observed, because the ice
being lighter, bulk for bulk, than water, will float on the surface, and so long
as a particle of it remains unmelted it will be distinctly seen.
Now it cannot be doubted that, during the whole of this process, the
mercury, supposed to be maintained at 200°, constantly imparts heat to the
ice; yet, from the moment the liquefaction begins, until it is completed, no
increased temperature is exhibited by the thermometer immersed in the
melting ice. If during this part of the process no heat were received by the
ice from the mercury, the consequence would be, that the application of the
lamp would cause the temperature of the mercury to rise above 200°, which
may be easily demonstrated by withdrawing the vessel of ice from the
mercurial bath during the process of liquefaction. The moment [Pg101] it is
withdrawn, the thermometer immersed in the mercury, instead of remaining
fixed at 200°, will begin to rise, although the action of the lamp remains the
same as before; from which it is evident that the heat which now causes the
mercury to rise above 200° was before received by the melting ice.
The heat which thus enters ice in the process of liquefaction, and which
is not indicated by the thermometer, is for this reason called latent heat. It
will be perceived that this phrase is the name of a fact, and not of an
hypothesis. That heat really enters the water, and is contained in it, has been
established by the experiments; and to declare that it is present there, is to
declare an established fact. To call it by the name latent heat, is to declare
another established fact, viz., that it is not sensible to the thermometer.
These facts show us that heat is capable of existing in bodies in two
distinct states, in one of which it is sensible to the thermometer, and in the
other not. Heat which is sensible to the thermometer is called, for
distinction, sensible or free heat. It may be here observed, that heat which is
sensible to the thermometer is also perceptible by the senses, and heat not

sensible to the thermometer is not perceptible by the senses. Thus, ice at 32°
and water at 32° feel equally cold, and yet we have seen that the latter
contains considerably more heat than the former.
Dr. Black, who first noticed the remarkable fact to which we have now
alluded, inferred that ice is converted into water by communicating to it a
certain quantity or dose of heat, which enters into combination with it in a
manner analogous to that which takes place when bodies combine
chemically. The heat, thus combined with the solid ice, loses its property of
affecting the senses or the thermometer, and the effects therefore bear a
resemblance to those cases of chemical combination in which the
constituent elements change their sensible properties when they form the
compound.
The fact that the thermometer immersed in the ice remains stationary
only as long as the process of liquefaction is going on, shows that this
absorption of heat is necessarily connected with that process, and that, were
it not for the conversion of [Pg102] the solid ice into liquid water, the heat which
is so received would be sensible, and would cause the thermometer
immersed in the ice to rise. Before the time of Black it was supposed that
the slightest addition of heat would cause solid ice to be converted into
water, and that the thermometer would immediately pass from the freezing
temperature to higher degrees. The experiments above described, however,
show the falsehood of such a supposition. If, while the mercurial bath, in
which the ice is immersed, is maintained at the temperature of 200°, the
length of time necessary to complete the liquefaction of the ice be observed,
it would be found that that time is about twenty-eight times the length of
time which it would take to raise the liquid water from 32° to 37°; and if it
be assumed that the same quantity of heat is imparted to the ice, during the
process of liquefaction, during each minute, as is imparted to the water,
during each minute, in rising from 32° to 37°, it will follow, that to liquefy
the ice requires twenty-eight times as much heat as is necessary to raise the
water from 32° to 37°. It appears, therefore, that, instead of a small quantity
of heat being necessary to melt the ice, a very considerable portion is
absorbed in that process.

Having ascertained the remarkable fact, that heat is absorbed in a large
quantity in the conversion of ice into water, without rendering the body so
absorbing it warmer, let us now inquire what the exact quantity of heat so
absorbed is. We have already stated that, if the quantity communicated in
equal times be the same, the heat necessary to liquefy a given weight of ice
would be twenty-eight times as much as would be necessary to raise the
same weight of water from 32° to 37°; or, if the heat necessary to raise
water through every 5° be the same, that quantity of heat would be
sufficient to raise water from 32° to 172°: and hence we infer, that as much
heat is absorbed in the liquefaction of a given quantity of ice as would raise
the same quantity of water through 140 degrees of the thermometric scale.

(52.)

Let us now examine the analogous effects produced by the continued
application of heat to water in the liquid state.
Let a small quantity of water be placed in a glass flask of considerable
size, and then closed so as to prevent the escape [Pg103] of any vapour. Let this
vessel be now placed over the flame of a spirit lamp, so as to cause the
water it contains to boil. For a considerable time the water will be observed
to boil, and apparently to diminish in quantity, until at length all the water
disappears, and the vessel is apparently empty. If the vessel be now
removed from the lamp, and suspended in a cool atmosphere, the whole of
the interior of its surface will presently appear to be covered with a dewy
moisture; and at length a quantity of water will collect in the bottom of it,
equal to that which had been in it at the commencement of the process. That
no water has at any period of the experiment escaped from it, may be easily
determined, by performing the experiment with the glass flask suspended
from the arm of a balance, counterpoised by a sufficient weight suspended
from the other arm. The equilibrium will be preserved throughout, and the
vessel will be found to have the same weight, when to all appearance it is
empty, as when it contains the liquid water. It is evident, therefore, that the
water exists in the vessel in every stage of the process, but that it becomes
invisible when the process of boiling has continued for a certain length of
time; and it may be shown that it will continue to be invisible, provided the

flask be exposed to a temperature considerably elevated. Thus, for example,
if it be suspended in a vessel of boiling water, the water which it contains
will continue to be invisible; but the moment it is withdrawn from the
boiling water, and exposed to the cold air, the water will again become
visible, as above mentioned, forming a dew on the inner surface, and finally
collecting in the bottom, as in the commencement of the experiment.
In fact, the liquid has, by the process of boiling, been converted into
vapour, or steam, which is a body similar in its leading properties to
common air, and, like it, is invisible. It will hereafter appear that it likewise
possesses the property of elasticity, and other mechanical qualities enjoyed
by gases in general.

(53.)

Again, let an open vessel be filled with water at 60°, and placed in a
mercurial bath, which is maintained, by a fire or lamp applied to it, at the
temperature of 230°. Place a thermometer in the water, and it will be
observed gradually to [Pg104] rise as the temperature of the water is increased
by the heat which it receives from the mercury in which it is immersed. The
water will steadily rise in this manner until it attains the temperature of
212°; but here the thermometer immersed in it will become stationary. At
the same time the water contained in the vessel will become agitated, and
its surface will present the same appearance as if bubbles of air were rising
from the bottom, and issuing at the top. A cloudy vapour will be given off in
large quantities from its surface. This process is called ebullition or boiling.
If it be continued for any considerable time, the quantity of water in the
vessel will be sensibly diminished; and at length every particle of it will
disappear, and the vessel will remain empty. During the whole of this
process, the thermometer immersed in the water will remain stationary at
212°.
Now, it will be asked, what has become of the water? It cannot be
imagined that it has been annihilated. We shall be able to answer this by
adopting means to prevent the escape of any particle of matter from the
vessel containing the water, into the atmosphere or elsewhere. Let us
suppose that the top of the vessel containing the water is closed, with the

exception of a neck communicating with a tube, and let that tube be carried
into another close vessel removed from the cistern of heated mercury, and
plunged in another cistern of cold water. Such an apparatus is represented in
fig. 15.

Fig. 15.

A is a cistern of heated mercury, in which the glass vessel B, containing
water, is immersed. From the top of the vessel B proceeds a glass tube C,
inclining downwards, and entering a glass vessel D, which is immersed in a
cistern E of cold water. If the process already described be continued until
the water by constant ebullition has disappeared, as already mentioned, [Pg105]
from the vessel B, it will be found that a quantity of water will be collected
in the vessel D; and if this water be weighed, it will be found to have exactly
the same weight as the water had which was originally placed in the vessel
B. It is, therefore, quite apparent that the water has passed by the process of
boiling from the one vessel to the other; but, in its passage, it was not
perceptible by the sight. The tube C and the upper part of the vessel B, had
the same appearance, exactly, as if they had been filled with atmospheric
air. That they are not merely filled with atmospheric air may, however, be
easily proved. When the process of boiling first commences, it will be
found that the tube C is cold, and the inner surface dry. When the process of
ebullition has continued a short time, the tube C will become gradually
heated, and the inner surface of it covered with moisture. After a time,
however, this moisture disappears, and the tube attains the temperature

212°. In this state it continues until the whole of the water is discharged
from the vessel B to the vessel D.

(54.)

These effects are easily explained. The water in the vessel B is incapable of
receiving any higher temperature than 212°, consistently with its retaining
the liquid form. Small portions, therefore, are constantly converted into
steam by the heat received from the surrounding mercury, and bubbles of
steam are formed on the bottom and sides of the vessel B. These bubbles,
being very much lighter, bulk for bulk, than water, rise rapidly through the
water, just in the same manner as bubbles of air would, and produce that
peculiar agitation at its surface which has been taken as the external
indication of boiling. They escape from the surface, and collect in the upper
part of the vessel. The steam thus collected, when it first enters the tube C, is
cooled below the temperature of 212° by the surface of the tube; and
consequently, being incapable of remaining in the state of vapour at any
lower temperature than 212°, it is reconverted into water, and forms the
dewy moisture which is observed in the commencement of the process on
the interior of the tube C. At length, however, the whole of the tube C is
heated to the temperature of 212°, and the moisture which was previously
collected upon its inner [Pg106] surface is again converted into steam. As the
quantity of steam evolved from the water in B increases, it drives before it
the steam previously collected in the tube C, and forces it into the vessel B.
Here it encounters the inner surface of this vessel, which is kept constantly
cold by being surrounded with the cold water in which it is immersed; and
the vapour, being thus immediately reduced below the temperature of 212°,
is reconverted into water. At first it collects in a dew on the surface of the
vessel D; but as this accumulates, it drops into the bottom of the vessel, and
forms a more considerable quantity. As the quantity of water is observed to
be gradually diminished in the vessel B, the quantity will be found to be
gradually increased in the vessel D; and if the operation be suspended at any
stage of the process, and the water in the two vessels weighed, it will be
found that the weight of the water in D is exactly equal to the weight which
the water in B has lost.

(55.)

The demonstration is, therefore, perfect, that the gradual diminution of the
boiling water in the vessel B is produced by the conversion of that water
into steam by the heat. In the process first described, when the top of the
vessel B was supposed to be open, this steam made its escape into the air,
where it was first dispersed, and subsequently cooled in separate particles,
and was deposited in minute globules of moisture on the ground and on
surrounding objects.

(56.)

In reviewing this process, we are struck by the fact, that the continued
application of heat to the vessel B is incapable of raising the temperature of
the water contained in it above 212°. This presents an obvious analogy to
the process of liquefaction, and leads to inquiries of a similar nature, which
are attended with a like result. We must either infer, that the water, having
arrived at 212°, received no more heat from the mercury; or that such heat,
if received, is incapable of affecting the thermometer; or, finally, that the
steam which passes off carries this heat with it. That the water receive heat
from the mercury, will be proved by the fact, that, if the vessel B be
removed from the mercury, other things remaining as before, the
temperature of the mercury will rapidly rise, and if the fire be continued, it
will even boil; but so long as the [Pg107] vessel B remains immersed, it prevents
the mercury from increasing in temperature. It therefore receives that heat
which would otherwise raise the temperature of the quicksilver.
If a thermometer be immersed in the steam which collects in the upper
part of the vessel B, it will show the same temperature (of 212°) as the water
from which it is raised. The heat, therefore, received from the mercury, is
clearly not imparted in a sensible form to the steam, which has the same
temperature in the form of steam as it had in the form of water. What has
been already explained respecting liquefaction would lead us, by analogy,
to suspect that the heat imparted by the mercury to the water has become
latent in the steam, and is instrumental to the conversion of water into
steam, in the same manner as heat has been shown to be instrumental to the

conversion of ice into water. As the fact
was in that case detected by mixing ice
with water, so we shall, in the present
instance, try it by a like test, viz. by
mixing water with steam. Let about five
ounces and a half of water, at the
temperature of 32°, be placed in a vessel A
(fig. 16.), and let another vessel B, in
which water is kept constantly boiling at Fig. 16.
the temperature of 212°, communicate
with A by a pipe C proceeding from the top, so that the steam may be
conducted from B, and escape from the mouth of the pipe at some depth
below the surface of the water in A. As the steam issues from the pipe, it
will be immediately reconverted into water by the cold water which it
encounters; and, by continuing this process, the water in A will be gradually
heated by the steam combined with it and received through the pipe C. If
this process be continued until the water in A is raised to the temperature of
212°, it will boil. Let it then be weighed, and it will be found to weigh six
ounces and a half: from whence we infer, that one ounce of water has been
received from the vessel B in the form of steam, and has been reconverted
into water by the inferior temperature of the water in A. Now, this ounce of
water received in the form of steam into the vessel A had, when in that
form, the temperature of 212°. It is now [Pg108] converted into the liquid form,
and still retains the same temperature of 212°; but it has caused the five
ounces and a half of water with which it has been mixed, to rise from the
temperature of 32° to the temperature of 212°,—and this, without losing
any temperature itself. It follows, therefore, that, in returning to the liquid
state, it has parted with as much heat as is capable of raising five times and
a half its own weight of water from 32° to 212°. This heat was combined
with the steam, though not sensible to the thermometer; and was, therefore,
latent. Had it been sensible in the water in B, it would have caused the water
to have risen through a number of thermometric degrees, amounting to five
times and a half the excess of 212° above 32°; that is, through five times
and a half 180°; for it has caused five times and a half its own weight of

water to receive an equal increase of temperature. But five times and a half
180° is 990°, or, to use round numbers (for minute accuracy is not here our
object), 1000°. It follows, therefore, that an ounce of water, in passing from
the liquid state at 212° to the state of steam at 212°, receives as much heat
as would be sufficient to raise it through 1000 thermometric degrees, if that
heat, instead of becoming latent, had been sensible.

(57.)

In order to derive all the knowledge from these experiments which they are
capable of imparting, it will be necessary to examine very carefully how
water comports itself under a variety of different circumstances.
If water be boiled in an open vessel, with a thermometer immersed, on
different days, it will be observed that the fixed temperature which it
assumes in boiling will be subject to a variation within certain small limits.
Thus, at one time, it will be found to boil at the temperature of 210°; while,
at others, the thermometer immersed in it will rise to 213°; and, on different
occasions, it will fix itself at different points within these limits. It will also
be found, if the same experiment be performed at the same time in distant
places, that the boiling points will be subject to a like variation. Now, it is
natural to inquire what cause produces this variation; and we shall be led to
the discovery of the cause, by examining what other physical effects
undergo a simultaneous change. [Pg109]
If we observe the height of the barometer at the time of making each
experiment, we shall find a very remarkable correspondence between it and
the boiling temperature. Invariably, whenever the barometer stands at the
same height, the boiling temperature will be the same. Thus, if the
barometer stands at 30 inches, the boiling temperature will be 212°. If the
barometer fall to 291⁄2 inches, the thermometer stands at a small fraction
above 211°. If the barometer rise to 301⁄2 inches, the boiling temperature
rises to nearly 213°. The variation in the boiling temperature is, then,
accompanied by a variation in the pressure of the atmosphere indicated by
the barometer; and it is constantly found that the boiling point will remain

unchanged, so long as the atmospheric pressure remains unchanged, and
that every increase in the one causes a corresponding increase in the other.

(58.)

From these facts it must be inferred, that the pressure excited on the surface
of the water has a tendency to resist its ebullition, and to make it necessary,
before it can boil, that it should receive a higher temperature; and, on the
contrary, that every diminution of pressure on the surface of the water will
give an increased facility to the process of ebullition, or will cause that
process to take place at a lower temperature. As these facts are of the utmost
importance in the theory of heat, it may be useful to verify them by direct
experiment.
If the variable pressure excited on the surface of the water by the
atmosphere be the cause of the change in the boiling temperature, it must
happen, that any change of pressure produced by artificial means on the
surface of the water must likewise change the boiling point, according to
the same law. Thus, if a pressure considerably greater than the atmospheric
pressure be excited on a liquid, the boiling point may be expected to rise
considerably above 212°; and, on the other hand, if the surface of the water
be relieved from the pressure of the atmosphere, and be submitted to a
considerably diminished pressure, the water would boil below 212°.
Let B (fig. 17.) be a strong spherical vessel of brass, supported on a stand
S, under which is placed a large spirit lamp L, or other means of heating it.
In the top of this vessel are three apertures, in two of which are screwed a
[Pg110] thermometer T, the bulb of which enters the hollow brass sphere, and a

stop-cock C, which may be closed or opened at pleasure, to confine the
steam, or allow it to escape. In the third aperture at the top, is screwed a
long barometer tube, open at both ends. The lower end of this tube extends
nearly to the bottom of the spherical vessel B. In the bottom of this vessel is
placed a quantity of mercury, the surface of which rises to some height
above the lower end of the tube A. Over the mercury is poured a quantity of
water, so as to half fill the vessel B. Matters being thus arranged, the screws
are made tight, so as to confine the water, and the lamp is allowed to act on

the vessel; the temperature of the water is
raised, and steam is produced, which, being
confined within the vessel, exerts its pressure
on the surface of the water, and resists its
ebullition. The pressure of the steam acting on
the surface of the water is communicated to
the surface of the mercury, and it forces a
portion of the mercury into the tube A, which
presently rises above the point where the tube
is screwed into the top of the vessel B. As the
action of the lamp continues, the thermometer
T exhibits a gradually increasing temperature;
while the column of mercury in A shows the
force with which the steam presses on the
surface of the water in B,—this column being
balanced by the pressure of the steam. Thus,
the temperature and pressure of the steam at
the same moment may always be observed by
inspecting the thermometer T and the tube A.
When the column in the tube A has risen to the
height of 30 inches above the level of the
mercury in the vessel B, then the pressure of
the steam will be equivalent to double the
pressure of the atmosphere, because, the tube
A being open at the top, the atmosphere
presses on the [Pg111] surface of the mercury in it. Fig. 17.
The thermometer T will be observed gradually
to rise until it attains the temperature of 212°; but it will not stop there, as it
would do if immersed in water boiled in an open vessel. It will, on the other
hand, continue to rise; and when the column of mercury in A has attained
the height of 30 inches, the thermometer T will have risen to 251°,—being
39° above the ordinary boiling point.
During the whole of this process, the surface of the water being
submitted to a constantly increasing pressure, its ebullition is prevented,

and it continues to receive heat without boiling. That it is the increased
pressure which resists its ebullition, and causes it to receive a temperature
above 212°, may be easily shown. Let the stop-cock C be opened;
immediately the steam in B, having a pressure considerably greater than that
of the atmosphere, will rush out, and will continue to issue from C, until its
pressure is balanced by the atmosphere. At the same time the column of
mercury in A will be observed rapidly to fall, and to sink below the orifice
by which it is inserted in the vessel B. The thermometer T will also fall until
it attains the temperature of 212°. At that point, however, it will remain
stationary; and the water will now be distinctly heard to be in a state of
rapid ebullition. If the stop-cock C be once more closed, the thermometer
will begin to rise, and the column of mercury ascending in A will be again
visible.
If, instead of a stop-cock being at C, the aperture were made to
communicate with a valve, like the safety-valve of a steam engine, loaded
with a certain weight,—say at the rate of 15 lbs. on the square inch,—then
the thermometer T, and the mercury in the tube A, would not rise
indefinitely as before. The thermometer would continue to rise till it
attained the temperature of 251°; and the mercury in the tube A would rise
to the height of 30 inches. At this limit the resistance of the valve would be
balanced by the pressure of the steam; and as fast as the water would have a
tendency to produce steam of a higher pressure, the valve would be raised
and the steam suffered to escape; the thermometer T and the column of
mercury in A remaining stationary during this process. If the valve were
loaded more heavily, the phenomena would be [Pg112] the same, only that the
mercury in T and A would become stationary at certain heights. But, on the
other hand, if the valve were loaded at a less pressure than 15 lbs. on the
square inch, then the mercury in the two tubes would become stationary at
lower points.

(59.)

These experiments show that every increase of pressure above the ordinary
pressure of the atmosphere causes an increase in the temperature at which

water boils. We shall now inquire whether a diminution of pressure will
produce a corresponding effect on the boiling point.
This may be easily accomplished by the aid of an air pump. Let water at
the temperature of 200° be placed in a glass vessel under the receiver of an
air pump, and let the air be gradually withdrawn. After a few strokes of the
pump, the water will boil; and if the mercurial gauge of the pump be
observed, it will be found that its altitude will be about 231⁄2 inches. Thus
the pressure to which the water is submitted has been reduced from the
ordinary pressure of the atmosphere expressed by the column of 30 inches
of mercury, to a diminished pressure expressed by 231⁄2 inches; and we find
that the temperature at which the water boils has been lowered from 212° to
200°. Let the same experiment be repeated with water at the temperature of
180°, and it will be found that a further rarefaction of the air is necessary,
but the water will at length boil. If the gauge of the pump be now observed,
it will be found to stand at about fifteen inches, showing, that at the
temperature of 180° water will boil under half the ordinary pressure of the
atmosphere. These experiments may be varied and repeated; and it will be
always found, that, as the pressure is diminished or increased, the
temperature at which the water will boil will be also diminished or
increased.

(60.)

The same effects may be exhibited in a striking manner without an air
pump, by producing a vacuum by the condensation of steam. Let a small
quantity of water be placed in a thin glass flask, and let it be boiled by
holding it over a spirit lamp. When the steam is observed to issue
abundantly from the mouth of the flask, let it be quickly corked and
removed from the lamp. The process of boiling will then cease, and the
water will become quiescent; but if the flask be plunged [Pg113] in a vessel of
cold water, the water it contains will again pass into a state of violent
ebullition, thus exhibiting the singular fact of water being boiled by cooling
it. This effect is produced by the cold medium in which the flask is
immersed, causing the steam above the surface of the water in it to be
condensed, and therefore relieving the water from its pressure. The water,

under these circumstances, boils at a lower temperature than when
submitted to the pressure of the uncondensed vapour.

(61.)

There is no limit to the temperature to which water may be raised, if it be
submitted to a sufficient pressure to resist its tendency to take the vaporous
form. If a strong metallic vessel be nearly filled with water, so as to prevent
the liquid from escaping by any force which it can exert, the water thus
inclosed may be heated to any temperature whatever without boiling; in
fact, it may be made red-hot; and the temperature to which it may be raised
will have no limit, except the strength of the vessel containing it, or the
point at which the metal of which it is formed may begin to soften or to be
fused.

(62.)

The following table will show the temperature at which water will boil
under different pressures of the atmosphere corresponding to the altitudes of
the barometer between 26 and 31 inches.
Barometer. Boiling
inches Point.
26 204°·91
26·5 205°·79
27 206°·67
27·5 207°·55
28 208°·43
28·5 209°·31
29 210°·19
29·5 211°·07
30 212°
30·5 212°·88
31 213°·76

From this table it appears, that, for every tenth of an inch which the
barometric column varies between these limits, the boiling temperature
changes by the fraction of a degree expressed by the decimal ·176, or nearly
by the vulgar fraction 1⁄6.

(63.)

In the experiment already described, by which the latent [Pg114] heat of steam
was determined, the water was supposed to be boiled under the ordinary
pressure of the atmosphere. Having seen, however, that water may boil at
different temperatures, under different pressures, the inquiry presents itself,
whether the heat absorbed in vaporisation at different temperatures, and
under different pressures, is subject to any variation? Experiments of the
same nature as those already described, instituted upon water in a state of
ebullition at different temperatures, as well below as above 212°, have led
to the discovery of a very remarkable fact in the theory of vapour. It has
been found that the heat absorbed by vaporisation is always less, the higher
the temperature at which the ebullition takes place; and less, by the same
amount as the temperature of ebullition is increased. Thus, if water boil at
312°, the heat absorbed in ebullition will be less by 100° than if it boiled at
212°; and again, if water be boiled under a diminished pressure, at 112°, the
heat absorbed in vaporisation will be 100° more than the heat absorbed by
water boiled at 212°. It follows, therefore, that the actual consumption of
heat in the process of vaporisation must be the same, whatever be the
temperature at which the vaporisation takes place; for whatever heat is
saved in the sensible form, is consumed in the latent form, and vice versâ.
Let us suppose a given weight of water at the temperature of 32° to be
exposed to any regular source by which heat may be supplied to it. If it be
under the ordinary atmospheric pressure, the first 180° of heat which it
receives will raise it to the boiling point, and the next 1000° will convert it
into steam. Thus, in addition to the heat which it contains at 32°, the steam
at 212° contains 1180° of heat. But if the same water be submitted to a
pressure equal to half the atmospheric pressure, then the first 148° of heat
which it receives will cause it to boil, and the next 1032° will convert it into
vapour. Thus, steam at the temperature of 180° contains a quantity of heat
more than the same quantity of water at 32°, by 1032° added to 148°, which
gives a sum of 1180°. Steam, therefore, raised under the ordinary pressure
of the atmosphere at 212°, and steam raised under half that pressure at 180°,
contain the same quantity of heat,—with this difference [Pg115] only—that the
one has more latent heat, and less sensible heat, than the other.

From this fact, that the sum of the latent and sensible heats of the vapour
of water is constant, it follows that the same quantity of heat is necessary to
convert a given weight of water into steam, at whatever temperature, or
under whatever pressure, the water may be boiled. It follows, also, that, in
the steam engine, equal weights of high-pressure and low-pressure steam
are produced by the same consumption of fuel; and that, in general, the
consumption of fuel is proportional to the quantity of water vaporised,
whatever the pressure of the steam may be.[18]

(64.)

Having explained the conditions under which, by supplying heat to water, it
is converted into steam, and, by abstracting heat from steam, it may be
reconverted into water, let us now consider the mechanical force which is
developed in these phenomena.
Let A B (fig. 18.) be a tube, or cylinder, the base of
which is equal to a square inch, and let a piston P move
in it so as to be steam-tight. Let it be supposed, that
under this piston there is, in the bottom of the cylinder,
a cubic inch of water between the bottom of the piston
and the bottom of the tube; let the piston be
counterbalanced by a weight W acting over a pulley,
which will be just sufficient to counterpoise the weight
of the piston, so as leave no force tending to keep the
piston down, except the force of the atmosphere acting
above it. Under the circumstances here supposed, the
piston being in contact with the water, and all air being
excluded, it will be pressed down by the weight of the
atmosphere, which we will suppose to be fifteen
pounds, the magnitude of the piston being a square
inch. [Pg116]

Fig. 18. Now let the flame of a lamp be applied at the bottom
of the tube; the water under the piston having its
temperature thereby gradually raised, and being

submitted to no pressure save that of the atmosphere above the piston, it
will begin to be converted into steam when it has attained the temperature
of 212°. According as it is converted into steam, it will cause the piston to
ascend in the tube until all the water has been evaporated. If the tube were
constructed of sufficient length, the piston then would be found to have
risen to the height of about seventeen hundred inches, or one hundred and
forty-two feet; since, as has been already explained, water passing into
steam under the ordinary pressure of the atmosphere undergoes an increase
of bulk in the proportion of about seventeen hundred to one.
Now in this process, the air above the piston, which presses on it with a
force equal to fifteen pounds, has been raised one hundred and forty-two
feet. It appears, therefore, that, by the evaporation of a cubic inch of water
under a pressure equal to fifteen pounds per square inch, a mechanical force
of this amount is developed.
It is evident that fifteen pounds raised one hundred and forty-two feet
successively, is equivalent to one hundred and forty-two times fifteen
pounds raised one foot. Now, one hundred and forty-two times fifteen is
two thousand one hundred and thirty, and therefore the force thus obtained
is equal to two thousand one hundred and thirty pounds raised one foot
high. This being within about 110 pounds of a ton, it may be stated, in
round numbers, that, by the evaporation of a cubic inch of water under these
circumstances, a force is obtained equal to that which would raise a ton
weight a foot high.
The augmentation of volume which water undergoes in passing into
steam under the pressure here supposed, may be easily retained in the
memory, from the accidental circumstance that a cubic inch of water is
converted into a cubic foot of steam, very nearly. A cubic foot contains one
thousand seven hundred and twenty-eight cubic inches,—which is little
different from the proportion which steam bears to water, when raised under
the atmospheric pressure. [Pg117]

(65.)

It will, therefore, be an advantage to retain in memory the following general
facts:—
1. A cubic inch of water evaporated under the ordinary atmospheric
pressure, is converted into a cubic foot of steam.
2. A cubic inch of water evaporated under the atmospheric pressure,
gives a mechanical force equal to what would raise about a ton weight a
foot high.

(66.)

Let us, again, suppose the piston P (fig. 23.) to be restored to its original
position, with the liquid water beneath it; and, in addition to the weight of
the atmosphere which before pressed it down, let us suppose another weight
of fifteen pounds laid upon it, so that the water below shall be pressed by
double the weight of the atmosphere. If the lamp were now applied, and at
the same time a thermometer were immersed in the water, it would be found
that the water would not begin to be converted into steam until it attained
the temperature of about 250°. The piston would then begin, as before, to
ascend, and the water to be gradually converted into vapour. The water
being completely evaporated, it would be found that the piston would be
raised to a height little more than half its former height, or 72 feet. The
mechanical effect, therefore, thus obtained, will be equivalent to double the
former weight raised half the former height.
In like manner, if the piston were loaded with thirty pounds in addition to
the atmosphere, the whole pressure on the water being then three times the
pressure first supposed, the piston would be raised to somewhat more than
one third of its first height by the evaporation of the water. This would give
a mechanical force equivalent to three times the original weight raised a
little more than one third of the original height.
In general, as the pressure on the piston is increased, the height to which
the piston would be raised by the evaporation of the water will be
diminished in a proportion somewhat less than the proportion in which the
pressure on the piston is increased. If the temperature at which the water is
converted into steam under these different pressures were the same, then the

height to which the piston would be raised by the evaporation of the water
would be diminished in precisely [Pg118] the same proportion as the pressure on
the piston is increased; and, in that case, the whole mechanical force
developed by the evaporation of the water would remain exactly the same
under whatever pressure the water might be boiled. We shall explain
hereafter the extent to which the variation of temperature in the water and
steam corresponding to the variation of pressure modifies this law; but, as
the effect of the difference of temperatures is not considerable, it will be
convenient to register in the memory the following important practical
conclusion:—

(67.)

A cubic inch of water converted into steam will supply a mechanical force
very nearly equal to a ton weight raised a foot high; and this force will not
be subject to considerable variation, whatever be the temperature or
pressure at which the water may be evaporated.

GLASGOW.

FOOTNOTES:
[18] The preceding paragraphs, and some other parts of the present volume on the general
properties of Heat, are taken from my Treatise on Heat, in the Cabinet Cyclopœdia,
to which those who desire more detailed explanation and more copious illustration
should refer.

GLASGOW COLLEGE.

CHAP. V.
[Pg119]

TOC INX

WATT FINDS THAT CONDENSATION IN THE CYLINDER IS INCOMPATIBLE WITH A
DUE ECONOMY OF FUEL.—CONCEIVES THE NOTION OF CONDENSING OUT OF
THE CYLINDER.—DISCOVERS SEPARATE CONDENSATION.—INVENTS THE AIR-
PUMP.—SUBSTITUTES STEAM PRESSURE FOR ATMOSPHERIC PRESSURE.—
INVENTS THE STEAM CASE, OR JACKET.—HIS FIRST EXPERIMENTS TO REALISE
THESE INVENTIONS.—HIS EXPERIMENTAL APPARATUS.—DIFFICULTIES OF
BRINGING THE IMPROVED ENGINES INTO USE.—WATT PRACTISES AS A CIVIL
ENGINEER.—HIS PARTNERSHIP WITH ROEBUCK.—HIS FIRST PATENT.—
DESCRIPTION OF HIS SINGLE-ACTING STEAM ENGINE.

(68.)

At the period to which we have now brought the history of the invention of
the steam engine, Watt had obtained, chiefly by his own experiments, a
sufficient knowledge of the phenomena which have been just explained, to
enable him to arrive at the conclusion that a very small proportion of the
whole mechanical effect attending the evaporation was really rendered
available by the atmospheric engine; and that, [Pg120] therefore, extensive and
injurious sources of waste existed in its machinery.
He perceived that the principal source of this wasteful expenditure of
power consisted in the quantity of steam which was condensed at each
stroke of the piston, in heating the cylinder previous to the ascent of the
piston. Yet, as it was evident that that ascent could not be accomplished in a
cold cylinder, it was apparent that this waste of power must be inevitable,
unless some expedient could be devised, by which a vacuum could be
maintained in the cylinder, without cooling it. But, to produce such a
vacuum, the steam must be condensed; and, to condense the steam, its
temperature must be lowered to such a point that the vapour proceeding
from it shall have no injurious pressure; yet, if condensed steam be
contained in a cylinder at a high temperature, it will return to the

temperature of the cylinder, recover its elasticity, and resist the descent of
the piston.
Having reflected on these circumstances, it became apparent to Watt, that
a vice was inherent in the structure of the atmospheric engine, which
rendered a large waste of power inevitable; this vice arising from the fact,
that the condensation of the steam was incompatible with the condition of
maintaining the elevated temperature of the cylinder in which that
condensation took place. It followed, therefore, either that the steam must
be imperfectly condensed, or that the condensation could not take place in
the cylinder. It was in 1765, that, pondering on these circumstances, the
happy idea occurred to him, that the production of a vacuum could be
equally effected, though the place where the condensation of the steam took
place were not the cylinder itself. He saw, that if a vessel in which a
vacuum was produced were put into communication with another
containing an elastic fluid, the elastic fluid would rush into the vacuum, and
diffuse itself through the two vessels; but if, on rushing into such vacuum,
this elastic fluid, being vapour, were there condensed, or restored to the
liquid form, that then the space within the two vessels would be equally
rendered a vacuum;—that, under such circumstances, one of the vessels
might be maintained at any temperature, however high, while [Pg121] the other
might be kept at any temperature, however low. This felicitous conception
formed the first step in that splendid career of invention and discovery
which has conferred immortality on the name of Watt. He used to say, that
the moment the idea of separate condensation occurred to him,—that is, of
condensing, in one vessel kept cold, the steam coming from another vessel
kept hot,—all the details of his improved engine rushed into his mind in
such rapid succession, that, in the course of a day, his invention was so
complete that he proceeded to submit it to experiment.
To explain the first conception of this memorable invention; let a tube or
pipe, S (fig. 19.), be imagined to proceed from the bottom of the cylinder A B
to a vessel, C, having a stop-cock, D, by which the communication between
the cylinder and the vessel C may be opened or closed at pleasure. If we
suppose the piston P at the top of the cylinder, and the space below it filled
with steam, the cylinder and steam being at the usual temperature, while the

vessel C is a vacuum, and maintained at a
low temperature. Then, on opening the cock
D, the steam will rush from the cylinder A B
through the tube S, and, passing into the cold
vessel C, will be condensed by contact with
its cold sides. This process of condensation
will be rendered instantaneous if a jet of
cold water is allowed to play in the vessel C.
When the steam thus rushing into C, has
been destroyed, and the space in the cylinder
A B becomes a vacuum, then the pressure of
the atmosphere being unobstructed, the
piston will descend with the force due to the
Fig. 19. excess of the pressure of the atmosphere
above the friction. When it has descended,
suppose the stop-cock D closed, and steam admitted from [Pg122] the boiler
through a proper cock or valve below the piston, the cylinder and piston
being still at the same temperature as before. The steam on entering the
cylinder, not being exposed to contact with any surface below its own
temperature, will not be condensed, and therefore will immediately cause
the piston to rise, and the piston will have attained the top of the cylinder
when as much steam shall have been supplied by the boiler as will fill the
cylinder. When this has taken place, suppose the communication with the
boiler cut off, and the cock D once more opened: the steam will again rush
through the pipe S into the vessel C, where encountering the cold surface
and the jet of cold water, it will be condensed, and the vacuum, as before,
will be produced in the cylinder A B; that cylinder still maintaining its
temperature, the piston will again descend, and so the process may be
continued.

(69.)

Having carried the invention to this point, Watt saw that the vessel C would
gradually become heated by the steam which would be continually
condensed in it. To prevent this, as well as to supply a constant jet of cold

water, he proposed to keep the vessel C submerged in a cistern of cold
water, from which a pipe should conduct a jet to play within the vessel, so
as to condense the steam as it would pass from the cylinder.
But here a difficulty presented itself, against which it was necessary to
provide. The cold water admitted through the jet to condense the steam,
mixed with the condensed steam itself, would gradually collect in the vessel
C, and at length choke it. To prevent this, Watt proposed to put the vessel C
in communication with a pump F, which might be wrought by the engine
itself, and by which the water, which would collect in the bottom of the
vessel C, would be constantly drawn off. This pump would be evidently
rendered the more necessary, since more or less atmospheric air, always
combined with water in its common state, would enter the vessel C by the
condensing jet. This air would be disengaged in the vessel C by the heat of
the steam condensed therein; and it would rise through the tube S, and
vitiate the vacuum in the cylinder;—an effect which would be rendered the
more injurious, [Pg123] inasmuch as, unlike steam, this elastic fluid would be
incapable of being condensed by cold. The pump F, therefore, by which
Watt proposed to draw off the water from the vessel C, might also be made
to draw off the air, or the principal part of it.
The vessel C was subsequently called a condenser; and, from the
circumstances just adverted to, the pump F has been called the air-pump.
These—namely, the cylinder, the condenser, and the air-pump—were the
three principal parts in the invention, as it first presented itself to the mind
of Watt—and even before it was reduced to a model, or submitted to
experiment. But, in addition to these, other two improvements offered
themselves in the very first stage of its progress.
In the atmospheric engine, the piston was maintained steam-tight in the
cylinder by supplying a stream of cold water above it, by which the small
interstices between the piston and cylinder would be stopped. It is evident
that the effect of this water as the piston descended would be to cool the
cylinder, besides which any portion of it which might pass between the
piston and cylinder and which would pass below the piston, would boil the
moment it would fall into the cylinder, which itself would be maintained at

the boiling temperature. This water, therefore, would produce steam, the
pressure of which would resist the descent of the piston.
Watt perceived, that even though this inconvenience were removed by
the use of oil or tallow upon the piston, still, that as the piston would
descend in the cylinder, the cold atmosphere would follow it; and would, to
a certain extent, lower the temperature of the cylinder. On the next ascent of
the piston, this temperature would have to be again raised to 212° by the
steam coming from the boiler, and would entail upon the machine a
proportionate waste of power.
If the atmosphere of the engine-house could be kept heated to the
temperature of boiling water, this inconvenience would be removed. The
piston would then be pressed down by air as hot as the steam to be
subsequently introduced into it. On further consideration, however, it
occurred to Watt that it would be still more advantageous if the cylinder
itself could be [Pg124] worked in an atmosphere of steam, having only the same
pressure as the atmosphere. Such steam would press the piston down as
effectually as the air would; and it would have the further advantage over
air, that if any portion of it leaked through between the piston and cylinder,
it would be condensed, which could not be the case with atmospheric air.
He therefore determined on surrounding the cylinder by an external casing,
the space between which and the cylinder he proposed to be filled with
steam supplied from the boiler. The cylinder would thus be enclosed in an
atmosphere of its own, independent of the external air, and the vessel so
enclosing it would only require to be a little larger than the cylinder, and to
have a close cover at the top, the centre of which might be perforated with a
hole to admit the rod of the piston to pass through, the rod being made
smooth, and so fitted to the perforation that no steam should escape
between them. This method would be attended also with the advantage of
keeping the cylinder and piston always heated, not only inside but outside;
and Watt saw that it would be further advantageous to employ the pressure
of steam to drive the piston in its descent instead of the atmosphere, as its
intensity or force would be much more manageable; for, by increasing or
diminishing the heat of the steam in which the cylinder was enclosed, its
pressure might be regulated at pleasure, and it might be made to urge the

piston with any force that might be required. The power of the engine
would therefore be completely under control, and independent of all
variations in the pressure of the atmosphere.

(70.)

This was a step which totally changed the character of the machine, and
which rendered it a STEAM ENGINE instead of an ATMOSPHERIC ENGINE. Not
only was the vacuum below the piston now produced by the property of
steam, in virtue of which it is reconverted into water by cold; but the
pressure which urged the piston into this vacuum was due to the elasticity
of steam.
The external cylinder, within which the working cylinder was enclosed,
was called THE JACKET, and is still very generally used.

Fig. 20.

(71.)

The first experiment in which Watt attempted to [Pg125] realise, on a small
scale, his conceptions, was made in the following manner. The cylinder of
the engine was represented by a brass syringe A B (fig. 20.) an inch and a
third in diameter, and ten inches in length, to which a top and a bottom of
tin plate was fitted. Steam was conveyed by a pipe, S, from a small boiler
into the lower end of this syringe, a communication being made with the
upper end of the syringe by a branch pipe D. For the greater convenience of
the experiment, it was found desirable to invert the position of the cylinder,
so that the steam should press the piston P upwards instead of downwards.
The piston-rod R therefore was presented downwards. An eduction pipe E
was also inserted in the top of the cylinder, which was carried to the
condenser. The piston-rod was made hollow, or rather a hole was drilled
longitudinally through it, and a valve was fitted at its lower end, to carry off
the water produced by the steam, which [Pg126] would be condensed in the
cylinder in the commencement of the process. The condenser used in this
experiment operated without injection, the steam being condensed by the
contact of cold surfaces. It consisted of two thin pipes F, G of tin, ten or
twelve inches in length, and the sixth of an inch in diameter, standing
beside each other perpendicularly, and communicating at the top with the
eduction pipe, which was provided with a valve opening upwards. At the
bottom these two pipes communicated with another tube I of about an inch
in diameter, by a horizontal pipe, having in it a valve, M, opening towards I,
fitted with a piston K, which served the office of the air-pump, being
worked by the hand. This piston, K, had valves in it opening upwards. These
condensing pipes and air-pump were immersed in a small cistern, filled
with cold water. The steam was conveyed by the steam-pipe S to the bottom
of the cylinder, a communication between the top and bottom of the
cylinder being occasionally opened by a cock, C, placed in the branch pipe.
The eduction pipe leading to the condenser also had a cock, L, by which the
communication between the top of the cylinder and the condenser might be

opened and closed at pleasure. In the commencement of the operation, the
cock N admitting steam from the boiler, and the cock L opening a
communication between the cylinder and the condenser, and the cock C
opening a communication between the top and bottom of the cylinder, being
all open, steam rushed from the boiler, passing through all the pipes, and
filling the cylinder. A current of mixed air and steam was thus produced
through the eduction pipe E, through the condensing pipes F and G, and
through the air-pump I, which issued from the valve H in the eduction pipe,
and from the valve in the air-pump piston, all of which opened upwards.
The steam also in the cylinder passed through the hole drilled in the piston-
rod, and escaped, mixed with air, through the valve in the lower end of that
rod. This process was continued until all the air in the cylinder, pipes, and
condenser, was blown out, and all these spaces filled with pure steam. The
cocks L, C, and N, were then closed, and the atmospheric pressure closed the
valve H and the valves in the air-pump piston. The cold surfaces condensing
the steam in [Pg127] the pipes F and G, and in the lower part of the air-pump, a
vacuum was produced in these spaces. The cock C being now closed, and
the cocks L and N being open, the steam in the upper part of the cylinder
rushed through the pipe E into the condenser, where it was reduced to
water, so that a vacuum was left in the upper part of the cylinder. The steam
from the boiler passing below the piston, pressed it upwards with such
force, that it lifted a weight of eighteen pounds hung from the end of the
piston-rod. When the piston reached the top of the cylinder, the cocks L and
N were closed, and the cock C opened. All communication between the
cylinder and the boiler, as well as between the cylinder and the condenser,
were now cut off, and the steam in the cylinder circulated freely above and
below the piston, by means of the open tube D. The piston, being subject to
equal forces upwards and downwards, would therefore descend by its own
weight, and would reach the bottom of the cylinder. The air-pump piston
meanwhile being drawn up, the air and the condensed steam in the tubes F
and G were drawn into the air-pump I, through the open horizontal tube at
the bottom. Its return was stopped by the valve M. By another stroke of the
air-pump, this water and air were drawn out through valves in the piston,
which opened upwards. The cock C was now closed, and the cocks L and N

opened, preparatory to another stroke of the piston. The steam in the upper
part of the cylinder rushed, as before, into the tubes F and G, and was
condensed by their cold surfaces, while steam from the boiler coming
through the pipe S, pressed the piston upwards. The piston again ascended
with the same force as before, and in the same manner the process was
continually repeated.

(72.)

The quantity of steam expended in this experimental model in the
production of a given number of strokes of the piston was inferred from the
quantity of water evaporated in the boiler; and on comparing this with the
magnitude of the cylinder and the weight raised by the pressure of the
steam, the contrivance was proved to affect the economy of steam, as far as
the imperfect conditions of such a model could have permitted. A larger
model was next constructed, having an outer cylinder, or steam case,
surrounding the working cylinder, and [Pg128] the experiments made with it
fully realised Watt's expectations, and left no doubt of the great advantages
which would attend his invention. The weights raised by the piston proved
that the vacuum in the cylinder produced by the condensation was almost
perfect; and he found that when he used water in the boiler which by long
boiling had been well cleared of air, the weight raised was not much less
than the whole amount of the pressure of the steam upon the piston. In this
larger model, the cylinder was placed in the usual position, with a working
lever and other apparatus similar to that employed in the Atmospheric
Engine.

(73.)

It was in the beginning of the year 1765, Watt being then in the twenty-
ninth year of his age, that he arrived at these great discoveries. The
experimental models just described, by which his invention was first
reduced to a rude practical test, were fitted up at a place called Delft House,
in Glasgow. It will doubtless, at the first view, be a matter of surprise that
improvements of such obvious importance in the economy of steam power,
and capable of being verified by tests so simple, were not immediately

adopted wherever atmospheric engines were used. At the time, however,
referred to, Watt was an obscure artisan, in a provincial town, not then
arrived at the celebrity to which it has since attained, and the facilities by
which inventions and improvements became public were much less than
they have since become. It should also be considered that all great and
sudden advances in the useful arts are necessarily opposed by the existing
interests with which their effects are in conflict. From these causes of
opposition, accompanied with the usual influence of prejudice and envy,
Watt was not exempt, and was not therefore likely suddenly to revolutionise
the arts and manufactures of the country by displacing the moving powers
employed in them, and substituting an engine, the efficacy and power of
which depended mainly on physical principles, then altogether new and but
imperfectly understood.
Not having the command of capital, and finding it impracticable to
inspire those who had, with the same confidence in the advantages of his
invention which he himself felt, he was [Pg129] unable to take any step towards
the construction of engines on a large scale. Soon after this, he gave up his
shop in Glasgow, and devoted himself to the business of a Civil Engineer.
In this capacity he was engaged to make a survey of the river Clyde, and
furnished an elaborate and valuable Report upon its projected
improvements. He was also engaged in making a plan of the canal, by
which the produce of the Monkland Colliery was intended to be carried to
Glasgow, and in superintending the execution of that work. Besides these,
several other engineering enterprises occupied his attention, among which
may be mentioned, the navigable canal across the isthmus of Crinan,
afterwards completed by Rennie; improvements proposed in the ports of
Ayr, Glasgow, and Greenock; the construction of the bridges at Hamilton,
and at Rutherglen; and the survey of the country through which the
celebrated Caledonian canal was intended to be carried.
"If, forgetful of my duties as the organ of this academy," says M. Arago
(whose eloquent observations on the delays of this great invention,
addressed to the assembled members of the National Institute of France, we
cannot forbear to quote), "I could think of making you smile, rather than
expressing useful truths, I would find here matter for a ludicrous contrast. I

would call to your recollection the authors, who at our weekly sittings
demand with all their might and main (à cor et à cris) an opportunity to
communicate some little remark—some small reflection—some trifling
note, conceived and written the night before; I would represent them to you
cursing their fate, when according to your rules, the reading of their
communication is postponed to the next meeting, although during this cruel
week, they are assured that their important communication is deposited in
our archives in a sealed packet. On the other hand, I would point out to you
the creator of a machine, destined to form an epoch in the annals of the
world, undergoing patiently and without murmur, the stupid contempt of
capitalists,—conscious of his exalted genius, yet stooping for eight years to
the common labour of laying down plans, taking levels, and all the tedious
calculations connected with the routine of common engineering. While in
this conduct you cannot fail to recognise the serenity, [Pg130] the moderation,
and the true modesty of his character, yet such indifference, however noble
may have been its causes, has something in it not altogether blameless. It is
not without reason that society visits with severe reprobation those who
withdraw gold from circulation and hoard it in their coffers. Is he less
culpable who deprives his country, his fellow citizens, his age, of treasures
a thousand times more precious than the produce of the mine; who keeps to
himself his immortal inventions, sources of the most noble and purest
enjoyment of the mind, who abstains from conferring upon labour those
powers, by which would be multiplied in an infinite proportion the products
of industry, and by which, with advantage to civilisation and human nature,
he would smooth away the inequalities of the conditions of man."[19]

(74.)

Although Watt was thus attracted by pursuits foreign to his recent
investigations respecting the improvement of steam power, he never lost
sight of that object. It was not until the year 1768, three years after his great
discoveries, that any step was taken to enable him to carry them into effect
on a large scale. At that time his friends brought him into communication
with Dr. Roebuck, the proprietor of the Carron Iron Works, who rented
extensive coal works at Kinneal from the Duchess of Hamilton. Watt was

first employed by Roebuck as a civil engineer; but when he made known to
him the improvements he had projected in the steam engine, Roebuck
proposed to take out a patent for an engine on the principle of the model
which had been fitted up at Delft House, and to join Watt in a partnership,
for the construction of such engines. Sensible of the advantages to be
derived from the influence of Roebuck, and from his command of capital,
Watt agreed to cede to him two thirds of the advantages to be derived from
the invention. A patent was accordingly taken out on the fifth of January,
1769, nearly four years after the invention had been completed; and an
experimental engine on a large scale was constructed by him, and fitted up
at Kinneal House. In the first trial this machine more than fulfilled Watt's
anticipations. Its [Pg131] success was complete. In the practical details of its
construction, however, some difficulties were still encountered, the greatest
of which consisted in packing the piston, so as to be steam-tight. The
principle of the new engine did not admit of water being kept upon the
piston, to prevent leakage, as in the old engines; he was therefore obliged to
have his cylinders much more accurately bored, and more truly cylindrical,
and to try a great variety of soft substances for packing the piston, which
would make it steam-tight without great friction, and maintain it so in a
situation perfectly dry, and at the temperature of boiling water.
While Watt was endeavouring to overcome these and other difficulties, in
the construction of the machine, his partner, Dr. Roebuck, became
embarrassed, by the failure of his undertaking in the Borrowstowness coal
and salt works; and he was unable to supply the means of prosecuting with
the necessary vigour the projected manufacture of the new engines.
The important results of Watt's labours having happily at this time
become more publicly known, Mr. Matthew Boulton, whose establishment
at Soho, near Birmingham, was at that time the most complete manufactory
for metal-work in England, and conducted with unexampled enterprise and
spirit, proposed to purchase Dr. Roebuck's interest in the patent. This
arrangement was effected in the year 1773, and in the following year Mr.
Watt removed to Soho, where a portion of the establishment was allotted to
him, for the erection of a foundery, and other works necessary to realise his
inventions on a grand scale.

The patent which had been granted in 1769 was limited to a period of
fourteen years, and would consequently expire about the year 1783. From
the small progress which had hitherto been made in the construction of
engines upon the new principle, and from the many difficulties still to be
encountered, and the large expenditure of capital which must obviously be
incurred before any return could be obtained, it was apparent that unless an
extension of the patent right could be obtained, Boulton and Watt could
never expect any advantage adequate to the risk of their great [Pg132]
enterprise. In the year 1774 an application was accordingly made to
parliament for an extension of the patent, which was supported by the
testimony of Dr. Roebuck, Mr. Boulton, and others, as to the merits and
probable utility of the invention. An Act was accordingly passed, in 1775,
extending the term of the patent until the year 1800.

(75.)

The following abstract of this Act may not be uninteresting at this time,
when the anticipations expressed in it have been so successfully and
extensively realised:—
"An Act for vesting in James Watt, engineer, his executors,
administrators, and assigns, the sole use and property of certain steam
engines, commonly called fire engines, of his invention, throughout his
majesty's dominions, for a limited time:
"And whereas the said James Watt hath employed many years, and a
considerable part of his fortune, in making experiments upon steam
engines, commonly called fire engines, with a view to improve those very
useful machines, by which several very considerable advantages over the
common steam engines are acquired; but upon account of the many
difficulties which always arise in the execution of such large and complex
machines, and of the long time requisite to make the necessary trials, he
could not complete his intention before the end of the year 1774, when he
finished some large engines as specimens of his construction, which have
succeeded, so as to demonstrate the utility of the said invention:

"And whereas, in order to manufacture these engines with the necessary
accuracy, and so that they may be sold at moderate prices, a considerable
sum of money must be previously expended in erecting mills and other
apparatus; and as several years and repeated proofs will be required before
any considerable part of the public can be fully convinced of the utility of
the invention, and of their interest to adopt the same, the whole term
granted by the said letters patent may probably elapse before the said James
Watt can receive an advantage adequate to his labour and invention:
"And whereas, by furnishing mechanical power at much less expense,
and in more convenient forms, than has hitherto been done, his engines may
be of great utility, in facilitating [Pg133] the operations in many great works and
manufactures of this kingdom; yet it will not be in the power of the said
James Watt to carry his invention into that complete execution which he
wishes, and so as to render the same of the highest utility to the public of
which it is capable, unless the term granted by the said letters patent be
prolonged, and his property in the said invention secured for such time as
may enable him to obtain an adequate recompense for his labour, time, and
expense:
"To the end, therefore, that the said James Watt may be enabled and
encouraged to prosecute and complete his said invention, so that the public
may reap all the advantages to be derived therefrom in their fullest extent: it
is enacted,
"That from and after the passing of this Act, the sole privilege and
advantage of making, constructing, and selling the said engines
hereinbefore particularly described, within the kingdom of Great Britain,
and his majesty's colonies and plantations abroad, shall be, and are hereby
declared to be, vested in the said James Watt, his executors, administrators,
and assigns, for and during the term of twenty-five years," &c. &c.

(76.)

Thus protected and supported, Watt now directed the whole vigour of his
mind to perfect the practical details of his invention, and the result was, the

construction on a large scale of the engine which has since been called his
Single acting Steam Engine.
It is necessary to recollect, that notwithstanding the extensive and various
application of steam power in the arts and manufactures, at the time to
which our narrative has now reached, the steam engine had never been
employed for any other purpose save that of raising water by working
pumps. The motion, therefore, which was required was merely an upward
force, such as was necessary to elevate the piston of a pump, loaded with
the column of water which it raised. The following then is a description of
the improved engine of Watt, by which such work was proposed to be
performed:—

Fig. 21.

In the cylinder represented at C (fig. 21.), the piston P moves steam-tight.
It is closed at the top, and the piston [Pg134] rod, being accurately turned, runs
in a steam-tight collar, B, furnished with a stuffing-box, and is constantly
lubricated with melted tallow. A funnel is screwed into the top of the
cylinder, through which, by opening a stop-cock, melted [Pg135] tallow is
permitted from time to time to fall upon the piston within the cylinder, so as
to lubricate it, and keep it steam-tight. Two boxes, A A, called the upper and
lower steam boxes, contain valves by which steam from the boiler may be
admitted and withdrawn. These steam boxes are connected by a tube of
communication T, and they communicate with the cylinder at the top and
bottom by short tubes represented in the figure. The upper steam box A
contains one valve, by which a communication with the boiler may be
opened or closed at pleasure. The lower valve box contains two valves. The
lower valve I communicates with the tube T′, leading to the condenser D,
which being opened or closed, a communication is made or cut off at
pleasure, between the cylinder C and the condenser D. A second valve, or
upper valve H, which is represented closed in the figure, may be opened so
as to make a free communication between the cylinder C and the tube T, and
by that means between the cylinder C, below the piston and the space above
the piston. The condenser D is submerged in a cistern of cold water. At the
side there enters it a tube, E, governed by a cock, which being opened or
closed to any required extent, a jet of cold water may be allowed to play in
the condenser, and may be regulated or stopped, at pleasure. This jet, when
playing, throws the water upwards in the condenser towards the mouth of
the tube T′, as water issues from the rose of a watering pot. The tube S
proceeds from the boiler, and terminates in the steam box A, so that the
steam supplied from the boiler constantly fills that box. The valve G is
governed by levers, whose pivots are attached to the framing of the engine,
and is opened or closed at pleasure, by raising or lowering the lever G′. The
valve G, when open, will therefore allow steam to pass from the boiler
through the short tube to the top of the piston, and this steam will also fill
the tube T. If the lower valve H be closed, its circulation beyond that point
will be stopped; but if the valve H be open, the valve I being closed, then the
steam will circulate equally in the cylinder, above and below the piston. If

the valve I be open, then steam will rush through the tube T′ into the
condenser; but this escape of the steam will be [Pg136] stopped, if the valve I be
closed. The valve H is worked by the lever H′, and the valve I by the lever I′.
The valve G is called the upper steam valve, H the lower steam valve, I
the exhausting valve, and E the condensing valve.
From the bottom of the condenser D proceeds a tube leading to the air-
pump, which is also submerged in the cistern of cold water. In this tube is a
valve M, which opens outwards from the condenser towards the air-pump.
In the piston of the air-pump N is a valve which opens upwards. The piston-
rod Q of the air-pump is attached to a beam of wood called a plug frame,
which is connected with the working beam by a flexible chain playing on
the small arch-head immediately over the air-pump. From the top of the air-
pump barrel above the piston proceeds a pipe or passage leading to a small
cistern, B, called the hot well. The pipe which leads to this well, is supplied
with a valve, K, which opens outwards from the air pump barrel towards the
well. From the nature of its construction, the valve M admits the flow of
water from the condenser towards the air-pump, but prevents its return; and,
in like manner, the valve K admits the flow of water from the upper part of
the air-pump barrel into the hot well B, but obstructs its return.
Let us now consider how these valves should be worked in order to move
the piston upwards and downwards with the necessary force. It is in the first
place necessary that all the air which fills the cylinder, the tubes and the
condenser shall be expelled. To accomplish this it is only necessary to open
at once the three valves G, H, and I. The steam then rushing from the boiler
through the steam-pipe S, and the open valve G will pass into the cylinder
above the piston, will fill the tube T, pass through the lower steam valve H,
will fill the cylinder C below the piston, and will pass through the open
valve I into the condenser. If the valve E be closed so that no jet shall play in
the condenser, the steam rushing into it will be partially condensed by the
cold surfaces to which it will be exposed; but if the boiler supply it through
the pipe S in sufficient abundance, it will rush with violence through the
cylinder and all the passages, and its pressure in the [Pg137] condenser D,

combined with that of the heated air with which it is mixed, will open the
valve M, and it will rush through mixed with the air into the air-pump barrel
N. It will press the valves in the air-pump piston upwards, and, opening
them, will rush through, and will collect in the air-pump barrel above the
piston. It will then, by its pressure, open the valve K, and will escape into
the cistern B.
Throughout this process the steam, which mixed with the air fills the
cylinder, condenser and air-pumps will be only partially condensed in the
last two, and it will escape mixed with air through the valve K, and this
process will continue until all the atmospheric air which at first filled the
cylinder, tubes, condenser and air-pump barrel shall be expelled through the
valve K, and these various spaces shall be filled with pure steam. When that
has happened let us suppose all the valves closed. In closing the valve I the
flow of steam to the condenser will be stopped, and the steam contained in
it will speedily be condensed by the cold surface of the condenser, so that a
vacuum will be produced in the condenser, the condensed steam falling in
the form of water to the bottom. In like manner, and for like reasons, a
vacuum will be produced in the air-pump. The valve M, and the valves in
the air-pump piston will be closed by their own weight.
By this process, which is called blowing through, the atmospheric air, and
other permanent gases, which filled the cylinder, tubes, condenser and air-
pump are expelled, and these spaces will be a vacuum. The engine is then
prepared to be started, which is effected in the following manner:—The
upper steam valve G is opened, and steam allowed to flow from the boiler
through the passage leading to the top of the cylinder. This steam cannot
pass to the bottom of the cylinder, since the lower steam valve H is closed.
The space in the cylinder below the piston being therefore a vacuum, and
the steam pressing above it the piston will be pressed downwards with a
corresponding force. When it has arrived at the bottom of the cylinder the
steam valve G must be closed, and at the same time the valve H opened. The
valve I leading to the condenser being also closed, the steam [Pg138] which fills
the cylinder above the piston is now admitted to circulate through the open
valve H below the piston, so that the piston is pressed equally upwards and

downwards by steam, and there is no force to resist its movement save its
friction with the cylinder. The weight of the pump rods on the opposite end
of the beam being more than equivalent to overcome this the piston is
drawn to the top of the cylinder, and pushes before it the steam which is
drawn through the tube T, and the open valve H, and passes into the cylinder
C below the piston.

When the piston has thus arrived once more at the top of the cylinder, let
the valve H be closed, and at the same time the valves G and I opened, and
the condensing cock E also opened, so as to admit the jet to play in the
condenser. The steam which fills the cylinder C below the piston, will now
rush through the open valve I into the condenser which has been hitherto a
vacuum, and there encountering the jet, will be instantly converted into
water, and a mixture of condensed steam and injected water will collect in
the bottom of the condenser. At the same time, the steam proceeding from
the boiler by the steam pipe S to the upper steam box A, will pass through
the open steam valve G to the top of the piston, but cannot pass below it
because of the lower steam valve H being closed. The piston, thus acted
upon above by the pressure of the steam, and the space in the cylinder
below it being a vacuum, its downward motion is resisted by no force but
the friction, and it is therefore driven to the bottom of the cylinder. During
its descent the valves G, I, and E remained open. At the moment it arrives at
the bottom of the cylinder, all these three valves are closed, and the valve H
opened. The steam which fills the cylinder above the piston is now
permitted to circulate below it, by the open valve H, and the piston being
consequently pressed equally upwards and downwards will be drawn
upwards as before by the preponderance of the pump rods at the opposite
end of the beam. The weight of these rods must also be sufficiently great to
draw the air-pump piston N upwards. As this piston rises in the air-pump, it
leaves a vacuum below it into which the water and air collected in the
condenser will be drawn through the valve M, which opens outwards. When
the [Pg139] air-pump piston has arrived at the top of the barrel, which it will do
at the same time that the steam piston arrives at the top of the cylinder, the
water and the chief part of the air or other fluids which may have been in
the condenser will be drawn into the barrel of the air-pump, and the valve M

being closed by its own weight, assisted by the pressure of these fluids they
cannot return into the condenser. At the moment the steam piston arrives at
the top of the cylinder, the valve H is closed, and the three valves G, I, and E
are opened. The effect of this change is the same as was already described
in the former case, and the piston will in the same manner and from the
same causes be driven downwards. The air-pump piston will at the same
time descend by the force of its own weight, aided by the weight of the
plug-frame attached to its rod. As it descends, the air below it will be
gradually compressed above the surface of the water in the bottom of the
barrel, until its pressure becomes sufficiently great to open the valves in the
air-pump piston. When this happens, the valves in the air-pump piston, as
represented on a large scale in fig. 22., will be opened, and the air will pass
through them above the piston. When the piston comes in contact with the
water in the bottom of the barrel, this water will likewise pass through the
open valves. When the piston has arrived at the bottom of the air-pump
barrel, the valves in it will be closed by the pressure of the fluids above
them. The next ascent of the steam piston will draw up the air-pump piston,
and with it the fluids in the pump barrel above it. As the air-pump [Pg140]
piston approaches the top of its barrel, the air and water above it will be
drawn through the valve K into the hot cistern B. The air will escape in
bubbles through the water in that cistern, and the warm water will be
deposited in it.
The magnitude of the opening in the condensing valve E, must be
regulated by the quantity of steam admitted to the cylinder. As much water
ought to be supplied through the injection valve as will be sufficient to
condense the steam contained in the cylinder, and also to reduce the
temperature of the water itself, when mixed with the steam, to a sufficiently
low degree to prevent it from producing vapour of a pressure which would
injuriously affect the working of the piston. It has been shown, that five and
a half cubic inches of ice-cold water mixed with one cubic inch of water in
the state of steam would produce six and a half cubic inches of water at the
boiling temperature. If then the cylinder contained one cubic inch of water
in the state of steam, and only five and a half cubic inches of water were
admitted through the condensing jet, supposing this water, when admitted,
to be at the temperature of 32°, then the consequence would be that six and

a half cubic inches of water at
the boiling temperature would be
produced in the condenser.
Steam would immediately arise
from this, and at the same time
the temperature of the remaining
water would be lowered by the
amount of the latent heat taken
up by the steam so produced.
This vapour would rise through
the open exhausting valve I,
would fill the cylinder below the
piston, and would impair the
efficiency of the steam above
pressing it down. The result of
the inquiries of Watt respecting
the pressure of steam at different Fig. 22.
temperatures, showed, that to
give efficiency to the steam acting upon the piston it would always be
necessary to reduce the temperature of the water in the condenser to 100°.
Let us then see what quantity of water at the common temperature would
be necessary to produce these effects.
If the latent heat of steam be taken at 1000°, a cubic inch of water in the
state of steam may be considered for the purposes of this computation, as
equivalent to one cubic inch of water at 1212°. Now the question is, how
many cubic inches of water at 60° must be mixed with this, in order that the
[Pg141] mixture may have the temperature of 100°? This will be easily

computed. As the cubic inch of water at 1212° is to be reduced to 100°, it
must be deprived of 1112° of its temperature. On the other hand, as many
inches of water at 60° as are to be added, must be raised in the same
mixture to the temperature of 100°, and therefore each of these must receive
40° of temperature. The number of cubic inches of water necessary to be
added will therefore be determined by finding how often 40° are contained
in 1112°. If 1112 be divided by 40, the quotient will be 27·8. Hence it

appears, that to reduce the water in the condenser to the temperature of
100°, supposing the temperature of the water injected to be 60°, it will be
necessary to supply by the injection cock very nearly twenty-eight times as
much water as passes through the cylinder in the state of steam; and
therefore if it be supposed that all the water evaporated in the boiler passes
through the cylinder, it follows that about twenty-eight times as much water
must be thrown into the condenser as is evaporated in the boiler.
From these circumstances it will be evident that the cold cistern in which
the condenser and air-pump are submerged, must be supplied with a
considerable quantity of water. Independently of the quantity drawn from it
by the injection valve, as just explained, the water in the cistern itself must
be kept down to a temperature of about 60°. The interior of the condenser
and air-pump being maintained by the steam condensed in them at a
temperature not less than 100°; the outer surfaces of these vessels
consequently impart heat to the water in the cold cistern, and have therefore
a tendency to raise the temperature of that water. To prevent this, a pump
called the cold pump, represented at L in fig. 21., is provided. By this pump
water is raised from any convenient reservoir, and driven through proper
tubes into the cold cistern. This cold pump is wrought by the engine, the rod
being attached to the beam. Water being, bulk for bulk, heavier the lower its
temperature, it follows that the water supplied by the cold pump to the
cistern will have a tendency to sink to the bottom, pressing upwards the
warmer water contained in it. A waste-pipe is provided, by which this [Pg142]
water is drained off, and the cistern therefore maintained at the necessary
temperature.
From what has been stated, it is also evident that the hot well B, into
which the warm water is thrown by the air-pump, will receive considerably
more water than is necessary to feed the boiler. A waste-pipe, to carry off
this, is also provided; and the quantity necessary to feed the boiler is
pumped up by a small pump, O, the rod of which is attached to the beam, as
represented in fig. 21., and which is worked by the engine. The water raised
by this pump is conducted to a reservoir from which the boiler is fed, by
means which will be hereafter explained.

We shall now explain the manner in which the machine is made to open
and close the valves at the proper times. By referring to the explanation
already given, it will be perceived that at the moment the piston reaches the
top of the cylinder, the upper steam valve G must be open, to admit the
steam to press it down; while the exhausting valve I must be opened, to
allow the steam to pass to the condenser; and the condensing valve E must
be opened, to let in the water necessary for the condensation of the steam;
and at the same time the lower steam valve H must be closed, to prevent the
passage of the steam which has been admitted through G. The valves G, I,
and E must be kept open, and the valve H kept closed, until the piston
arrives at the bottom of the cylinder, when it will be necessary to close all
the three valves, G, I, and E, and to open the valve H, and the same effects
must be produced each time the piston arrives at the top and bottom of the
cylinder. All this is accomplished by a system of levers, which are exhibited
in fig. 21. The pivots on which these levers play are represented on the
framing of the engine, and the arms of the levers G′, H′, and I′,
communicating with the corresponding valves G, H, and I, are represented
opposite a bar attached to the rod of the air-pump, called the plug frame.
This bar carries certain pegs and detents, which act upon the arms of the
several levers in such a manner that, on the arrival of the beam at the
extremities of its play upwards and downwards, the levers are so struck that
the valves are opened and closed at the proper [Pg143] times. It is needless to
explain all the details of this arrangement. Let it be sufficient, as an
example of all, to explain the method of working the upper steam valve G.
When the piston reaches the top of the cylinder, a pin strikes the arm of the
lever G′, and throws it upwards: this, by means of the system of levers, pulls
the arm of the valve G downwards, by which the upper steam valve is raised
out of its seat, and a passage is opened from the steam pipe to the cylinder.
The valve is maintained in this state until the piston reaches the bottom of
the cylinder, when the arm G′ is pressed downwards, by which the arm G is
pressed upwards, and the valve restored to its seat. By similar methods the
levers governing the other three valves, H, I, and E, are worked.
The valves used in these engines were of the kind called spindle valves.
They consisted of a flat circular plate of bell metal, A B, fig. 23., with a

round spindle passing perpendicularly through its centre, and
projecting above and below it. This valve, having a conical
form, was fitted very exactly, by grinding into a
corresponding circular conical seat, A B C D, fig. 24., which
forms the passage which it is the office of the valve to open
and close. When the valve falls into its seat, it fits the aperture
like a plug, so as entirely to stop it. The spindle plays in
Fig. 23. sockets or holes, one above and the other below the aperture
which the valve stops; these holes keep the valve in its proper
position, so as to cause it to drop exactly
into its place.
In the experimental engine made by Mr.
Watt at Kinneal, he used cocks, and
sometimes sliding covers, like the regulator
described in the old engines; but these he
found very soon to become leaky. He was,
therefore, obliged to change them for the
spindle valves just described, which, being
Fig. 24. truly [Pg144] ground, and accurately fitted in
the first instance, were not so liable to go
out of order. These valves are also called puppet clacks, or button valves.
In the earlier engines constructed by Watt, the condensation was
produced by the contact of cold surfaces, without injection. The reason of
rejecting the method of condensing by injection was, doubtless, to avoid the
injurious effects of the air, which would always enter the condenser, in
combination with the water of condensation, and vitiate the vacuum. It was
soon found, however, that a condenser acting by cold surfaces without
injection, being necessarily composed of narrow pipes or passages, was
liable to incrustation from bad water, by which the conducting power of the
material of the condenser was diminished; so that, while its outer surface
was kept cold by the water of the cold cistern, the inner surface might,
nevertheless, be so warm that a very imperfect condensation would be
produced.

SOHO, BIRMINGHAM.

FOOTNOTES:
[19] Eloge, p. 308.

BIRMINGHAM.

CHAP. VI.
[Pg145]

TOC INX

CORRESPONDENCE OF WATT WITH SMEATON.—FAILURE OF CONDENSATION BY
SURFACE.—IMPROVEMENTS IN CONSTRUCTION OF PISTON.—METHOD OF
PACKING.—IMPROVEMENTS IN BORING THE CYLINDERS.—DISADVANTAGES
OF THE NEW COMPARED WITH THE OLD ENGINES.—GREATLY INCREASED
ECONOMY OF FUEL.—EXPEDIENTS TO FORCE THE NEW ENGINES INTO USE.—
CORRESPONDENCE WITH SMEATON.—EFFICIENCY OF FUEL IN THE NEW
ENGINES.—DISCOVERY OF THE EXPANSIVE ACTION OF STEAM.—WATT STATES
IT IN A LETTER TO DR. SMALL.—ITS PRINCIPLE EXPLAINED.—MECHANICAL
EFFECT RESULTING FROM IT.—COMPUTED EFFECT OF CUTTING OFF STEAM AT
DIFFERENT PORTIONS OF THE STROKE.—PRODUCES A VARIABLE POWER.—
EXPEDIENTS FOR EQUALISING THE POWER.—LIMITATION OF THE EXPANSIVE
PRINCIPLE IN WATT'S ENGINES.—ITS MORE EXTENSIVE APPLICATION IN THE
CORNISH ENGINES.

(77.)

In a letter addressed by Watt to Smeaton, dated April, 1766, Watt refers to
some of these practical difficulties which he had to encounter. "I have
been," says he, "tormented with exceedingly bad health, resulting from the
operation of an anxious mind, the natural consequence of staking
everything [Pg146] upon the cast of a die; for in that light I look upon every
project which has not received the sanction of repeated success.
"I have made considerable alterations in our engine lately, particularly in
the condenser. That which I used at first was liable to be impaired, from
incrustations from bad water; therefore we have substituted one which
works by an injection. In pursuing this idea I have tried several kinds, and
have at last come to one, which I am not inclined to alter. It consists of a
jack-head pump, shut at bottom, with a common clack bucket, and a valve
in the cover of the pump, to discharge the air and water. The eduction steam
pipe, which comes from the cylinder, communicates with this pump both
above and below the bucket, and has valves to prevent anything from going
back from the pump to the eduction pipe. The bucket descends by its own

weight, and is raised by the engine when the great piston descends, being
hung to the outer end of the great lever: the injection is made both into the
upper part of this pump and into the eduction pipe, and operates beyond my
ideas in point of quickness and perfection."
Besides the difficulty arising from incrustation, Watt found the tubulated
condensers, and indeed all other expedients for condensing by cold
surfaces, subject to a fatal objection. They did not condense
instantaneously, and although they were capable of ultimately effecting the
condensation, yet that process was not completed until a great part of the
stroke of the piston was made. Thus during more or less of the stroke the
uncondensed steam resisted the piston, and robbed the moving power of a
part of its effect. This objection has ever attended condensation by surface.

(78.)

Another source of difficulty arose from the
necessity of constructing the piston and cylinder
with greater precision than had been usual in the old
engines. To fit the cover to the cylinder so as to be
steam-tight; to construct the piston rod so as to
move through it without allowing the escape of
steam, and yet at the same time without injurious
friction; to connect the piston rod with the piston,
so as to drive the [Pg147] latter through the cylinder
with a perfectly straight and parallel motion; to
make such connection perfectly centrical and firm,
and yet to allow the piston in its ascent to come Fig. 25.
nearly into contact with the cover of the cylinder—
were all difficulties peculiar to the new engine. In the atmospheric engine
the shank of the piston rod was rough and square, and the rod was secured
to the piston by two or four branches or stays, as represented in fig. 25. It is
evident that such a construction would be inadmissible in an engine in
which the piston in its ascent must be brought nearly into contact with the
close cover of the cylinder. Besides this the piston rod of an atmospheric
engine might throughout its whole length have any form which was most

convenient, and required no other property than
the strength necessary to work the beam. In the
new engine, on the contrary, it was necessary
that it should be accurately turned and finely
polished, so as to pass through the hole in the
top of the cylinder, and be maintained in it
steam-tight. This was effected by a contrivance
called a stuffing-box B, represented in fig. 26. A
hole is made in the cover of the cylinder very Fig. 26.
little greater in magnitude than the diameter of
the piston rod. Above this hole is a cup in
which, around the piston, is placed a stuffing of hemp or tow, which is
saturated with oil or melted tallow. This collar of hemp is pressed down by
another piece, also perforated with a hole through which the piston rod
plays, and which is screwed down on the said collar of hemp.

(79.)

Although the imperfect manner in which the interior of the cylinders was
then formed impaired the efficiency of the [Pg148] new engines, yet such
imperfections were not so injurious as in the old atmospheric engines. Any
imperfection of form of the inner surface of the cylinder would necessarily
cause more or less steam or air to escape between the piston and cylinder. In
the improved engine this steam passing into the vacuum below the piston
would rush into the condenser, and be there condensed, so that its effect in
resisting the motion of the piston would necessarily be trifling. But on the
other hand, any escape of air between the piston and cylinder of an
atmospheric engine would introduce an elastic fluid under the piston, which
would injuriously affect the action of the machine.
To make the pistons move sufficiently steam-tight in these early
imperfect cylinders, Watt contrived a packing formed of a collar of hemp,
or tow, as represented in fig. 27. The bottom of the piston was formed of a
circular plate of a diameter nearly, but not altogether equal to the interior
diameter of the cylinder. The part of the piston above this was considerably
less in diameter, so that the piston was surrounded by a circular groove or

channel two inches wide, into which
hemp or soft rope, called gasket, was
run, so as to form the packing. The top
of the piston was placed over this,
having a rim or projecting part, which
entered the circular groove and pressed
upon the packing, the cover being
Fig. 27. pressed downwards by screws passing
through the piston. The lower part of the
groove round the piston was rounded with a curve, so that the pressure on
the packing might force the latter against the inner surface of the cylinder.
This packing was kept supplied with melted tallow, as already described,
from the funnel, screwed into the top of the cylinder. The metallic edges of
the piston were by this means prevented from coming into contact with the
surface of the cylinder, which was only pressed upon by the stuffing or
packing projecting beyond these.

(80.)

Improved methods of boring soon, however, relieved [Pg149] the engine from a
part of these imperfections, and Watt writes to Mr. Smeaton in the letter
above quoted as follows:—
"Mr. Wilkinson has improved the art of boring cylinders; so that I
promise, upon a 72 inch cylinder, being not further distant from absolute
truth than the thickness of a thin sixpence in the worst part. I am labouring
to improve the regulators; my scheme is to make them acute conical valves,
shut by a weight, and opened by the force of the steam. They bid fair for
success, and will be tried in a few days."
The person here alluded to was Mr. John Wilkinson, of Bersham near
Chester, who, about the year 1775, contrived a new machine for accurately
boring the insides of cylinders. The cylinder being first obtained from the
foundery with a surface as accurate as the process of casting would admit,
had its inner surface reduced to still greater accuracy by this machine,
which consists of a straight central bar extended along the axis of the
cylinder, which was made to revolve slowly round it. During the operation

of boring, the borer or cutter was fitted to slide along this bar, which being
perfectly straight, served as a sort of ruler to guide the borer or cutter in its
progress through the cylinder. In this manner the interior surface of the
cylinder was rendered not only true and straight in its longitudinal direction,
but also perfectly circular in its cross section.
The grease found to be most eligible for lubrication was the tallow of
beef or mutton; but in the earlier cylinders this was soon consumed by
reason of the imperfection of the boring, and the piston being left dry
ceased to be steam-tight. To prevent this, Watt sought for some substance,
which while it would thicken the tallow, and detain it around the piston,
would not be subject to decomposition by heat. Black lead dust was used
for this purpose, but was soon found to wear the cylinder. In the mean
while, however, the improved method of boring supplied cylinders which
rendered this expedient unnecessary.
When the inner surface of the cylinder is perfectly true and smooth, the
packing of the piston is soon rendered solid and hard, being moulded to the
cylinder by working, so as to fit it perfectly. When by wear it became loose,
it was [Pg150] only necessary to tighten the screws by which the top and bottom
of the piston were held together. The packing being compressed by those
means, was forced outwards towards the surface of the cylinder, so as to be
rendered steam-tight.

(81.)

It was not until about the year 1778, nine years after the date of the patent,
and thirteen after the invention of separate condensation, that any
impression was produced on the mining interests by the advantages which
were presented to them by these vast improvements. This long interval,
however, had not elapsed without considerable advantage; for although all
the great leading principles of the contrivance were invented so early as the
year 1765, yet the details of construction had been in a state of progressive
and continued improvement from the time Watt joined Dr. Roebuck, in
1769, to the period now adverted to.

The advantages which the engine offered in the form in which it has been
just described, were numerous and important, as compared even with the
most improved form of the atmospheric engine; and it should be
remembered, that that machine had also gone on progressively improving,
and was probably indebted for some of its ameliorations to hints derived
from the labours of Watt, and to the adoption of such of his expedients as
were applicable to this imperfect machine, and could be adopted without an
infraction of his patent.
In the most improved forms to which the atmospheric engine had then
attained, the quantity of steam wasted at each stroke of the piston was equal
to the contents of the cylinder. Such engines, therefore, consumed twice the
fuel which would be requisite, if all sources of waste could have been
removed. In Watt's engines, the steam consumed at each stroke of the piston
amounted only to 11⁄4 times the contents of the cylinder. The waste steam,
therefore, per stroke, was only a quarter of what was usefully employed.
The absolute waste, therefore, of the best atmospheric engines was four
times that of the improved engine, and consequently the saving of fuel in
the improved engines amounted to about three eighths of all the fuel
consumed in atmospheric engines of the same power. [Pg151]

(82.)

But independently of this saving of steam, which would otherwise be
wasted, the power of Watt's engine, as compared with the atmospheric
engine, was so much augmented that the former would work against a
resistance of ten pounds on the square inch under the same circumstances in
which the latter would not move against more than seven pounds. The cause
of this augmentation of power is easily explained. In the atmospheric
engine the temperature of the condensed steam could not be reduced below
152° without incurring a greater loss than would be compensated by the
advantage to be obtained from any higher degree of condensation. Now
steam raised from water at 152° has a pressure of nearly four pounds per
square inch. This pressure, therefore, acted below the piston resisting the
atmospheric pressure above. In Watt's engine, however, the condenser was
kept at a temperature of about 100°, at which temperature steam has a

pressure of less than one pound per square inch. A resisting force upon the
piston of three pounds per square inch was therefore saved in Watt's engine
as compared with the atmospheric engine.

(83.)

Besides these direct sources of economy, there were other advantages
incidental to Watt's engine. An atmospheric engine possessed very limited
power of adaptation to a varying load. The moving power being the
atmospheric pressure, was not under control, and, on the other hand, was
subject to variations from day to day and from hour to hour, according to
the changes of the barometer. In the first construction of such an engine,
therefore, its power being necessarily adapted to the greatest load which it
would have to move, whenever the load upon its pumps was diminished,
the motion of the piston in descending would be rapidly accelerated in
consequence of the moving power exceeding the resistance. By this the
machinery would be subject to sudden shocks, which were productive of
rapid wear, and exposed the machinery to the danger of fracture. To remedy
this inconvenience, the following expedient was provided in the
atmospheric engine: whenever the load on the engine was materially
diminished, the quantity of water admitted through the injection valve to
condense the steam was proportionally [Pg152] diminished. An imperfect
condensation being therefore produced, vapour remained in the cylinder
under the piston, the pressure of which resisted the atmosphere, and
mitigated the force of the machine. Besides this, a cock was provided in the
bottom of the cylinder, called an air cock, by which atmospheric air could
be admitted to resist the piston whenever the motion was too rapid.
These expedients, however, were all attended with a waste of fuel in
relation to the work done by the engine; for it is evident that the
consumption of steam was necessarily the same, whether the engine was
working against its full load or against a reduced resistance.
On the other hand, in the improved engine of Watt, when the load, to
work against which the engine exerted its full power, was diminished, a
cock or valve was provided in the steam pipe leading from the boiler, which
was called a throttle valve, by adjusting which the passage in that pipe

could be more or less contracted. By regulating this cock the supply of
steam from the boiler was checked, and the quantity transmitted to the
cylinder diminished, so that its effect upon the piston might be rendered
equal to the amount of the diminished resistance. By this means the quantity
of steam transmitted to the cylinder was rendered exactly proportional to
the work which the engine had to perform. If, under such circumstances, the
boiler was worked to its full power, so as to produce steam as fast as it
would when the engine was working at full power, then no saving of fuel
would be effected, since the surplus steam produced in the boiler would
necessarily escape at the safety valve. But in such case the fireman was
directed to limit the fuel of the furnace until the discharge at the safety
valve ceased.
By these expedients, the actual consumption of fuel in one of these
improved engines was always in the exact proportion of the work which it
performed, whether it worked at full power or at any degree under its
regular power.

(84.)

Notwithstanding these and other advantages attending the new engines,
Boulton and Watt experienced difficulties all but insurmountable in getting
them into use. No manufactory existed in the country possessing machinery
capable of [Pg153] executing with the necessary precision the valves and other
parts which required exact execution, and the patentees were compelled to
construct machinery at Soho for this purpose; and even after they succeeded
in getting the cylinders properly bored, the piston rods exactly turned and
polished, the spindle valves constructed so as to be steam-tight, and every
other arrangement completed which was necessary for the efficiency of the
machine, the novelty of the engine, and the difficulty which was supposed
to attend its maintenance in good working order, formed strong objections
to its adoption.
To remove such objections, great sacrifices were necessary on the part of
Boulton and Watt; and they accordingly resolved to undertake the
construction of the new engines without any profit, giving them to the

parties requiring their use at first cost, on the condition of being
remunerated by a small share of what they would save in fuel.
"We have no objection," writes Mr. Boulton, "to contract with the Carron
Company to direct the making of an engine to return the water for their
mills. * * * * We do not aim at profits in engine building, but shall
take our profits out of the saving of fuel; so that if we save nothing, we shall
take nothing. Our terms are as follows: we will make all the necessary
plans, sections, and elevations for the building, and for the engine with its
appurtenances, specifying all cast and forged iron work, and every other
particular relative to the engine. We will give all necessary directions to
your workmen, which they must implicitly obey. We will execute, for a
stipulated price, the valves, and all other parts which may·require exact
execution, at Soho; we will see that all the parts are put together, and set to
work, properly; we will keep our own work in repair for one year, and we
have no other objection to seven years than the inconvenience of the
distance. We will guarantee that the engine so constructed shall raise at least
20,000 cubic feet of water twenty-four feet high with each hundred weight
of coals burnt.
"When all this is done, a fair and candid comparison shall be made
between it, and your own engine, or any other engine in Scotland, from
which comparison the amount of savings in fuel shall be estimated, and that
amount being [Pg154] divided into three parts, we shall be entitled to one of
those parts, in recompense for our patent licence, our drawings, &c. &c.
Our own share of savings shall be estimated in money, according to the
value of your coals delivered under the boiler, and you shall annually pay us
that sum, during twenty-five years from the day you begin to work;
provided you continue the use of the engine so long. And in case you sell
the engine, or remove it to any other place, you must previously give us
notice, for we shall then be entitled to our third of the savings of fuel,
according to the value of coals at such new place. This is a necessary
condition, otherwise the engine which we make for you at an expense of
two thousand pounds may be sold in Cornwall for ten thousand pounds.
"Such parts of the engine as we execute at Soho we will be paid for at a
fair price; I conclude, from all the observations I have had an opportunity of

making, that our engines are four times better than the common engines. In
boilers, which are a very expensive article, the savings will be in proportion
to the savings of coal. If you compare our engine with the common engine
(not in size, but in power), you will find the original expense of erecting
one to be nearly the same.
"Mr. Wilkinson has bored us several cylinders, almost without error; that
of fifty inches diameter, which we put up at Tipton, does not err the
thickness of an old shilling in any part; so that you must either improve
your method of boring, or we must furnish the cylinder to you."
The reluctance of mining companies to relinquish the old engines, even
on these terms, led them to propose to Mr. Watt to grant them a licence for
the use of his condenser, to be applied to the atmospheric engine, without
the introduction of other improvements. Such a proposition was made to
him by Mr. Smeaton, in the year 1778, to which he returned the following
answer:—
"I have several times considered the propriety of the application of my
condensers to common engines, and have made experiments with that view
upon our engine at Soho, but have never found such results as would induce
me to try [Pg155] it any where else; and, in consequence, we refused to make
that application to Wheal Virgin engines in Cornwall, and to some others;
our reasons were, that though it might have enabled them to have gone
deeper with their present engines, yet, the savings of fuel would not have
been great, in comparison to the complete machine. By adding condensers
to engines that were not in good order, our engine would have been
introduced into that country (which we look upon as our richest mine) in an
unfavourable point of view, and without such profits as would have been
satisfactory either to us or to the adventurers; and if we had granted the use
of condensers to one, we must have done so to all, and thereby have
curtailed our profits, and perhaps injured our reputation. Besides, where a
new engine is to be erected, and to be equally well executed in point of
workmanship and materials, an engine of the same power cannot be
constructed materially cheaper on the old plan than on ours; for our boiler
and cylinder are much smaller, and the building, the lever, the chains,

together with all the pump and pit work, are only the same.
* * * *
"We charge our profits in proportion to the saving made in fuel by our
engine, when compared with a common one which burns the same kind of
coals; we ask one third of these savings to be paid us annually, or half
yearly; the payment being redeemable in the option of our employer, at ten
years' purchase; and when the coals are low priced, we should also make
some charge as engineers. In all these comparisons our own interest has
made us except your (Mr. Smeaton) improved engines, unless we were
allowed a greater proportion of the savings."
Their exertions to improve the manufacture of engines at Soho is shown
by the following letter from Mr. Boulton, in the same correspondence to Mr.
Smeaton:—
"We are systematising the business of engine making, as we have done
before in the button manufactory; we are training up workmen, and making
tools and machines to form the different parts of Mr. Watt's engines with
more accuracy, and at a cheaper rate than can possibly be done by the
ordinary methods of working. Our workshop and apparatus will be of [Pg156]
sufficient extent to execute all the engines which are likely to be soon
wanted in this country; and it will not be worth the expense for any other
engineers to erect similar works, for that would be like building a mill to
grind a bushel of corn.
"I can assure you from experience, that our small engine at Soho is
capable of raising 500,000 cubic feet of water 1 foot high with every 112
lbs. of coals, and we are in hopes of doing much more. Mr. Watt's engine
has a very great advantage in mines, which are continually working deeper:
suppose, for instance, that a mine is 50 fathoms deep, you may have an
engine which will be equal to draining the water when the mine is worked,
to 100 fathoms deep, and yet you can constantly adapt the engine to its
load, whether it be 50 or 100 fathoms, or any intermediate depth; and the
consumption of coals will be less in proportion when working at the lesser
than at the greater depths; supposing it works, as our engines generally do,
at 11 lbs. per square inch, when the mine becomes 100 fathoms deep."

(85.)

The great improvement which has been introduced within the last half
century, in the details of Watt's steam engine, will be rendered manifest by
comparing the effects of a given weight of fuel here supplied by Mr.
Boulton with the effects which the same weight of fuel is now known to
produce in the best pumping engines worked in Cornwall. One of these
engines, in good working order, has been known to raise 125,000,000 lbs. 1
foot high, by the combustion of a bushel of coals. But the average
performance of even the best engines is below this amount. If we take it at
90,000,000, this will be equivalent to the weight of about 11⁄2 million cubic
feet of water, a bushel of coals being 3⁄4 cwt. It will therefore follow that,
with the present engines, one hundred weight of coals is capable of raising
about two million cubic feet of water one foot high, being a duty four times
that assigned to the early engines by Mr. Boulton.

(86.)

At the time that Watt, in conjunction with Dr. Roebuck, obtained the patent
for his improved engine, the idea occurred to him, that the steam which had
impelled the piston in its descent rushed from the cylinder with a
mechanical force much more than sufficient to overcome any resistance [Pg157]
which it had to encounter in its passage to the condenser; and that such
force might be rendered available as a moving power, in addition to that
already obtained from the steam during the stroke of the piston. This notion
involved the whole principle of the expansive action of steam, which
subsequently proved to be of such importance in the performance of steam
engines. Watt was, however, so much engrossed at that time, and
subsequently, by the difficulties he had to encounter in the construction of
his engines, that he did not attempt to bring this principle into operation. It
was not until after he had organised that part of the establishment at Soho
which was appropriated to the manufacture of steam engines, that he
proceeded to apply the expansive principle. Since the date of the patent
which he took out for this (1782), was subsequent to the application of the
same principle by another engineer, named Hornblower, it is right to state,

that the claim of Mr. Watt to this important step in the improvement of the
steam engine, is established by a letter addressed by him to Dr. Small, of
Birmingham, dated Glasgow, May, 1769:—
"I mentioned to you a method of still doubling the effect of the steam,
and that tolerably easy, by using the power of steam rushing into a vacuum,
at present lost. This would do little more than double the effect, but it would
too much enlarge the vessels to use it all: it is peculiarly applicable to wheel
engines, and may supply the want of a condenser, where the force of steam
only is used; for open one of the steam valves, and admit steam until one
fourth of the distance between it and the next valve is filled with steam,
then shut the valve, and the steam will continue to expand, and to press
round the wheel, with a diminishing power, ending in one fourth of its first
exertion. The sum of the series you will find greater than one half, though
only one fourth of steam was used. The power will indeed be unequal, but
this can be remedied by a fly, or by several other means."
In 1776 the engine, which had been then recently erected at Soho, was
adapted to act upon the principle of expansion. When the piston had been
pressed down in the cylinder for a certain portion of the stroke, the further
supply of steam [Pg158] from the boiler was cut off, by closing the upper steam
valve, and the remainder of the stroke was accomplished by the expansive
power of the steam which had already been introduced into the cylinder.

(87.)

To make this method of applying the force of steam intelligible, some
previous explanation of mechanical principles will be necessary.
If a body which offers a certain resistance be urged by a certain moving
force, the motion which it will receive will depend on the relation between
the energy of the moving force and the amount of the resistance opposed to
it. If the moving force be precisely equal to the resistance, the motion which
the body will receive will be perfectly uniform.
If the energy of the moving force be greater than the resistance, then its
surplus or excess above the amount of resistance will be expended in
imparting momentum to the mass of the body moved, and the latter will,
consequently, continually acquire augmented speed. The motion of the body
will, therefore, be in this case accelerated.
If the energy of the moving force be less in amount than the resistance,
then all that portion of the resistance which exceeds the amount of the
moving force will be expended in depriving the mass of the body of
momentum, and the body will therefore be moved with continually
diminished speed until it be brought to rest.

(88.)

Whenever, therefore, a uniform motion is produced in a body, it may be
taken as an indication of the equality of the moving force to the resistance;
and, on the other hand, according as the speed of the body is augmented or
diminished, it may be inferred that the energy of the moving force has been
greater or less than the resistance.
It is an error to suppose that rest is the only condition possible for a body
to assume when under the operation of two or more mechanical forces
which are in equilibrium. By the laws of motion the state of a body which is
not under the operation of any external force must be either in a state of rest
or of uniform motion. Whichever be its state, it will suffer no change if the
body be brought under the operation of two or more forces which are in

equilibrium; for to suppose [Pg159] such forces to produce any change in the
state of the body, whether from rest to motion, or vice versâ, or in the
velocity of the motion which the body may have previously had, would be
equivalent to a supposition that the forces applied to the body being in
equilibrium were capable of producing a dynamical effect, which would be
a contradiction in terms. This, though not always clearly understood by
mere practical men, or by persons superficially informed, is, in fact, among
the fundamental principles of mechanical science.

(89.)

When the piston is at the top of the cylinder, and about to commence its
motion downwards, the steam acting upon it will have not only to overcome
the resistance arising from the friction of the various parts of the engine, but
will also have to put in motion the whole mass of matter of the piston pump
rods, pump pistons, and the column of water in the pump barrels. Besides
imparting to this mass the momentum corresponding to the velocity with
which it will be moved, it will also have to encounter the resistance due to
the preponderance of the weight of the water and pump rods over that of the
steam piston. The pressure of steam, therefore, upon the piston at the
commencement of the stroke must, in accordance with the mechanical
principles just explained, have a greater force than is equal to all the
resistances which it would have to overcome, supposing the mass to be
moving at a uniform velocity. The moving force, therefore, being greater
than the resistance, the mass, when put in motion, will necessarily move
with a gradually augmented speed, and the piston of the engine which has
been described in the last chapter would necessarily move from the top to
the bottom of the cylinder with an accelerated motion, having at the
moment of its arrival at the bottom a greater velocity than at any other part
of the stroke. As the piston and all the matter which it has put in motion
must at this point come to rest, the momentum of the moving mass must
necessarily expend itself on some part of the machinery, and would be so
much mechanical force lost. It is evident, therefore, independently of any
consideration of the expansive principle, to which we shall presently refer,
that the action of the [Pg160] moving power in the descent of the piston ought to

be suspended before the arrival of the piston at the bottom of the cylinder,
in order to allow the momentum of the mass which is in motion to expend
itself, and to allow the piston to come gradually to rest at the termination of
the stroke.
Thus, if we were to suppose that after the piston had descended through
three fourths of the whole length of the cylinder, and had acquired a certain
velocity, the steam above it were suddenly condensed, so as to leave a
vacuum both above and below it, the piston, being then subject to no
impelling force, would still move downwards, in virtue of the momentum it
had acquired, until the resistance would deprive it of that momentum, and
bring it to rest; and if the remaining fourth part of the cylinder were
necessary for the accomplishment of this, then it is evident that that part of
the stroke would be accomplished without further expenditure of the
moving power.
In fact, this part of the stroke would be made by the expenditure of that
excess of moving power, which, at the commencement of the stroke, had
been employed in putting the machinery and its load in motion, and in
subsequently accelerating that motion.
Although under such circumstances the resistance, during the operation
of the moving power, shall not have been at any time equal to the moving
power, since while the motion was accelerated it was less, and while
retarded greater than that power, yet as the whole moving power has been
expended upon the resistance, the mechanical effect which the moving
power has produced under such circumstances will be equal to the actual
amount of that power. If in an engine of this kind the steam was not cut off
till the conclusion of the stroke, a part of the moving power would be lost
upon those fixed points in the machinery which would sustain the shock
produced by the instantaneous cessation of motion at the end of the stroke.
Independently, therefore, of any consideration of the expansive principle,
it appears that, in an engine of this kind, the steam ought to be cut off before
the completion of the stroke. [Pg161]

(90.)

To render the expansive action of steam intelligible, let A
B (fig. 28.) represent a cylinder whose area we will
suppose, for the sake of illustration, to be a square foot,
and whose length, A B, shall also be a foot. If steam of a
pressure equal to the atmosphere be supplied to this
cylinder, it will exert a pressure of about one ton on the
piston; and if such steam be uniformly supplied from the
boiler, the piston will be moved from A to B with the
force of one ton, and that motion will be uniform if the
piston be opposed throughout the same space by a
resistance equal to a ton. When the piston has arrived at
B, let us suppose that the further supply of steam from
the boiler is stopped by closing the upper steam valve, Fig. 28.
and let us also suppose the cylinder to be continued
downwards so that B C shall be equal to A B, and suppose that B C has been
previously in communication with the condenser, and is therefore a
vacuum. The piston at B will then be urged with a force of one ton
downwards, and as it descends the steam above it will be diffused through
an increased volume, and will consequently acquire a diminished pressure.
We shall, for the present, assume that this diminution of pressure follows
the law of elastic fluids in general; that it will be decreased in the same
proportion as the volume of the steam is augmented. While the piston,
therefore, moves from B downwards it will be urged by a continually
decreasing force. Let us suppose, that by some expedient, it is also subject
to a continually decreasing resistance, and that this resistance decreases in
the same proportion as the force which urges the piston. In that case the
motion of the piston would continue uniform. When the piston would arrive
at P′, the middle of the second cylinder, then the space occupied by the
steam being increased in the proportion of 2 to 3, the pressure on the piston
would be diminished in the proportion of 3 to 2, and the pressure at B being
one ton, it would be two-thirds of a ton at P′. In like manner when the piston
would arrive at C, the space occupied by the steam being double that which
[Pg162] it occupied when the piston was at B, the pressure of the steam would be

half its pressure at B, and therefore at the termination of the stroke, the
pressure on the piston would be half a ton.
If the space from B to C, through which the steam is here supposed to act
expansively, be divided into ten equal parts, the pressure on the piston at the
moment of passing each of those divisions would be calculated upon the
same principle as in the cases now mentioned. After moving through the
first division, the volume of the steam would be increased in the proportion
of 10 to 11, and therefore its pressure would be diminished in the proportion
of 11 to 10. The pressure, therefore, driving the piston at the end of the first
of these ten divisions would be 10⁄11ths of a ton. In like manner, its pressure
at the second of the divisions would be 10⁄12ths of a ton, and the third
10
⁄13ths of a ton; and so on, as indicated in the figure.
Now if the pressure of the steam through each of these divisions were to
continue uniform, and, instead of gradually diminishing, to suffer a sudden
change in passing from one division to another, then the mechanical effect
produced from B to C would be obtained by taking a mean or average of the
several pressures throughout each of the ten divisions. In the present case it
has been supposed that the force on the piston at B was 2240 pounds. To
obtain the pressure in pounds corresponding to each of the successive
divisions, it will therefore only be necessary to multiply 2240 by 10, and to
divide it successively by 11, 12, 13, &c. The pressures, therefore, in pounds,
at each of the ten divisions, will be as follows:—
1st 2036·3
2d 1866·6
3d 1723·1
4th 1600·0
5th 1493·3
6th 1400·0
7th 1317·6
8th 1244·4
9th 1179·0
10th 1120·0

If the mean of these be taken by adding them together [Pg163] and dividing
by 10, it will be found to be 1498 pounds. It appears, therefore, that the

pressures through each of the ten divisions being supposed to be uniform
(which however, strictly, they are not,) the mechanical effect of the steam
from B to C would be the same as if it acted uniformly throughout that space
upon the piston with a force of about 1500 pounds, being rather less than
three-fourths of its whole effect from A to B.
But it is evident that this principle will be equally applicable if the second
cylinder had any other proportion to the first. Thus it might be twice the
length of the first; and in that case, a further mechanical effect would be
obtained from the expansion of the steam.
The more accurate method of calculating the effect of the expansion from
B to C, would involve more advanced mathematical principles than could
properly be introduced here; but the result of such a computation would be
that the actual average effect of the steam from B to C would be equal to a
uniform pressure through that space, amounting to one thousand five
hundred and forty-five pounds, being greater than the result of the above
computation, the difference being due to the expansive action through each
of the ten divisions, which was omitted in the above computation.

(91.)

It is evident that the expansive principle, as here explained, involves the
condition of a variation in the intensity of the moving power. Thus, if the
steam act with a uniform energy on the piston so long as its supply from the
boiler continues, the moment that supply is stopped, by closing the steam
valve, the steam contained in the cylinder will fill a gradually increasing
volume by the motion of the piston, and therefore will act above the piston
with a gradually decreasing energy. If the resistance to the moving power
produced by the load, friction, &c. be not subject to a variation
corresponding precisely to such variation in the moving power, then the
consequence must be that the motion imparted to the load will cease to be
uniform. If the energy of the moving power at any part of the stroke be
greater than the resistance, the motion produced will be accelerated; if it be
less, the motion will be retarded; and if it be at one time greater, and another
[Pg164] time less, as will probably happen, then the motion will be alternately

accelerated and retarded. This variation in the speed of the body moved will
not, however, affect the mechanical effect produced by the power, provided
that the momentum imparted to the moving mass be allowed to expend
itself at the end of the stroke, so that the piston may be brought to rest as
nearly as possible by the resistance of the load, and not by any shock on any
fixed points in the machine. This is an object which, consequently, should
be aimed at with a view to the economy of power, independently of other
considerations connected with the wear and tear of the machinery. So long
as the engine is only applied to the operation of pumping water, great
regularity of motion is not essential, and, therefore, the variation of speed
which appears to be an almost inevitable consequence of any extensive
application of the expansive principle, is of little importance. In the patent
which Watt took out for the application of the expansive principle, he
specified several methods of producing a uniform effect upon a uniform
resistance, notwithstanding the variation of the energy of the power which
necessarily attended the expansion of the steam. This he proposed to
accomplish by various mechanical means, some of which had been
previously applied to the equalisation of a varying power. One consisted in
causing the piston to act on a lever, which should have an arm of variable
length, the length increasing in the same proportion as the energy of the
moving power diminished. This was an expedient which had been already
applied in mechanics for the purpose of equalising a varying power. A well-
known example of it is presented in the main-spring and fuzee of a watch.
According as the watch goes down, the main-spring becomes relaxed, and
its force is diminished; but, at the same time, the chain by which it drives
the fuzee acts upon a wheel or circle, having a diameter increased in the
same proportion as the energy of the spring is diminished.
Another expedient consisted in causing the moving power, when acting
with greatest energy, to lift a weight which should be allowed to descend
again, assisting the piston when the energy of the moving force was
diminished. [Pg165]
Another method consisted in causing the moving force, when acting with
greatest energy, to impart momentum to a mass of inert matter, which
should be made to restore the same force when the moving power was more

enfeebled. We shall not more than allude here to these contrivances
proposed by Watt, since their application has never been found
advantageous in cases where the expansive principle is used.

(92.)

The application of the expansive principle in the engines constructed by
Boulton and Watt, was always very limited, by reason of their confining
themselves to the use of steam having a pressure not much exceeding that
of the atmosphere. If the principle of expansion, as above explained, be
attentively considered, it will be evident that the extent of its application
will mainly depend on the density and pressure of the steam admitted from
the boiler. If the density and pressure be not considerable when the steam is
cut off, the extent of its subsequent expansion will be proportionally
limited. It was in consequence of this, that this principle from which
considerable economy of power has been derived, was applied with much
less advantage by Mr. Watt than it has since been by others, who have
adopted the use of steam of much higher pressure. In the engines of Boulton
and Watt, where the expansive principle was applied, the steam was cut off
after the piston had performed from one half to two thirds of the stroke,
according to the circumstances under which the engine was worked. The
decreasing pressure produced by expansion was, in this case, especially
with the larger class of engines, little more than would be necessary to
allow the momentum of the mass moved to spend itself, before the arrival
of the piston at the end of the stroke.
Subsequently, however, boilers producing steam of much higher pressure
were applied, and the steam was cut off when the piston had performed a
much smaller part of the whole stroke. The great theatre of these
experiments and improvements has been the mining districts in Cornwall,
where, instead of working with steam of a pressure not much exceeding that
of the atmosphere, it has been found advantageous to use steam whose
pressure is at least four times as great as [Pg166] that of the atmosphere; and
instead of limiting its expansion to the last half or fourth of the stroke, it is
cut off after the piston has performed one fourth part of the stroke or less,

all the remainder of the stroke being accomplished by the expansive power
of the steam, and by momentum.

BRIDGE OVER THE CLYDE AT HAMILTON,
DESIGNED BY WATT.

DOUBLE-ACTING ENGINE,
ZINC WORKS, CITY ROAD, LONDON.

CHAP. VII.
[Pg167]

TOC INX

PROPERTIES OF STEAM.—COMMON STEAM.—SUPERHEATED STEAM.—LAW OF
DALTON AND GAY LUSSAC.—LAW OF MARIOTTE.—RELATION BETWEEN
TEMPERATURE AND PRESSURE OF COMMON STEAM.—EFFECTS OF THE
EXPANSION OF COMMON STEAM.—MECHANICAL EFFECTS OF STEAM.—
METHOD OF EQUALISING THE EXPANSIVE FORCE.—HORNBLOWER'S ENGINE.
—WOOLF'S ENGINE.—WATT'S ATTEMPTS TO EXTEND THE STEAM ENGINE TO
MANUFACTURES.—PAPIN'S PROJECTED APPLICATIONS OF THE STEAM ENGINE.
—SAVERY'S APPLICATIONS OF THE ENGINE TO MOVE MACHINERY.—
JONATHAN HULL'S APPLICATION TO WATER WHEELS.—STEWART'S
APPLICATION OF THE ENGINE TO MILL WORK.—WASHBOROUGH'S
APPLICATION OF THE FLY WHEEL AND CRANK.—WATT'S SECOND PATENT.—
DOUBLE-ACTION VALVES.

(93.)

Since the application of the expansive action of steam involves the
consideration of its properties when it ceases to be in contact with the water
from which it was produced, and likewise the variation of its pressure in
different states of [Pg168] density and at different temperatures, it is necessary
here to explain some of the most important of these properties of vapour.
Steam may exist in two states, distinguished from each other by the
following circumstances:—
1st. It may be such that the abstraction from it of any portion of heat,
however small, will cause its partial condensation.
2d. It may be such as to admit of the abstraction of heat from it without
undergoing any other change than that which air would undergo under like
circumstances, viz. a diminution of temperature and pressure.

(94.)

We shall call, for distinction, the former Common Steam, and the latter
Superheated Steam.
To explain the circumstances out of which these properties
arise, let B (fig. 29.) be imagined to be a vessel filled with
water, communicating by a pipe and stopcock with another
vessel A, which in the commencement of the process may be
conceived to be filled with air. Let D be a pipe and stopcock at
the top of this vessel. If the vessel B be heated, and the two
cocks be opened, the steam proceeding from the water in B
will blow the air out of the vessel A through the open stopcock
D, in the same manner as air is blown from a steam engine.
When the vessel A by these means has been filled with pure
steam, let both stopcocks be closed. If the steam in A, under
these circumstances, have a pressure of 15 lbs. per square inch,
Fig. 29. its temperature will be found to be 213°. Now, if any heat be
abstracted from this steam, its temperature will fall, and a
portion of it will be reconverted into water.
Again, suppose the vessel A to be filled with pure steam which has been
produced from the heated water in B, the stopcock C being open. Let the
stopcock C be then closed, and the water in B be heated to a higher
temperature, the temperature and pressure of the steam in A being observed.
If the stopcock C be now opened, the steam in A will be immediately
observed to rise to the more elevated temperature which has been imparted
to the water in B, and at the same time it will acquire an increased pressure.
[Pg169]

The increase of temperature which it has received would of itself produce
an increased pressure; but that this is not the sole cause of the augmented
pressure in the present case might be proved by weighing the vessel A. It
would be found to have increased weight, which could only arise from its
having received from the water in B an additional quantity of vapour. The
increased pressure therefore, which the steam in A has acquired, is due
conjointly to its increased density and its increased temperature. In general,

if the water in the vessel B be raised or lowered in temperature, the steam in
the vessel A will rise and fall in temperature in a corresponding manner,
always having the same temperature as the water in B. If the weight of the
vessel A were observed, it would be found to increase with every increase of
temperature, and to diminish with every diminution of temperature, proving
that the augmented temperature of the water in B produces an augmented
density of the steam in A. The same pressure would be found always to
correspond to the same temperature and density, so that if the numerical
amount of any one of the three quantities, the temperature, the pressure, or
the density, were known, the other two must necessarily be determined, the
same temperature always corresponding to the same pressure, and vice
versâ. And in like manner, steam produced under these circumstances of the
same density cannot have different pressures. It must be observed that the
steam here produced receives all the heat which it possesses from the water
from which it is raised. Now it is easily demonstrable, that this is the least
quantity of heat which is compatible with the steam maintaining the
vaporous form; for if the stopcock C be closed so as to separate the steam in
A from the water in B, and that any portion of heat, however small, be then
abstracted from the steam in A, some portion of the steam will be
reconverted into water.
This then, according to the definition already given, is Common Steam.

(95.)

Let us now suppose that the vessel A, being in communication with the
vessel B by the open stopcock, has been filled with pure steam of any given
temperature. The steam which it thus contains will be common steam, and,
as has been [Pg170] shown (94.), it cannot lose any portion of heat, however
small, without being partially condensed; but let the stopcock C be closed,
and let the steam in A be then exposed to any source of heat by which its
temperature may be raised any required number of degrees. From the steam
thus obtained heat may be abstracted without producing any condensation;
and such abstraction of heat may be continued without producing
condensation, until the steam is cooled down to that temperature at which it

was raised from the water in B, when the stopcock C was opened. Any
further reduction of temperature would be attended with condensation.
If after increasing the temperature of the steam in A, the stopcock C being
shut so as to render it superheated steam, its pressure be observed, the
pressure will be found to be increased, but not to that amount which it
would have been increased had the steam in A been raised to the same
temperature by heating the water in B to that temperature, and keeping the
stopcock open. In fact, its present augmented pressure will be due only to
its increased temperature, since its density remains unchanged. But if in
these circumstances the stopcock C be suddenly opened, the pressure of the
steam in A will as suddenly rise to that pressure which in common steam
corresponds to its temperature; and if the vessel A were weighed, it would
be found to have increased in weight, proving that the steam contained in it
has received increased density by an increased quantity of vapour
proceeding from the water in A. In fact, by opening the stopcock the steam
which was before superheated steam, has become common steam. It has the
greatest density which steam of that temperature can have; and
consequently, if any heat be abstracted from it, a partial condensation will
ensue.
To render these general principles more intelligible, let us suppose that
the water in B is raised to the temperature of 213°, the stopcock C being
open; the vessel A will then be filled with steam of the same temperature,
and having a pressure of 15 lbs. per square inch. This will be common
steam. If the stopcock be now closed, and the whole apparatus be exposed
to the temperature of 243°; the steam in A will preserve the same density,
but its pressure will be [Pg171] increased from 15 lbs. to a little more than 16
lbs. per square inch. Let the stopcock C be then opened and while the
temperature of the steam in A shall continue to be 243°, the pressure will
suddenly rise from 16 lbs. to about 26 lbs. per square inch. The weight of
the steam in A will be at the same time increased in the same proportion of
16 to 26 as its pressure. The steam thus produced in A will then be common
steam, and any abstraction of heat from it would be attended with partial
condensation.

(96.)

The law, according to which the pressure of elastic fluids in general,
whether gases or vapours, increases with their temperature, was
simultaneously discovered by Dalton and Gay Lussac. If the pressure which
the gas or vapour would have at the temperature of melting ice, were
expressed by 10,000, then the increase of pressure which it would receive
for every degree of temperature by which it would be raised, its volume
being supposed to be preserved, would be expressed by 2081⁄3. Thus, if the
pressure of gas, or vapour, on a surface of a certain magnitude at the
temperature of 32° were 10,000 ounces, then the same gas or vapour would
acquire an additional pressure of 2081⁄3 ounces for every degree of
temperature which would be imparted to it above 32°. This law is common
to all gases and vapours.
It may be objected that water cannot exist in the state of vapour under the
usual pressures at so low a temperature as melting ice. This, however, does
not hinder the application of the above law, for that law will equally hold
good by computing the pressure which the vapour would have if it were a
permanent gas, and if it could therefore exist in the elastic form at that low
temperature.

(97.)

Another law, common to all elastic fluids, and of equal importance with the
former, was discovered by Mariotte. By this law it appears that every gas or
vapour, so long as its temperature is unchanged, will have a pressure
directly proportional to its density. If therefore, while we compress steam
into half its volume, we could preserve its temperature unaltered, we should
increase its pressure in a two-fold proportion; but if the process of
compression should cause its temperature to increase, [Pg172] then its increase
of pressure will be greater than its increase of density, since it will be due
conjointly to the increase of density and to the increase of temperature. In
this case the increased pressure may be deduced from the combined
application of the two laws just explained; that of Mariotte will determine
that increase of pressure which is due to the increase of density, and that of

Dalton and Gay Lussac will determine the further increase of pressure
which will be due to the increase of temperature. The full investigation of
these effects, and the formulæ expressing them, will be found in the
Appendix to this volume.

(98.)

The fixed relations which exist between the temperatures of common steam
and its pressure and density, have never been discovered from any general
physical principles. The pressures and the densities however, which
correspond to a great variety of temperatures throughout the thermometric
scale, have been ascertained by extensive series of experiments instituted by
philosophers of this and other countries. From a comparison of the
temperatures and pressures thus found by experiment, empirical formulæ
have been constructed, which exhibit, with an approximation sufficiently
close for practice, this relation; and these formulæ may accordingly be used
for the computation of tables exhibiting the pressures, temperatures, and
densities of common steam; and such tables will have sufficient numerical
accuracy for all practical purposes. These formulæ, and the tables resulting
from them, will be found in the Appendix to this volume.

(99.)

It has been explained, that to effect the conversion of water into steam, it is
only necessary to impart to it as much heat as, added to the temperature
which it has, would, if it continued in the liquid form, raise it to the
temperature of 1212°. This condition is necessary, and sufficient to effect
the transition of water into vapour. If, for example, as much heat were
imparted to the water evaporated, as would maintain it in the liquid state to
1300°, then the steam so produced would be superheated steam, having 80°
of heat more than is necessary to maintain it in the vaporous form. From
such steam, therefore, 80° of heat may be abstracted without producing any
condensation. [Pg173]

(100.)

Common steam being raised from water at any pressure and temperature,
and being afterwards separated from the water, if the same steam be
compressed into a small volume, or allowed to expand into a greater
volume, it will still maintain its quality of common steam, and will have the
same pressure and temperature, whatever volume it may assume, as it
would have if immediately raised from water at that pressure. Thus if steam
be raised from water under a pressure of 30 lbs. per square inch, and, being
separated from the water, be allowed to dilate, until its pressure is reduced
to 15 lbs. per square inch, its temperature will then be reduced to 213°,
which is that temperature which it would have if immediately raised from
water under a pressure of 15 lbs. per square inch; and if any heat be
abstracted from such steam, whether under its original pressure, or under
the diminished pressure of 15 lbs. per square inch, a condensation will be
produced, the amount of which will be the same, if the same quantity of
heat be abstracted from the steam. These are consequences which
immediately flow from the fact, that the sum of the latent and sensible heats
of steam is always the same.[20]
It appears, therefore, that supposing the steam used in an engine to
receive no additional heat after it leaves the boiler, however it may be
changed in its density by subsequent expansion, it will still retain its
character of common steam, and cannot lose any portion of heat, however
small, without suffering partial condensation. The mechanical force also
exerted by such steam, after expansion, must be computed in the same
manner as if it were raised immediately.

(101.)

If the law of Mariotte were strictly applicable to steam, its mechanical
effect would be the same as has been already explained in all states of
density; but since its temperature will rise and fall as its density is increased
or diminished, a corresponding change will be produced in its [Pg174]
mechanical efficacy. It is therefore necessary in the calculation of the
mechanical effect of steam, whether it be used at a uniform pressure
without the principle of expansion, or with the application of that principle
to any given extent, to take into account the combined operation of the laws

of Mariotte and Dalton. Formulæ exhibiting the relation between the
temperatures, pressures, volumes, and densities of steam, and the
mechanical effect produced by the evaporation of water, whether acting
with or without expansion, together with the tables necessary for the
practical application of these, will be found in the Appendix.

(102.)

One of the methods of equalising the varying force of expanding steam,
would be to work it at the same time in two cylinders connected with the
same beam; so that while its force in one would be augmented, its force in
the other would be diminished, the combination of the two producing a
uniform effect. Soon after the expansive principle was promulged by Mr.
Watt, this expedient was accordingly resorted to by an engineer named
Hornblower.
In the year 1781, Hornblower
conceived the notion of working an
engine with two cylinders of different
sizes, by allowing the steam to flow freely
from the boiler until it fills the smaller
cylinder, and then permitting it to expand
into the greater one, employing it thus to
press down two pistons in the following
manner.
Let C, fig. 30., be the centre of the great
working-beam, carrying two arch heads,
on which the chains of the piston rods
play. The distances of these arch heads
from the centre C must be in the same
proportion as the length of the cylinders,
Fig. 30. in order that the same play of the beam
may correspond to [Pg175] the plays of both
pistons. Let F be the steam-pipe from the boiler, and G a valve to admit the
steam above the lesser piston. H is a tube by which a communication may

be opened by the valve I, between the top and bottom of the lesser cylinder
B. K is a tube communicating by the valve L, between the bottom of the
lesser cylinder B and the top of the greater cylinder A. M is a tube
communicating, by the valve N, between the top and bottom of the greater
cylinder A; and P a tube leading to the condenser by the exhausting valve O.
At the commencement of the operation, suppose all the valves opened,
and steam allowed to flow through the engine until the air be completely
expelled, and then let all the valves be closed. To start the engine, let the
exhausting valve O and the steam valves G and L be opened, as in fig. 30.
The steam will flow freely from the boiler, and press upon the lesser piston,
and at the same time the steam below the greater piston will flow into the
condenser, leaving a vacuum in the greater cylinder. The valve L being
opened, the steam which is under the piston in the lesser cylinder will flow
through K, and press on the greater piston, which, having a vacuum beneath
it, will consequently descend. At the commencement of the motion, the
lesser piston is as much resisted by the steam below it, as it is urged by the
steam above it; but after a part of the descent has been effected, the steam
below the piston, in the lesser cylinder, passing into the greater, expands
into an increased space, and therefore loses part of its elastic force. The
steam above the lesser piston retaining its full force by having a free
communication with the boiler by the valve G, the lesser piston will be
urged by a force equal to the excess of the pressure of this steam above the
diminished pressure of the expanded steam below it. As the pistons
descend, the steam which is between them is continually increasing in its
bulk, and therefore decreasing in its pressure, from whence it follows, that
the force which resists the lesser piston is continually decreasing, while that
which presses it down remains the same, and therefore the effective force
which impels it must be continually increasing. [Pg176]
On the other hand, the force which urges the greater piston is continually
decreasing, since there is a vacuum below it, and the steam which presses it
is continually expanding into an increased bulk.
Impelled in this way, let us suppose the pistons to have arrived at the
bottoms of the cylinders, and let the valves G, L, and O, be closed, and the

valves I and N opened. No steam is allowed to
flow from the boiler, G being closed, nor any
allowed to pass into the condenser, since O is
closed, and all communication between the
cylinders is stopped by closing L. By opening the
valve I, a free communication is made between
the top and bottom of the lesser piston through
Fig. 31. the tube H, so that the steam which presses above
the lesser piston will exert the same pressure
below it, and the piston is in a state of indifference. In the same manner the
valve N being open, a free communication is made between the top and
bottom of the greater piston, and the steam circulates above and below the
piston, and leaves it free to rise. A counterpoise attached to the pump-rods,
in this case, draws up the piston, as in Watt's single engine; and when they
arrive at the top, the valves I and N are closed, and G, L, and O, opened, and
the next descent of the pistons is produced in the manner already described,
and so the process is continued.
The valves are worked by the engine itself, by means similar to some of
those already described. By computation, we find the power of this engine
to be nearly the same as a similar engine on Watt's expansive principle. It
does not, however, appear, that any adequate advantage was gained by this
modification of the principle, since no engines of this construction are now
made.

(103.)

The use of two cylinders was revived by Arthur Woolf in 1804, who, in this
and the succeeding year, obtained patents for the application of steam raised
under a high pressure to double-cylinder engines. The specification of his
patent states, that he has proved by experiment that steam raised [Pg177] under a
safety-valve loaded with any given number of pounds upon the square inch
will, if allowed to expand into as many times its bulk as there are pounds of
pressure on the square inch, have a pressure equal to that of the atmosphere.
Thus, if the safety-valve be loaded with four pounds on the square inch, the
steam, after expanding into four times its bulk, will have the atmospheric

pressure; if it be loaded with 5, 6, or 10 lbs. on the square inch, it will have
the atmospheric pressure when it has expanded into 5, 6, or 10 times its
bulk, and so on. It was, however, understood in this case, that the vessel
into which it was allowed to expand should have the same temperature as
the steam before it expands.
It is very unaccountable how a person of Mr. Woolf's experience in the
practical application of steam could be led into errors so gross as those
involved in the averments of this patent; and it is still more unaccountable
how the experiments could have been conducted which led him to
conclusions not only incompatible with all the established properties of
elastic fluids, but even involving in themselves palpable contradiction and
absurdity. If it were admitted that every additional pound avoirdupois which
should be placed upon the safety-valve would enable steam, by its
expansion into a proportionally enlarged space, to attain a pressure equal to
the atmosphere, the obvious consequence would be, that a physical relation
would subsist between the atmospheric pressure and the pound avoirdupois!
It is wonderful that it did not occur to Mr. Woolf, that, granting his principle
to be true at any given place, it would necessarily be false at another place,
where the barometer would stand at a different height! Thus, if the principle
were true at the foot of a mountain, it would be false at the top of it; and if it
were true in fair weather, it would be false in foul weather, since these
circumstances would be attended by a change in the atmospheric pressure,
without making any change in the pound avoirdupois.[21]
[Pg178]

(104.)

For several years after the extension of Watt's first patent had been obtained
from parliament, he was altogether engrossed by the labour of bringing to
perfection the application of the steam-engine to the drainage of mines, and
in surmounting the numerous difficulties which presented themselves to its
general adoption, even after its manifold advantages were established and
admitted. When, however, these obstacles had been overcome, and the
works for the manufacture of engines for pumping water, at Soho, had been
organised and brought into active operation, he was relieved from the

pressure of these anxieties, and was enabled to turn his attention to the far
more extensive and important uses of which he had long been impressed
with the conviction that the engine was capable. His sagacious mind
enabled him to perceive that the machine he had created was an infant
force, which by the fostering influence of his own genius would one day
extend its vast power over the arts and manufactures, the commerce and the
civilisation of the world. Filled with such aspirations, he addressed his
attention about the year 1779, to the adaptation of the steam-engine to move
machinery, and thereby to supersede animal power, and the natural agents,
wind and water.
The idea that steam was capable of being applied extensively as a prime
mover, had prevailed from a very early period; and now that we have seen
its powers so extensively brought to bear, it will not be uninteresting to
revert to the faint traces by which its agency was sketched in the crude
speculations of the early mechanical inventors.

(105.)

Papin, to whom the credit of discovering the method of producing a vacuum
by the condensation of steam is due, was the earliest and most remarkable
of those projectors. With very limited powers of practical application, he
was, nevertheless, peculiarly happy in his mechanical conceptions; and had
his experience and opportunities been proportionate to the clearsighted
character of his mind, he would doubtless have anticipated some of the
most memorable of his successors in the progressive improvement of the
steam engine.
In his work already cited, after describing his method of imparting an
alternate motion to a piston by the atmospheric [Pg179] pressure acting against a
vacuum produced by the condensation of steam, he stated that his invention,
besides being applicable to pumping water, could be available for rowing
vessels against wind and tide, which he proposed to accomplish in the
following manner.
Paddle-wheels, such as have since been brought into general use, were to
be placed at the sides, and attached to a shaft extending across the vessel.

Within the vessel, and under this shaft, he proposed to place several
cylinders supplied with pistons, to be worked by the atmospheric pressure.
On the piston-rods were to be constructed racks furnished with teeth: these
teeth were to work in the teeth of wheels or pinions, placed on the shaft of
the paddle-wheels. These pinions were not to be fixed on the shaft, but to be
connected with it by a ratchet; so that when they turned in one direction,
they would revolve without causing the shaft to revolve; but when driven in
the other direction, the catch of the ratchet-wheel would act upon the shaft
so as to compel the shaft and paddle-wheels to revolve with the motion of
the pinion or wheel upon it. By this arrangement, whenever the piston of
any cylinder was forced down by the atmospheric pressure, the rack
descending would cause the corresponding pinion of the paddle-shaft to
revolve; and the catch of the ratchet wheel, being thus in operation, would
cause the paddle-shaft and paddle-wheels also to revolve; but whenever the
piston would rise, the rack driving the pinion in the opposite direction, the
catch of the ratchet wheel would merely fall from tooth to tooth, without
driving the paddle-shaft.
It is evident that by such an arrangement a single cylinder and piston
would give an intermitting motion to the paddle-shaft, the motion of the
wheel being continued only during the descent of the piston; but if several
cylinders were provided, then their motion might be so managed, that when
one would be performing its ascending stroke, and therefore giving no
motion to the paddle-shaft, another should be performing its descending
stroke, and therefore driving the paddle-shaft. As the interval between the
arrival of the piston at the bottom of the cylinder and the commencement
[Pg180] of its next descent would have been, in the imperfect machine
conceived by Papin, much longer than the time of the descent, it was
evident that more than two cylinders would be necessary to insure a
constantly acting force on the paddle-shaft, and, accordingly, Papin
proposed to use several cylinders.
In addition to this, Papin proposed to construct a boiler having a fireplace
surrounded on every side by water, so that the heat might be imparted to the
water with such increased rapidity as to enable the piston to make four

strokes per minute. These projects were promulged in 1690, but it does not
appear that they were ever reduced to experiment.

(106.)

Savery proposed, in his original patent, in 1698, to apply his steam engine
as a general prime mover for all sorts of machinery, by causing it to raise
water to make an artificial fall, by which overshot water-wheels might be
driven. This proposal was not acted on during the lifetime of Savery, but it
was at a subsequent period partially carried into effect. Mr. Joshua Rigley
erected several steam engines on this principle at Manchester, and other
parts of Lancashire, to impel the machinery of some of the earliest
manufactories and cotton mills in that district. The engines usually raised
the water from sixteen to twenty feet high, from whence it was conveyed to
an overshot wheel, to which it gave motion. The same water was repeatedly
elevated by the engine, so that no other supply was necessary, save what
was sufficient to make good the waste. These engines continued in use for
some years, until superseded by improved machines.[22]

(107.)

In 1736, Jonathan Hulls obtained a patent for a method of towing ships into
or out of harbour against wind and tide. This method was little more than a
revival of that proposed by Papin in 1690. The motion, however, was to be
communicated to the paddle-shaft by a rope passing over a pulley fixed on
an axis, and was to be maintained during the returning stroke of the piston
by the descent of a weight which was elevated during the descending
stroke. There is no record, however, of this plan, any more than that of
Papin, ever having been reduced to experiment.

(108.)

During the early part of the last century the [Pg181] manufactures of this
country had not attained to such an extent as to render the moving power
supplied by water insufficient or uncertain to any inconvenient degree; and
accordingly mills, and other works in which machinery required to be

driven by a moving power, were usually built along the streams of rivers.
About the year 1750 the general extension of manufactures, and their
establishment in localities where water power was not accessible, called the
steam engine into more extensive operation. In the year 1752, Mr.
Champion, of Bristol, applied the atmospheric engine to raise water, by
which a number of overshot wheels were driven. These were applied to
move extensive brass-works in that neighbourhood, and this application
was continued for about twenty years, but ultimately given up on account of
the expense of fuel and the improved applications of the steam engine.
About this time Smeaton applied himself with great activity and success
to the improvement of wind and water mills, and succeeded in augmenting
their useful effect in a twofold proportion with the same supply of water.
From the year 1750 until the year 1780 he was engaged in the construction
of his improved water mills, which he erected in various parts of the
country, and which were imitated so extensively that the improvement of
such mills became general. In cases where a summer drought suspended the
supply of water, horse machinery was provided, either to work the mill or to
throw back the water. These improvements necessarily obstructed for a time
the extension of steam power to mill work; but the increase of manufactures
soon created a demand for power greatly exceeding what could be supplied
by such limited means.
In the manufacture of iron, it is of great importance to keep the furnaces
continually blown, so that the heat may never be abated by day or night. In
the extensive ironworks at Colebrook Dale, several water-wheels were used
in the different operations of the manufacture of iron, especially in driving
the blowers of the iron furnaces. These wheels were usually driven by the
water of a river, but in the summer months the supply became so short that
it was insufficient to work them all. Steam engines were accordingly
erected to [Pg182] return the water for driving these wheels. This application of
the engine as an occasional power for the supply of water-wheels having
been found so effectual, returning engines were soon adopted as the
permanent and regular means of supplying water-wheels. The first attempt
of this kind is recorded to have been made by Mr. Oxley, in 1762, who
constructed a machine to draw coals out of a pit at Hartley colliery, in

Northumberland. It was originally intended to turn the machine by a
continuous circular motion received from the beam of the engine; but that
method not being successful, the engine was applied to raise water for a
wheel by which the machine was worked. This engine was continued in use
for several years, and though it was at length abandoned, on account of its
defective construction, it nevertheless established the practicability of using
steam power as a means of driving water wheels.[23]

(109.)

In the year 1777, Mr. John Stewart read a paper before the Royal Society,
describing a method for obtaining a continued circular motion for turning
all kinds of mills from the reciprocating motion of a steam engine. He
proposed to accomplish this by means of two endless chains passing over
pulleys, which should be moved upwards and downwards by the motion of
the engine, in the manner of a window sash. The joint pins of the links of
the two chains worked in teeth at the opposite sides of a cog wheel, to
which they imparted a circular motion, first by one chain, and then by the
other, acting alternately on opposite sides of the wheel. One chain impelled
it during the descent of the piston, and the other during the ascent; but one
of these chains always passed over its pulleys so as to produce no effect on
one side of the cog wheel, whilst the other chain worked on the opposite
side to turn it round. For this purpose each chain was provided with a catch,
to prevent its circulating over its pulleys in one direction, but to allow it free
motion in the other. The cog wheel thus kept in revolution might be applied
to the axis of any mill which the engine was required to work. Thus, if it
were applied to a flour-mill, the millstone itself would perform the office of
a fly-wheel to regulate the intermission of [Pg183] the power, and in other mills
a fly-wheel might be added for this purpose.
The hints obtained by Mr. Stewart from Papin's contrivance, before
mentioned, will not fail to be perceived. In Mr. Stewart's paper he notices
indirectly the method of obtaining a continued circular motion from a
reciprocating motion by means of a crank or winch, which, he says, occurs
naturally in theory, but in practice would be impossible, from the nature of
the motion of the engine, which depends on the force of the steam, and

cannot be ascertained in its length. Therefore, on the first variation, the
machine would be either broken in pieces or turned back. Such an opinion,
pronounced by a man of considerable mechanical knowledge and ingenuity,
against a contrivance which, as will presently appear, proved in practice,
not less than in theory, to be the most effectual means of accomplishing the
end here pronounced to be impossible, is sufficiently remarkable. It might
cast some doubt on the extent of Mr. Stewart's practical knowledge, if it did
not happen to be in accordance with a judgment so generally
unimpeachable as that of Mr. Smeaton. This paper of Mr. Stewart's was
referred by the council of the Royal Society to Mr. Smeaton, who remarked
upon the difficulty arising from the absolute stopping of the whole mass of
moving power, whenever the direction of the motion is changed; and
observed, that although a fly-wheel might be applied to regulate the motion,
it must be such a large one as would not be readily controlled by the engine
itself; and he considered that the use of such a fly-wheel would be a greater
incumbrance to a mill than a water-wheel to be supplied by water pumped
up by the engine. This engineer, illustrious as he was, not only fell into the
error of Mr. Stewart in respect of the crank, but committed the further
blunder of condemning the very expedient which has since rendered the
crank effectual. It will presently appear that the combination of the crank
and fly-wheel have been the chief means of establishing the dominion of
the steam engine over manufactures.

(110.)

In 1779, Mr. Matthew Wasbrough, an engineer at Bristol, took out a patent
for the application of a steam engine [Pg184] to produce a continuous circular
motion by means of ratchet wheels, similar to those previously used by Mr.
Oxley, at Hartley colliery; to which, however, Mr. Wasbrough added a fly-
wheel to maintain and regulate the motion. Several machines were
constructed under this patent; and among others, one was erected at Mr.
Taylor's saw-mills and block manufactory at Southampton. In 1780, one
was erected at Birmingham, where the ratchet work was found to be subject
to such objections, that one of the persons about the works substituted for it
the simple crank, which has since been invariably used. A patent was taken
out for this application of the crank in the same year, by Mr. James Pickard,

of Birmingham. It will presently appear, however, that the suggestion of this
application of the crank was derived from the proceedings of Watt, who was
at the same time engaged in similar experiments.

(111.)

The single-acting steam engine, as constructed by Watt, was not adapted to
produce continuous uniform motion of rotation, for the following reasons:
—

First. The effect required was that of an uniformly acting force. The
steam engine, on the other hand, supplied an intermitting force. Its
operation was continued during the descending motion of the piston, but
it was suspended during the ascent of the piston. To produce the
continued effect now required, either its principle of operation should be
altered, or some expedient should be devised for maintaining the motion
of the revolving shaft during the ascent of the piston, and the consequent
suspension of the moving power.
Secondly. The action of the steam engine was rectilinear. It was a power
which acted in a straight line, viz., in the direction of the cylinder. The
motion, however, required to be produced, was a circular motion—a
motion of rotation around the axis or shaft of the mill.

The steps by which Watt proceeded to accomplish these objects have
been recorded by himself as follows, in his notes upon Dr. Robison's article
on the steam engine:—
"I had very early turned my mind to the producing of continued motion
round an axis; and it will be seen, by reference to my first specification in
1769, that I there described [Pg185] a steam wheel, moved by the force of
steam, acting in a circular channel against a valve on one side, and against a
column of mercury, or some other fluid metal, on the other side. This was
executed upon a scale of about six feet diameter at Soho, and worked
repeatedly, but was given up, as several practical objections were found to
operate against it; similar objections lay against other rotative engines,
which had been contrived by myself and others, as well as to the engines
producing rotatory motions by means of ratchet wheels.

"Having made my single reciprocating engines very regular in their
movements, I considered how to produce rotative motions from them in the
best manner; and amongst various schemes which were subjected to trial, or
which passed through my mind, none appeared so likely to answer the
purpose as the application of the crank, in the manner of the common
turning lathe; but as the rotative motion is produced in that machine by
impulse given to the crank in the descent of the foot only, it requires to be
continued in its ascent by the energy of the wheel, which acts as a fly; being
unwilling to load my engine with a fly-wheel heavy enough to continue the
motion during the ascent of the piston (or with a fly-wheel heavy enough to
equalise the motion, even if a counterweight were employed to act during
that ascent), I proposed to employ two engines, acting upon two cranks
fixed on the same axis, at an angle of 120° to one another, and a weight
placed upon the circumference of the fly-wheel at the same angle to each of
the cranks, by which means the motion might be rendered nearly equal, and
only a very light fly-wheel would be requisite.
"This had occurred to me very early; but my attention being fully
employed in making and erecting engines for raising water, it remained in
petto until about the year 1778 or 1789, when Mr. Wasbrough erected one
of his ratchet-wheel engines at Birmingham, the frequent breakages and
irregularities of which recalled the subject to my mind, and I proceeded to
make a model of my method, which answered my expectations; but having
neglected to take out a patent, the invention was communicated by a
workman employed to [Pg186] make the model, to some of the people about Mr.
Wasbrough's engine, and a patent was taken out by them for the application
of the crank to steam engines. This fact the said workman confessed, and
the engineer who directed the works acknowledged it; but said,
nevertheless, that the same idea had occurred to him prior to his hearing of
mine, and that he had even made a model of it before that time; which
might be a fact, as the application to a single crank was sufficiently
obvious.
"In these circumstances, I thought it better to endeavour to accomplish
the same end by other means, than to enter into litigation; and if successful,
by demolishing the patent, to lay the matter open to every body.

Accordingly, in 1781, I invented and took out a patent for several methods
of producing rotative motions from reciprocating ones; amongst which was
the method of the sun-and-planet wheels. This contrivance was applied to
many engines, and possesses the great advantage of giving a double
velocity to the fly-wheel; but is perhaps more subject to wear, and to be
broken under great strains, than a simple crank, which is now more
commonly used, although it requires a fly-wheel of four times the weight, if
fixed upon the first axis; my application of the double engine to these
rotative machines rendered the counterweight unnecessary, and produced a
more regular motion."

(112.)

Watt's second patent here referred to, was dated 25th October, 1781, and
was entitled "A patent for certain new methods of applying the vibrating or
reciprocating motions of steam or fire engines to produce a continued
rotative or circular motion round an axis or centre, and thereby to give
motion to the wheels of mills and other machines."
All the methods specified in this patent were intended to be worked by
the single-acting engine, already described, a counterweight being applied
to impel the machinery during the returning stroke of the engine, which
weight would be elevated during the descent of the piston. There were five
different expedients proposed in the specification for producing a rotatory
motion; but, of these five, two only were ever applied in practice. [Pg187]

(113.)

Suppose a rod or bar attached by a pin or joint at the upper extremity to the
working end of the beam of the engine, and by a similar pin or joint at the
lower extremity to an iron wheel fixed on the extremity of the axis of the
fly-wheel. One half of this wheel is formed of a solid semicircle of cast
iron, while the other half is constructed of open spokes, so as to be as light
as is consistent with strength. The position of the wheel on the axis is such
that during the returning stroke of the piston, when the operation of the
steam is suspended, the heavy semicircle of the wheel will be descending,
and by its weight will draw down the connecting bar, and thereby draw

down the working end of the beam, and draw up the piston in the cylinder.
When the piston descends and is driven by the power of the steam, the
heavy semicircle of the above-mentioned wheel will be drawn upwards, and
in the same way the motion will be continued.

(114.)

The second method of
producing a rotatory motion,
which was subsequently
continued for many years in
practical operation, was that
which was called the Sun-
and-planet Wheels. A
toothed wheel A (fig. 32.),
called the sun wheel, was
fixed on the axle of the fly-
wheel, to which rotation was
to be imparted. The wheel B, Fig. 32.
called the planet wheel,
having an equal diameter, was fastened on the end I of the connecting rod H
I, so as to be incapable of revolving. During the descent of the piston, the
working end of the beam was drawn upwards, and the end I of the
connecting rod travelled from C to D, through the dotted semicircle C I D.
The wheel B not being capable of revolving on the centre I, would, during
this motion, drive the sun wheel A. During the ascent of the steam piston,
the working end of the beam would descend, and the centre I of [Pg188] the
planet wheel B would be driven downwards from D to C, through the other
dotted semicircle, and would consequently continue to drive the sun wheel
round in the same direction.
This contrivance, although in the main inferior to the more simple one of
the crank, is not without some advantages; among others, it gives to the sun
wheel double the velocity which would be communicated by the crank; for
in the crank one revolution only on the axle is produced by one revolution

of the crank, but in the sun-and-planet wheel, two revolutions of the sun
wheel are produced by one of the planet wheel; thus a double velocity is
obtained from the same motion of the beam. This will be evident from
considering that when the planet wheel is in its highest position, its lowest
tooth is engaged with the highest tooth of the sun wheel; as the planet wheel
passes from the highest position, its teeth drive those of the sun wheel
before them, and when it comes into the lowest position, the highest tooth
of the planet wheel is engaged with the lowest of the sun wheel: but then
half of the sun wheel has rolled off the planet wheel, and, therefore, the
tooth which was engaged with it in its highest position, must now be distant
from it by half the circumference of the wheel, and must, therefore, be
again in the highest position; so that while the planet wheel has been carried
from the top to the bottom, the sun wheel has made a complete revolution.
This advantage of giving an increased velocity may be obtained also by
the crank, by placing toothed wheels on its axle. Independently of the
greater expense attending the construction of the sun-and-planet wheel, its
liability to go out of order, and the rapid wear of the teeth, and other
objections, rendered it inferior to the crank, which has entirely superseded
it.

(115.)

Although by these contrivances Watt succeeded in obtaining a continuous
circular motion from the reciprocating motion of the steam engine, the
machine was still one of intermitting, instead of continuous action. The
expedient of a counterweight, elevated during the descending stroke, and
giving back the power expended on it in the interval of the returning stroke,
did not satisfy the [Pg189] fastidious mechanical taste of Watt. He soon
perceived that all which he proposed to accomplish by the application of
two cylinders and pistons working alternately, could be attained with
greater simplicity and effect by a single cylinder, if he could devise means
by which the piston might be impelled by steam upwards as well as
downwards. To accomplish this, it was only necessary to throw the lower
end of the cylinder into alternate communication with the boiler, while the
upper end would be put into communication with the condenser. If, for

example, during the descent of the piston, the upper end of the cylinder
communicated with the boiler, and the lower end with the condenser; and,
on the other hand, during the ascent of the piston, the lower end
communicated with the boiler, and the upper end with the condenser; then
the piston would be driven continually, whether upwards or downwards, by
the power of steam acting against a vacuum. Watt obtained his third patent
for this contrivance, on the 12th of March, 1782.
This change in the principle of the machine involved several other
changes in the details of its mechanism.

(116.)

It was necessary, in the first place, to provide means for
admitting and withdrawing the steam at either end of
the cylinder. For this purpose let B and B′ (fig. 33.) be
two steam-boxes, B the upper, and B′ the lower,
communicating respectively with the top and bottom of
the cylinder by proper passages D D′. Let two valves be
placed in B, one, S, above the passage D, and the other,
C, below it; and in like manner two other valves in the
lower valve-box, B′, one, S′, above the passage D′, and Fig. 33.
the other, C′, below it. Above the valve S in the upper
steam-box is an opening at which the steam-pipe from the boiler enters, and
below the valve C is another opening, at which enters the exhausting-pipe
leading to the condenser. In like manner, above the valve S′ in the lower
steam-box enters a steam-pipe leading from the boiler, and below the valve
C′ enters an exhausting-pipe leading to [Pg190] the condenser. It is evident,
therefore, that steam can always be admitted above the piston by opening
the valve S, and below it by opening the valve S′; and, in like manner, steam
can be withdrawn from the cylinder above the piston, and allowed to pass to
the condenser, by opening the valve C, and from below it by opening the
valve C′.
Supposing the piston P to be at the top of the cylinder, and the cylinder
below the piston to be filled with pure steam, let the valves S and C′ be

opened, the valves C and S′ being closed as represented
in fig. 34. Steam from the boiler will, therefore, flow in
through the open valve S, and will press the piston
downwards, while the steam that has filled the cylinder
below the piston will pass through the open valve C′
into the exhausting-pipe leading to the condenser, and
being condensed will leave the cylinder below the
piston a vacuum. The piston will, therefore, be pressed
downwards by the action of the steam above it, as in
the single-acting engine. Having arrived at the bottom
of the cylinder, let the valves S and C′ be both closed, Fig. 34.
and the valves S′ and C be opened, as represented in fig.
34. Steam will now be admitted through the open valve S′ and through the
passage D′ below the piston, while the steam which has just driven the
piston downwards, filling the cylinder above the piston, will be drawn off
through the open valve C, and the exhausting-pipe, into the condenser,
leaving the cylinder above the piston a vacuum. The piston will, therefore,
be pressed upwards by the action of the steam below it, against the vacuum
above it, and will ascend with the same force as that with which it had
descended.
This alternate action of the piston upwards and downwards may
evidently be continued by opening and closing the valves alternately in
pairs. Whenever the piston is at the top of the cylinder, as represented in fig.
33., the valves S and C′, that is, the upper steam-valve and the lower
exhausting-valve, are opened, and the valves C and S′, that is, the upper
exhausting-valve and the lower steam-valve, are closed; and [Pg191] when the
piston has arrived at the bottom of the cylinder, as represented in fig. 34.,
the valves C and S′, that is, the upper exhausting-valve and the lower steam-
valve, are opened, and the valves S and C′, that is, the upper steam-valve and
the lower exhausting-valve, are closed.
If these valves, as has been here supposed, be opened and closed at the
moments at which the piston reaches the top and bottom of the cylinder, it
is evident that they may be all worked by a single lever connected with

them by proper mechanism. When the piston arrives at the top of the
cylinder, this lever would be made to open the valves S and C′, and at the
same time to close the valves S′ and C; and when it arrives at the bottom of
the cylinder, it would be made to close the valves S and C′, and to open the
valves S′ and C.
If, however, it be desired to cut off the steam before the arrival of the
piston at the termination of its stroke, whether upwards or downwards, then
the steam-valves must be closed before the arrival of the piston at the end of
its stroke; and as the exhausting-valve ought to be left open until the stroke
is completed, these valves ought to be moved at different times. In that case
separate levers should be provided for the different valves. We shall,
however, return again to the subject of the valves which regulate the
admission of steam to the cylinder and its escape to the condenser.

(117.)

It will be remembered that in the single-acting engine the process of
condensation was suspended while the piston ascended in the cylinder, and
therefore the play of the jet of cold water in the condenser was stopped
during this interval. In the double-acting engine, however, the flow of steam
from the cylinder to the condenser is continued, whether the piston ascends
or descends, and therefore a constant condensation of steam must be
produced. The condensing jet, therefore, does not in this case, as in the
former, play with intervals of intermission. A constant jet of cold water
must be maintained in the condenser.
It will presently appear that in the double-acting engine applied to
manufactures, the motion of the piston was subject to more or less variation
of speed, and the quantity of steam [Pg192] admitted to the cylinder was subject
to a corresponding change. The quantity of steam, therefore, drawn into the
condenser was subject to variation, and required a considerable change in
the quantity of cold water admitted through the jet to condense it. To
regulate this, the valve or cock by which the water was admitted into the
condenser was worked in the double-acting engine by a lever furnished with
an index, by which the quantity of condensing water admitted into the

condenser could be regulated. This index played upon a graduated arch, by
which the engine-man was enabled to regulate the supply.

HEATHFIELD HOUSE, NEAR BIRMINGHAM,
THE RESIDENCE OF WATT.

FOOTNOTES:
[20] These effects are explained in my Treatise on Heat; and they have lately been verified
by experiments made with locomotive engines by M. de Pambour, who found that
the steam raised from the boiler of a locomotive engine, under a pressure of above 50
lbs. per square inch, was in the state of common steam as it issued from the chimney
at a very diminished pressure and at a lower pressure.
[21] It is strange that this absurdity has been repeatedly given as unquestionable fact in
various encyclopædias, as well as in by far the greater number of treatises expressly
on the subject.
[22] Farey, Treatise on the Steam Engine, p. 122.
[23] Farey on the Steam Engine, p. 297.

DOUBLE-ACTING ENGINE.—CITY SAW-MILLS.

CHAP. VIII.
[Pg193]

TOC INX

METHODS OF CONNECTING THE PISTON-ROD AND BEAM IN THE DOUBLE-ACTING
ENGINE.—RACK AND SECTOR.—PARALLEL MOTION.—CONNECTING ROD AND
CRANK.—FLY-WHEEL.—THROTTLE-VALVE.—GOVERNOR.—CONSTRUCTION
AND OPERATION OF THE DOUBLE-ACTING ENGINE.—ECCENTRIC.—COCKS
AND VALVES.—SINGLE-CLACK VALVE.—DOUBLE-CLACK VALVE.—CONICAL
VALVES.—SLIDE VALVES.—MURRAY'S SLIDES.—THE D VALVE.—SEAWARD'S
SLIDES.—SINGLE COCK.—FOUR-WAY COCK.—PISTONS.—COMMON HEMP-
PACKED PISTON.—WOOLFE'S PISTON.—METALLIC PISTONS.—CARTWRIGHT'S
ENGINE.—CARTWRIGHT'S PISTON.—BARTON'S PISTON.

(118.)

In the single-acting engine, the force of the piston acted on the beam only
during its descent; and this force was transmitted from the piston to the
beam, as we have seen, by a flexible chain, extended from the end of the
piston-rod, [Pg194] and playing upon the arch head of the beam. In the double-
acting engine, however, the force of the steam pressing the piston upwards
must likewise be transmitted to the beam, so as to drive the latter upwards
while the piston ascends. This action could not be accomplished by a chain
connecting the piston with the arch head of the beam.
Where the mechanical action to be transmitted is a pull, and not a push, a
flexible chain, cord, or strap, is sufficient; but if a push or thrust is required
to be transmitted, then the flexibility of the medium of mechanical
communication afforded by a chain renders it inapplicable. In the double-
acting engine, during the descent, the piston-rod still pulls the beam down;
and so far a chain connecting the piston-rod with the beam would be
sufficient to transmit the action of the one to the other; but in the ascent, the
beam no longer pulls up the piston-rod, but is pushed up by it. A chain from
the piston-rod to the arch head, as described in the single-acting engine,
would fail to transmit this force. If such a chain were used with the double
engine, where there is no counterweight on the opposite end of the beam,

the consequence would be, that in the ascent of the piston the chain would
slacken, and the beam would still remain depressed. It is therefore
necessary that some other mechanical connection be contrived between the
piston-rod and the beam, of such a nature that in the descent the piston-rod
may pull the beam down, and may push it up in the ascent.
Watt first proposed to effect this by attaching to the end of the
piston-rod a straight rack, faced with teeth, which should work in
corresponding teeth raised on the arch head of the beam, as
represented in fig. 35. If his improved steam engines required no
further precision of operation and construction than the
atmospheric engines, this might have been sufficient; but in these
engines it was indispensably necessary that the piston-rod should
Fig.
be guided with a smooth and even motion through the stuffing-
35.
box in the top of the cylinder, otherwise any shake or irregularity
would cause it to work loose in the stuffing-box, and either to
admit the air, or to let the steam escape. Under these circumstances, the
motion of [Pg195] the rack and toothed arch head were inadmissible, since it
was impossible by such means to impart to the piston-rod that smooth and
equable motion which was requisite. Another contrivance which occurred to
Watt was, to attach to the top of the piston-rod a bar, which should extend
above the beam, and to use two chains or straps, one extending from the top
of the bar to the lower end of the arch head, and the other from the bottom
of the bar to the upper end of the arch head. By such means the latter strap
would pull the beam down when the piston would descend, and the former
would pull the beam up when the piston would ascend. These contrivances,
however, were superseded by the celebrated mechanism since called the
Parallel Motion, one of the most ingenious mechanical combinations
connected with the history of the steam engine.

(119.)

It will be observed that the object was to connect by some inflexible means
the end of the piston-rod with the extremity of the beam, and so to contrive
the mechanism, that while the end of the beam would move alternately up
and down in part of a circle, the end of the piston-rod connected with the

beam should move up and down in a straight line. If the end of the piston-
rod were fastened upon the end of the beam by a pivot without any other
connection, it is evident that, being moved up and down in the arch of a
circle, it would be drawn to the left and the right alternately, and would
consequently either be broken or bent, or would work loose in the stuffing-
box. Instead of connecting the end of the rod immediately with the end of
the beam by a pivot, Watt proposed to connect them by certain moveable
rods, so arranged that, as the end of the beam would move up and down in
the circular arch, the rods would so accommodate themselves to that
motion, that the end connected with the piston-rod should not be disturbed
from its rectilinear course.
To explain the principle of the mechanism called the parallel motion, let
us suppose that O P (fig. 36.) is a rod or lever moveable on a centre O, and
that the end P of this rod shall move through a circular arch P P′ P″ P‴ a
vertical plane, and let its play be limited by two stops S, which shall prevent
its ascent above the point P, and its descent below [Pg196] the point P‴. Let the
position of the rod and the limitation of its play be such that the straight line
A B drawn through P and P‴, the extreme positions of the lever O P, shall be a
vertical line.

Fig. 36.

Let o be a point on the other side of the vertical line A B, and let the
distance of O to the right of A B be the same as the distance of o to the left of
A B. Let o p be a rod equal in length to O P, moving like O P on the centre o,
so that its [Pg197] extremity p shall play upwards and downwards through the
arch p p′ p″ p‴, its play being limited in like manner by stops s.

Now, let us suppose that the ends P p of these two rods are joined by a
link P p, the connection being made by a pivot, so that the angles formed by
the link and the rods shall be capable of changing their magnitude. This link
will make the motion of one rod depend on that of the other, since it will
preserve their extremities P p always at the same distance from each other.
If, therefore, we suppose the rod O P to be moved to the position O P‴, its
extremity P tracing the arch P P′ P″ P‴, the link connecting the rods will at the
same time drive the extremity p of the rod o p through the arch p p′ p″ p‴ so
that when the extremity of the one rod arrives at P‴, the extremity of the
other rod will arrive at p‴. By this arrangement, in the simultaneous motion
of the rods, whether upwards or downwards, through the circular arches to
which their play is limited, the extremities of the link joining them will
deviate from the vertical line A B in opposite directions. At the limits of their
play, the extremities of the link will always be in the line A B; but in all
intermediate positions, the lower extremity of the link will be to the right of
A B, and its upper extremity to the left of A B. So far as the derangement of
the lower extremity of the link is concerned, the matter composing the link
would be transferred to the right of A B, and so far as the upper extremity of
the link is concerned, the matter composing it would be transferred to the
left of A B.
By the combined effects of these contrary derangements of the
extremities of the link from the vertical line, it might be expected that a
point would exist, in the middle of the link, where the two contrary
derangements would neutralise each other, and which point would therefore
be expected to be disturbed neither to the right nor to the left, but to be
moved upwards and downwards in the vertical line A B. Such is the
principle of the parallel motion; and in fact the middle point of the link will
move for all practical purposes accurately in the vertical line A B, provided
that the angular play of the levers O P and o p does not exceed a certain [Pg198]
limit, within which, in practice, their motion may always be restrained.
To trace the motion of the middle point of the link more minutely, let P P′
P″ P‴ be four positions of the lever O P, and let p p′ p″ p‴ be the four
corresponding positions of the lever o p. In the positions O P o p, the link

will take the position P p, in which the entire link will be vertical, and its
middle point x will therefore be in the vertical line A B.
When the one rod takes the position O P′, the other rod will have the
position o p′; and the link will have the position P′ p′. The middle point of
the link will be at x′, which will be found to be on the vertical line A B. Thus
one half of the link P′ x′ will be to the left of the vertical line A B; while the
other half, p′ x′, will be to the right of the vertical line; the derangement
from the vertical line affecting each half of the link in contrary directions.
Again, taking the one rod in the position O P″, the corresponding position
of the other rod will be o p″, and the position of the link will be P″ p″. If the
middle point of the link in this position be taken, it will be found to be at x″,
on the vertical line A B; and, as before, one half of the link P″ x″ will be
thrown to the left of the vertical line, while the other half p″ x″, will be
thrown to the right of the vertical line.
Finally, let the one rod be in its lowest position, O P‴, while the other rod
shall take the corresponding position, o p‴. The direction of the link P‴ p‴
will now coincide with the vertical line; and its middle point x‴ will
therefore be upon that line. The previous derangement of the extremities of
the rod, to the right and to the left, are now redressed, and all the parts of
the rod have assumed the vertical position.
It is plain, therefore, that by such means the alternate motion of a point
such as P or p, upwards and downwards in a circular arch, may be made to
produce the alternate motions of another point x, upwards and downwards
in a straight line.

(120.)

Although the guidance of the air-pump rod in a true vertical line is not so
necessary as that of the steam piston, [Pg199] and as the air-pump piston is
always brought down by its own weight and that of its rod, the connection
of the air-pump piston-rod with the beam, by any contrivance of the kind
now described, was not so necessary. Nevertheless, by a slight addition to
the mechanical contrivance which has been just described, Watt obtained

the means of at once preserving the true rectilinear motion of both piston-
rods.

Fig. 37.

Let the lever represented by O P in fig. 36. be conceived to be prolonged
to twice its length, as represented in fig. 37., so that O P′ shall be twice O P.
Let the points P p be connected by a link as before. Let a link P′ x′, equal in
length to the link P p be attached to the point P′, and let the extremity x′ of
this link be connected with the point p by another link, equal in length to P
P′, by pivots at x′ and p, so that the figure P P′ x′ p shall be a jointed
parallelogram, the angles of which will be capable of altering their
magnitude with every change of position of the rods o p and O P. Thus,
when the rod O P descends, the angles of the parallelogram at P and x′ will
be diminished in magnitude, while the angles at P′ and p will be increased in
magnitude. Now, let a line be conceived to be drawn from O to x′. It is
evident that that line will pass through the middle point of the link p P, for
the triangle O P x is in all respects similar to the greater triangle O P′ x′ only
on half the scale, so that every side of the one is [Pg200] half the corresponding
side of the other. Therefore P x is half the length of P′ x′; but P′ x′ was made

equal to P p, and therefore p x is half of P p, that is to say, x is the middle
point of P p.
It has been already shown, that in the alternate motion of the rods o p, O P
in ascending and descending, the point x is moved upwards and downwards
in a true vertical line. Now since the triangle O P x is in all respects similar
to O P′ x′, and subject to a similar motion during the ascent and descent of
the rods, it is apparent that the point x′ must be subject to a motion in all
respects similar to that which affects the points x, except that the point x′
will move through double the space. In fact, the principle of the mechanism
is precisely similar to that of the common pantograph, where two rods are
so connected as that the motion of the one governs the motion of the other,
so that whatever line or figure may be described by one, a similar line or
figure must be described by the other. Since, then, the point x is moved
upwards and downwards in a vertical straight line, the point x′ will also be
moved in a vertical straight line of double the length.
If such an arrangement of mechanism as has been here described can be
connected with the beam of the steam engine, so that while the point x′ is
attached to the top of the steam piston, and the space through which it
ascends and descends shall be equal to the length of the stroke of that
piston, the point x shall be attached to the rod of the air-pump piston, the
stroke of the latter being half that of the steam piston, then the points x′ and
x will guide the motion of the two pistons so as to preserve them in true
vertical straight lines.
The manner in which these ideas are reduced to practice admits of easy
explanation: let the point O be the centre of the great working beam, and let
O P′ be the arm of the beam on the side of the steam cylinder. Let P be a
pivot upon the beam, at the middle point between its centre O and its
extremity P′; and let the links P p, P′ x′, and P p be jointed together, as
already described. Let the point or pivot o be attached to some part of the
fixed framing of the engine or engine house, and let the rod o p, equal to
half the arm of the beam, be attached by a pivot to the corner of the
parallelogram at [Pg201] p. Let the end of the steam piston-rod be attached to
the corner of the parallelogram x′, and let the end of the air-pump be

attached to the middle point x of the link P p; by which arrangement it is
evident that the rectilinear motion of the two piston-rods will be rendered
compatible with the alternate circular motions of the points P′ and P on the
beam.
Among the many mechanical inventions produced by the fertile genius of
Watt, there is none which has excited such universal, such unqualified, and
such merited admiration as that of the parallel motion. It is indeed
impossible, even for an eye unaccustomed to view mechanical
combinations, to behold the beam of a steam engine moving the pistons,
through the instrumentality of the parallel motion, without an instinctive
feeling of pleasure at the unexpected fulfilment of an end by means having
so little apparent connection with it. When this feeling was expressed to
Watt himself, by those who first beheld the performance of this exquisite
mechanism, he exclaimed with his usual vivacity, that he himself, when he
first beheld his own contrivance in action, was affected by the same sense
of pleasure and surprise at its regularity and precision. He said, that he
received from it the same species of enjoyment that usually accompanies
the first view of the successful invention of another person.
"Among the parts composing the steam engine, you have doubtless," says
M. Arago, "observed a certain articulated parallelogram. At each ascent and
descent of the piston, its angles open and close with the sweetness—I had
almost said with the grace—which charms you in the gestures of a
consummate actor. Follow with your eye alternately the progress of its
successive changes, and you will find them subject to the most curious
geometrical conditions. You will see, that of the four angles of the jointed
parallelogram, three describe circular arches, but the fourth which holds the
piston-rod is moved nearly in a straight line. The immense utility of this
result strikes mechanicians with even less force than the simplicity of the
means by which Watt has attained it."
The parallel motion, of which there are several other varieties,
depending, however, generally upon the same [Pg202] principle, formed part of
a patent which Mr. Watt obtained in the year 1784, another part of which
patent was for a locomotive engine, by which a carriage was to be propelled

on a road. In a letter to Mr. Smeaton dated 22d October, in the same year,
Watt says,—
"I have lately contrived several methods of getting entirely rid of all the
chains and circular arches about the great levers of steam engines, and
nevertheless making the piston-rods ascend and descend perpendicularly,
without any sliding motions or right-lined guides, merely by combinations
of motions about centres; and with this further advantage, that they answer
equally well to push upwards as to pull downwards, so that this method is
applicable to our double engines which act both in the ascent and descent of
their pistons.
"A rotative engine of this species with the new motion which is now at
work in our manufactory (but must be sent away very soon) answers
admirably. It has cost much brain work to contrive proper working gear for
these double engines, but I have at last done it tolerably well, by means of
the circular valves, placed in an inverted position, so as to be opened by the
force of the steam; and they are kept shut by the working gear. We have
erected an engine at Messrs. Goodwyne and Co.'s brewery, East Smithfield,
London."

(121.)

By the contrivance which has been explained above, the force of the piston
in ascending and descending would be conveyed to the working end of the
beam; and the next problem which Watt had to solve was, to produce by the
force exerted by the working end of the beam in ascending and descending
a continuous motion of rotation. In the first instance he proposed to
accomplish this by a crank placed upon the axle to which rotation was to be
imparted, and driven by a rod connecting it with the working end of the
beam. Let K (fig. 38.) be the centre, to which motion is to be imparted by
the working end H of the beam. On the axle K suppose a short lever K I to be
fixed so that when K I is turned round the centre K, the axle must turn with
it. Let an iron rod, the weight of which shall balance the piston and piston-
rod at the other end of the beam, be connected by joints with the working
end H of the beam, and the extremity I of the [Pg203] lever K I. As the end H of

the beam is moved upwards and
downwards, the lever K I will be
turned round the centre K, taking
successively the positions
represented by faint lines in the
figure; and thus a motion of
continued rotation will be imparted
to the axle K.
This simple and effectual
expedient of producing a continued
rotatory motion by a crank was
abandoned by Watt, as already
explained, by reason of a patent
having been obtained upon
information of his experiments
surreptitiously procured. To avoid
litigation, he therefore substituted
for the crank the sun and planet
wheel already described; but at the
expiration of the patent, which
restricted the use of the crank, the
sun and planet wheel was
discontinued in Watt's engine, and
the crank restored.

(122.) Fig. 38.

Whether the crank or the sun and planet wheel be used, there is still a
difficulty in the maintenance of a regular motion of rotation. In the various
positions which the crank and connecting rod assume throughout a
complete revolution, there are two in which the moving power loses all
influence in impelling the crank. These positions are those which the crank
assumes when the piston is at the top and bottom of the [Pg204] cylinder, and is
just about to change the direction of its motion. When the piston is at the
bottom of the cylinder, the pivot I (fig. 38.), by which the connecting rod H I

is attached to the end of the crank, is immediately over the axle K of the
crank, and under the pivot H, which joins the upper end of the connecting
rod with the beam. In fact, in this position the connecting rod and crank are
in the same straight line, extending from the end of the beam to the axle of
the crank. The steam, on entering the cylinder below the piston, and
pressing it upwards, would produce a corresponding downward force on the
connecting rod at H, which would be continued along the connecting rod
and crank to the axle K. It is evident that such a force could have no
tendency to turn the crank round, but would expend its whole energy in
pressing the axle K downwards.
The other position in which the power loses its effect upon the crank is
when the piston is at the top of the cylinder. In this case, the working end of
the beam will be at the lowest point of its play, and the crank-pin I will be
immediately below the axle K; so that K will be placed immediately between
H and I. When the steam presses on the top of the piston, it will expend its
force in drawing the end H of the connecting rod upwards, by which the
crank-pin I will likewise be drawn upwards. It is evident that this force can
have no effect in turning the crank round, but will expend its whole energy
in producing an upward strain on the axle K.
If the crank were absolutely at rest in either of the positions above
described, it is apparent that the engine could not be put in motion by the
steam; but if the engine has been previously in motion, then the mass of
matter forming the crank, and the axle on which the crank is formed, having
already had a motion of rotation, will have a tendency to preserve the
momentum it has received, and this tendency will be sufficient to throw the
crank K I out of either of those critical positions which have been described.
Having once escaped these dead points, then the connecting rod forming an
angle, however obtuse or acute, with the crank, the pressure or pull upon
the former will have a tendency to produce rotation in the latter. As the
crank revolves, however, the influence [Pg205] of the connecting rod upon it
will vary according to the angle formed by the connecting rod and crank.
When that angle is a right angle, then the effect of the connecting rod on the
crank is greatest, since the force upon it has the advantage of the whole

leverage of the crank; but according as the angle formed by the crank and
connecting rod becomes more or less acute or obtuse in the successive
attitudes which they assume in the revolution of the crank, the influence of
the connecting rod over the crank varies, changing from nothing at the two
dead points already described, to the full effect produced in the two
positions where they are at right angles. In consequence of this varying
leverage, by which the force with which the connecting rod is driven by the
steam is transmitted to the axle on which the crank revolves, a
corresponding variation of speed would necessarily be produced in the
motion imparted to the crank. The speed at the dead points would be least,
being due altogether to the momentum already imparted to the revolving
mass of the crank and axle; and it would gradually increase and be greatest
at the points where the effect of the crank on the connecting rod is greatest.
Although this change of speed would not affect the actual mechanical
efficacy of the machine, and although the same quantity of steam would
perform the same work at the varying velocity as it would do if the velocity
were regulated, yet this variation of speed would be incompatible with the
purposes to which it was now proposed that the steam engine should be
applied in manufactures. In these a regular uniform motion should be
imparted to the main axle.

(123.)

One of the expedients which Watt proposed for the attainment of this end
was, by placing two cranks on the same axle, in different positions, to be
worked by different cylinders, so that while one crank should be at its dead
points, the other should be in the attitude most favourable for its action.
This expedient has since, as we shall see, been carried into effect in steam
vessels; but one more simple and efficient presented itself in the use of a
fly-wheel.
On the main axle driven by the crank Watt placed a large wheel of metal,
as represented in fig. 43., called a fly-wheel. This wheel being well
constructed, and nicely balanced on its [Pg206] axle, was subject to very little
resistance from friction; any moving force which it would receive it would
therefore retain, and would be ready to impart such moving force to the

main axle whenever that axle ceased to be driven by the power. When the
crank, therefore, is in those positions in which the action of the power upon
it is most efficient, a portion of the energy of the power is expended in
increasing the velocity of the mass of matter composing the fly-wheel. As
the crank approaches the dead points, the effect of the moving power upon
the axle and upon the crank is gradually enfeebled, and at these points
vanishes altogether. The momentum which has been imparted to the fly-
wheel then comes into play, and carries forward the axle and crank out of
the dead points with a velocity very little less than that which it had when
the crank was in the most favourable position for receiving the action of the
moving power.
By this expedient, the motion of revolution received by the axle from the
steam piston is subject to no other variation than just the amount of change
of momentum in the great mass of the fly-wheel, which is sufficient to
extricate the crank twice in every revolution from the mechanical dilemma
to which its peculiar form exposes it; and this change of velocity may be
reduced to as small an amount as can be requisite by giving the necessary
weight and magnitude to the fly-wheel.

(124.)

By such arrangements the motion imparted to the main axle K would be
uniform, provided that the moving power of the engine be always
proportionate to the load which it drives. But in the general application of
the steam engine to manufactures it was evident that the amount of the
resistance to which any given machine would be subject must be liable to
variation. If, for example, the engine drive a cotton-mill, it will have to
impart motion to all the spinning frames in that mill. The operation of one
or more of these may from time to time be suspended, and the moving
power would be relieved from a corresponding amount of resistance. If,
under such circumstances, the energy of the moving power remained the
same, the velocity with which the machines would be driven would be
subject to variation, being increased whenever the operation of any portion
of the machines usually [Pg207] driven by it is suspended; and, on the other
hand, diminished when any increased number of machines are brought into

operation. In fine, the speed would vary nearly in the inverse proportion of
the load driven, increasing as the load is diminished, and vice versâ.
On the other hand, supposing that no change took place in the amount of
the load driven by the engine, and that the same number of machines of
whatever kind would have to be continually driven, the motion imparted to
the main axle would still be subject to variation by the changes inevitable to
the moving power. The piston of the engine being subject to an unvaried
resistance, a uniform motion could only be imparted to it, by maintaining a
corresponding uniformity in the impelling power. This would require a
uniform supply of steam from the boiler, which would further imply a
uniform rate of evaporation in the boiler, unless means were provided in the
admission of steam from the boiler to the cylinder to prevent any excess of
steam which might be produced in the boiler from reaching the cylinder.
This end was attained by a contrivance afterwards
called the throttle-valve. An axis A B (figs. 39, 40.) was
placed across the steam pipe in a ring of cast-iron D E, of
proper thickness. On this axis was fastened a thin circular
plate T, of nearly the same diameter as the steam pipe. On
Fig. 39. the outer end B of this axle was placed a short lever or
handle B C, by which it could be turned. When the
circular plate T was turned into such a position as to be
at right angles to the length of the tube, it stopped the
passage within the tube altogether, so that no steam
could pass from the boiler to the engine. On the other
hand, when the handle was turned through a fourth of a
revolution from this position, then the circular plate T
had its plane in the direction of the length of the tube,
so that its edge would be presented towards the current
Fig. 40. of steam flowing from the boiler to the cylinder. In that
position the passage within the tube [Pg208] would be
necessarily unobstructed by the throttle-valve. In intermediate positions of
the valve, as that represented in figs. 39, 40., the passage might be left more
or less opened, so that steam from the boiler might be admitted to the

cylinder in any regulated quantity according to the position given to the
lever B C.
A view of the throttle-valve taken by a section across the steam pipe is
exhibited in fig. 40., and a section of it through the axis of the steam pipe is
represented in fig. 39. The form of the valve is such, that, if accurately
constructed, the steam in passing from the boiler would have no effect by its
pressure to alter any position which might be given to the valve; and any
slight inaccuracy of form which might give a tendency to the steam to alter
the position would be easily counteracted by the friction of the valve upon
its axle. The latter might be regulated at pleasure.
By this expedient, however the evaporation of water in the boiler might
vary within practical limits, the supply of steam to the cylinder would be
rendered regular and uniform. If the boiler became too active, and produced
more steam than was necessary to move the engine with its load at the
requisite speed, then the throttle-valve was shifted so as to contract the
passage and limit the supply of steam. If, on the other hand, the process of
evaporation in the boiler was relaxed, then the throttle-valve was placed
with its edge more directed towards the steam. Independently of the boiler,
if the load on the engine was lightened, then the same supply of steam to
the cylinder would unduly accelerate the motion. In this case, likewise, the
partial closing of the throttle-valve would limit the supply of steam and
regulate the motion; and if, on the other hand, the increase of load upon the
engine rendered necessary an increased supply of steam, then the opening
of the throttle-valve would accomplish the purpose. By these means,
therefore, a uniform motion might be maintained, provided the vigilance of
the engine-man was sufficient for the due management of the lever B C, and
provided that the furnace under the boiler was kept in sufficient activity to
supply the greatest amount of steam which would be necessary [Pg209] for the
maintenance of a uniform motion with the throttle-valve fully opened.

(125.)

Watt, however, soon perceived that the proper manipulation of the lever B C
would be impracticable with any degree of vigilance and skill which could
be obtained from the persons employed to attend the engine. He, therefore,

adapted to this purpose a beautiful application of a piece of mechanism,
which had been previously used in the regulation of mill-work, and which
has since been well known by the name of the Governor, and has always
been deservedly a subject of much admiration.
The governor is an apparatus by which the axle of the fly-wheel is made
to regulate the throttle-valve, so that the moment that the axle begins to
increase its velocity, it shifts the position of the throttle-valve, so as to limit
the supply of steam from the boiler, and thereby to check the increase of
speed. And on the other hand, whenever the velocity of the axle is
diminished, the lever B C is moved in the contrary direction, so as to open
more fully the passage for the steam, and accelerate the motion of the
engine.
A small grooved wheel A B (fig. 41.) is attached to a vertical spindle
supported in pivots or sockets C and D, in which it is capable of revolving.
An endless cord works in the groove A B, and is carried over proper pulleys
to the axle of the fly-wheel, where it likewise works in a groove. When this
cord is properly tightened the motion of the fly-wheel will give motion to
the wheel A B, so that the velocity of the one will be subject to all the
changes incidental to the velocity of the other. By this means the speed of
the grooved wheel A B may be considered as representing the speed of the
fly-wheel, and of the machinery which the axle of the fly-wheel drives.

Fig. 41.

It is evident that the same end might be attained by substituting for the
grooved wheel A B a toothed wheel, which might be connected by other
toothed wheels, and proper shafts, and axles with the axle of the fly-wheel.
A ring or collar E is placed on the upright spindle, so as to be capable of
moving freely upwards and downwards. To this ring are attached by pivots
two short levers, E F, the [Pg210] pivots or joints at E allowing these levers to
play upon them. At F these levers are joined by pivots to other levers F G,
which cross each other at H, where an axle or pin passes through them, and
attaches them to the upright spindle C D. These intersecting levers are
capable, however, of playing on this axle or pin H. To the ends G of these
levers are attached two heavy balls of metal I. The levers F G pass through
slits in a metallic arch attached to the upright spindle, so as to be capable of
revolving upon it. If the balls I are drawn outwards from the vertical axis, it
is evident that the ends F of the levers will be drawn down, and therefore the
pivots E likewise drawn down. In fact, the angles E F H will become more
acute, and the angle F E F more obtuse. By these means the sliding ring E
will be drawn down. To this sliding ring E, and immediately above it, is
attached a grooved collar, which slides on the vertical spindle upwards and

downwards with the ring E. In the grooved collar are inserted the prongs of
a fork K, formed at the end of the lever K L, the fulcrum or pivot of the lever
being at L. By this arrangement, when the divergence of the balls I causes
the collar E to be drawn down, the fork K, whose prongs are inserted in the
groove of that collar, is likewise drawn down; and, on the other hand, when,
by reason of the balls I falling towards the [Pg211] vertical spindle, the collar E
is raised, the fork K is likewise raised.
The ascent and descent of the fork K necessarily produce a contrary
motion in the other end N of the lever. This end is connected by a rod, or
system of rods, with the end M of the short lever which works the throttle-
valve T. By such means the motion of the balls I, towards or from the
vertical spindle, produces in the throttle-valve a corresponding motion; and
they are so connected that the divergence of the balls I will cause the
throttle-valve to close, while their descent towards the vertical spindle will
cause it to open.
These arrangements being comprehended, let us suppose that, either by
reason of a diminished load upon the engine or an increased activity of the
boiler, the speed has a tendency to increase. This would impart increased
velocity to the grooved wheel A B, which would cause the balls I to revolve
with an accelerated speed. The centrifugal force which attends their motion
would therefore give them a tendency to move from the axle, or to diverge.
This would cause, by the means already explained, the throttle-valve T to be
partially closed, by which the supply of steam from the boiler to the
cylinder would be diminished, and the energy of the moving power,
therefore, mitigated. The undue increase of speed would thereby be
prevented.
If, on the other hand, either by an increase of the load, or a diminished
activity in the boiler, the speed of the machine was lessened, a
corresponding diminution of velocity would take place in the grooved
wheel A B. This would cause the balls I to revolve with less speed, and the
centrifugal force produced by their circular motion would be diminished.
This force being thus no longer able fully to counteract their gravity, they
would fall towards the spindle, which would cause, as already explained,

the throttle-valve to be more fully opened. This would produce a more
ample supply of steam to the cylinder, by which the velocity of the machine
would be restored to its proper amount.

Fig. 42.

(126.)

The principle which renders the governor so perfect a regulator of the
velocity of the machine is difficult to be [Pg212] explained without having
recourse to the aid of the technical language of mathematical physics. As,
however, this instrument is of such great practical importance, and has
attracted such general admiration, it may be worth while here to attempt to
render intelligible the mechanical principles which govern its operation. Let
S (fig. 42.) be the point of suspension of a common pendulum S P, and let P O
P′ be the arch of its vibration, so that the ball P shall swing or vibrate
alternately to the east and to the west of the lowest point O, through the
arches O P′ and O P. It is a property of such an instrument that, provided the
arch in which it vibrates be not considerable in magnitude, the time of its
vibration will be the same whether the arch be long or short. Thus, for
example, if the pendulum, instead of vibrating in the arch P P′, vibrated in
the arch p p′, the time which it would take to perform its vibrations would
be the same. If, however, the magnitude of the arch of vibration be

increased, then a variation will take place in the time of vibration; but
unless the arch of vibration be considerably increased, this variation will
not be great.
Now let it be supposed that while the pendulum P P′ continues to vibrate
east and west through the arch P P′, it shall receive such an impulse from
north and south as would, if it were not in a state of previous vibration,
cause it to vibrate between north and south, in an arch similar to the arch P
P′. This second vibration between north and south [Pg213] would not prevent the
continuance of the other vibration between east and west; but the ball P
would be at the same time affected by both vibrations. While, in virtue of
the vibration from east to west, the ball would swing from P to P′, it would,
in virtue of the other vibration, extend its motion towards the north to a
distance from the line W E equal to half a vibration, and will return from that
distance again to the position P′. While returning from P′ to P, its second
vibration will carry it towards the south to an equal distance on the southern
side of W E, and it will return again to the position P. If the combination of
these two motions or vibrations be attentively considered, it will be
perceived that the effect on the ball will be a circular motion, precisely
similar to the circular motion of the balls of the governor already described.
Now the time of vibration of the pendulum S P between east and west will
not in any way be affected by the second vibration, which it is supposed to
receive between north and south, and therefore the time the pendulum takes
in moving from P to P′ and back again from P′ to P will be the same whether
it shall have simultaneously or not the other vibration between north and
south. Hence it follows that the time of revolution of the circular pendulum
will be equal to the time of similar vibrations of the same pendulum, if,
instead of having a circular motion, it were allowed to vibrate in the manner
of a common pendulum.
If this point be understood, and if it also be remembered that the time of
vibration of a common pendulum is necessarily the same whether the arch
of vibration be small or great, it will be easily perceived that the revolving
pendulum or governor will have nearly the same time of revolution whether
it revolve in a large circle or a small one: in other words, whether the balls

revolve at a greater or a less distance from the central spindle or axis. This,
however, is to be understood only approximately. When the angle of
divergence of the balls is as considerable as it usually is in governors, the
time of revolution at different distances from the axis will therefore be
subject to some variation, but to a very small one. [Pg214]
The centrifugal force (which is the name given in mechanics to that
influence which makes a body revolving in a circle fly from the centre)
depends conjointly on the velocity of revolution, and on the distance of the
revolving body from the centre of the circle. If the velocity of revolution be
the same, then the centrifugal force will increase in the same proportion as
the distance of the revolving body from the centre. If, on the other hand, the
distance of the revolving body from the centre remain the same, the
centrifugal force will increase in the same proportion as the square of the
time of vibration diminishes, or, in other words, it will increase in the same
proportion as the square of the number of revolutions per minute. It follows
from this, therefore, that the greater is the divergence of the balls of the
governor, and the more rapidly they revolve, the greater will be their
centrifugal force. Now this centrifugal force, if it were not counterbalanced,
would give the balls a constant tendency to recede from the centre; but from
the construction of the apparatus, the further they are removed from the
centre the greater will be the effect of their gravitation in resisting the
centrifugal force.
It is evident that the ball at P will have a greater tendency to fall by
gravitation towards O than it would have at p, because the acclivity of the
arch descending towards O at P is greater than its acclivity at p. The
gravitation, therefore, or tendency of the ball to fall towards the central axis
being greater at P than at p it will be able to resist a greater centrifugal
force. This increased centrifugal force, which the ball would have revolving
at the distance P above what it would have at the distance p, is produced
partly by the greater distance of the ball from the central axis, and partly by
the greater velocity of its motion. But it will be evident that the time of its
revolution may nevertheless be the same, or nearly the same, at both
distances. If it should appear that the actual velocity of its motion of
revolution at P be greater than its velocity at p, in the same proportion as the

circles in which they revolve, then it is evident that the time of revolution
would be as much increased by the greater space which P will have to travel
over, as it will have to be [Pg215] diminished by the greater speed with which
that space is traversed. The time of revolution, therefore, may be the same,
or nearly the same, in both cases.
If this explanation be comprehended, it will not be difficult to apply it to
the actual case of the governor. If a sudden increase of the energy of the
moving power, or a diminution of the load, should give the machine an
increased velocity, then the increased speed of the balls of the governor will
give them an increased centrifugal force, which for the moment will be
greater than the tendency of their gravitation to make them fall towards the
vertical axis. This centrifugal force, therefore, prevailing, the balls will
recede from the axis; but as they recede, their gravitation towards the
vertical axis will, as has been already explained, be increased, and will
become equal to the centrifugal force produced by the increased velocity,
provided that velocity do not exceed a certain limit. When the balls, by
diverging, get such increased gravitation as to balance the centrifugal force,
then they will continue to revolve at a fixed distance from the vertical axis.
When this happens, the time of the revolution must be nearly the same as it
was before their increased divergence; in other words, the proportion of the
moving power to the load will be so restored by the action of the levers of
the governor on the throttle-valve that the machine will move at its former
velocity, or nearly so.
The principle on which the governor acts, as just explained, necessarily
supposes temporary disarrangements of the speed. In fact, the governor,
strictly speaking, does not maintain a uniform velocity, but restores it after
it has been disturbed. When a sudden change of motion of the engine takes
place, the governor being immediately affected will cause a corresponding
alteration in the throttle-valve; and this will not merely correct the change
of motion, but it will, as it were, overdo it, and will cause a derangement of
speed of the opposite kind. Thus if the speed be suddenly increased to an
undue amount, then the governor being affected will first close the throttle-
valve too much, so as to reduce the speed below the proper limit. This
second error will again affect the governor in the contrary way, and the

speed [Pg216] will again be increased rather too much. In this way a succession
of alterations of effect will ensue until the governor settles down into that
position in which it will maintain the engine at the proper speed.
To prevent the inconvenience which would attend any excess of such
variations, the governor is made to act with great delicacy on the throttle-
valve, so that even a considerable change in the divergence of the balls shall
not produce too much alteration in the opening of that valve: the steam in
the boiler should have at least 2 lbs. per square inch pressure more than is
generally required in the cylinder. This excess is necessary to afford scope
for that extent of variation of the power which it is the duty of the throttle-
valve to regulate.
The governor is usually so adjusted as to make thirty-six revolutions per
minute, when in uniform motion; but if the motion is increased to the rate of
thirty-nine revolutions, the balls will fly to the utmost extent allowed them,
being the limitation of the grooves in which their rods move; and if, on the
other hand, the speed be diminished to thirty-four revolutions per minute,
they will collapse to the lowest extent of their play. The duty of the
governor, therefore, is to correct smaller casual derangements of the
velocity; but if any permanent change to a considerable extent be made
either in the load driven by the machine or in the moving power supplied to
it from the boiler, then a permanent change is necessary to be made in the
connection between the governor and the throttle-valve, so as to render the
governor capable of regulating those smaller changes to which the speed of
the machine is liable.

(127.)

Having thus explained the principal mechanical contrivances provided by
Watt for the maintenance and regulation of the rotatory motion to be
produced by his double-acting steam engine, let us now consider the
machine as a whole, and investigate the process of its operation. A section
of this engine is represented in fig. 43.

Fig. 43.

Steam is supplied from the boiler to the cylinder by the steam pipe S. The
throttle-valve T in that pipe, near the cylinder, is regulated by a system of
levers connected with [Pg217] the governor. The piston P is accurately fitted in
the steam cylinder C by packing, as already described in the single-acting
engine. This piston, as it moves, divides the cylinder into two
compartments, between which there is no communication by which steam
or any other elastic fluid can pass. The upper steam box B is divided into
three compartments by the two valves. Above the upper steam valve V is a
compartment communicating with the steam pipe; below the upper
exhausting valve E is another compartment communicating with the

eduction pipe which leads to the condenser. By the valves V and E a
communication may be opened or closed between the boiler on the one
hand, or the condenser on the other, and the top of the cylinder. The
continuation S′ of the steam pipe leads to the lower box B′, which, like the
upper, is divided into three compartments by two valves V′ and E′. The upper
compartment communicates with the steam pipe, and thereby with the
boiler; and the lower compartment communicates with the eduction pipe,
and thereby with the condenser. By means of the two valves V′ and E′, a
communication may be opened or closed between the steam pipe on the one
hand, or the exhausting pipe on the other, and the lower part of the cylinder.
The four valves V, E, V′, and E′ are connected by a system of levers with a
handle or spanner m, which, being driven downwards or upwards, is
capable of opening or closing the valves in pairs, in the manner already
described (116.). The condensers, the air-pump, and the hot-water pump,
are in all respects similar to those already described in the single-acting
engine, except that the condensing jet is governed by a lever I, by which it
is allowed to play continually in the condenser, and by which the quantity
of water admitted through it is regulated. The cold-water pump N is worked
by the engine as already described in the single-acting engine, and supplies
the cistern in which the air-pump and condenser are submerged, so as to
keep down its temperature to the proper limit. On the air-pump rod R are
two pins properly placed, so as to strike the spanner m, upwards and
downwards, at the proper times, when the piston approaches the termination
of the stroke at the top or bottom of the cylinder. The pump L [Pg218] conducts
the warm water drawn by the air-pump from the condenser to a proper
reservoir for feeding the boiler. The vertical motion of the piston-rod in a
straight line is rendered compatible with the circular motion of the end of
the beam by the parallel motion already described. The point b, on the
beam, moves upwards and downwards in a circular arch, of which the axis
of the beam is the centre. In like manner the point d of the rod d c moves
upwards and downwards, in a similar arch of which the fixed pivot c is the
centre. The joint or bar d b, which joins these two pivots, will be moved so
that its middle point e will ascend and descend nearly in a straight line, as
has been already explained (120.); [Pg219] opposite this point e is attached the

piston-rod of the air-pump, which is accordingly guided upwards and
downwards by this means. The jointed parallelogram b d g f is attached to
the beam by pivots; and, as has been explained (120.), the point g will be
moved upwards and downwards in a straight line, through twice the space
through which the point e is moved. To the point g the rod of the steam
piston is attached. Thus, the rods of the steam piston and air-pump are
moved by the same system of jointed bars, and moved through spaces
which are in the proportion of two to one.
Although this system of jointed rods forming the parallel motion, appears
in the figure to consist only of one parallelogram b d g f, and one rod c d,
called the radius rod, it is, in fact, double, a similar parallelogram and
radius rod being attached to corresponding points, and in the same manner
on the other side of the beam; but from the view given in the cut, the one set
of rods hides the other. The two systems of rods thus attached to opposite
sides of the beam at several inches asunder, are connected by cross rods, the
ends of which form the pivots or joints, and extend between the
parallelograms. The ends of these rods are only visible in the figure. It is to
the middle of one of these rods, the end of which is represented at e, that the
air-pump piston-rod is attached; and it is to the middle of another, the end of
which is represented at g, that the steam piston-rod is attached. These two
piston-rods, therefore, are driven, not immediately by either of the
parallelograms forming the parallel motion, but by the bars extending
between them.
To the working end of the beam H is attached a rod of cast-iron O, called
the connecting rod, the lower end of which is attached to the crank by a
pivot. The weight of the connecting rod is so made, that it shall balance the
weight of the piston-rods of the air-pump and cylinder on the other side of
the beam; and the weight of the piston-rod of the cold-water pump N nearly
balances the weight of the piston-rod of the hot-water pump L. Thus, so far
as the weights of the machinery are concerned, the engine is in equilibrium,
and the piston would rest in any position indifferently in the cylinder.
The axis of the fly-wheel on which the crank is formed is [Pg220] square in
the middle part, where the fly-wheel is attached to it, but has cylindrical
necks at each end, which rest in sockets or bearings supported by the