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Title: How it Works

Author: Archibald Williams

Release date: April 10, 2009 [eBook #28553]
Most recently updated: January 4, 2021

Language: English

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WORKS ***

Transcriber’s Note

The punctuation and spelling from the original text have been
faithfully preserved. Only obvious typographical errors have been
corrected.

HOW IT WORKS

AUTHOR'S NOTE.

I beg to thank the following gentlemen and firms for the help they have
given me in connection with the letterpress and illustrations of "How It
Works"—

Messrs. F.J.C. Pole and M.G. Tweedie (for revision of MS.); W.
Lineham; J.F. Kendall; E. Edser; A.D. Helps; J. Limb; The Edison Bell
Phonograph Co.; Messrs. Holmes and Co.; The Pelton Wheel Co.; Messrs.
Babcock and Wilcox; Messrs. Siebe, Gorman, and Co.; Messrs. Negretti
and Zambra; Messrs. Chubb; The Yale Lock Co.; The Micrometer
Engineering Co.; Messrs. Marshall and Sons; The Maignen Filter Co.;
Messrs. Broadwood and Co.

ON THE FOOTPLATE OF A LOCOMOTIVE.

How It Works
Dealing in Simple Language with Steam, Electricity,
Light, Heat, Sound, Hydraulics, Optics, etc.
and with their applications to Apparatus
in Common Use

By

ARCHIBALD WILLIAMS
Author of "The Romance of Modern Invention,"
"The Romance of Mining," etc., etc.

THOMAS NELSON AND SONS
London, Edinburgh, Dublin, and New York

P R E FA C E .

How does it work? This question has been put to me so often by persons
young and old that I have at last decided to answer it in such a manner that
a much larger public than that with which I have personal acquaintance may
be able to satisfy themselves as to the principles underlying many of the
mechanisms met with in everyday life.

In order to include steam, electricity, optics, hydraulics, thermics, light,
and a variety of detached mechanisms which cannot be classified under any
one of these heads, within the compass of about 450 pages, I have to be
content with a comparatively brief treatment of each subject. This brevity
has in turn compelled me to deal with principles rather than with detailed
descriptions of individual devices—though in several cases recognized
types are examined. The reader will look in vain for accounts of the Yerkes
telescope, of the latest thing in motor cars, and of the largest locomotive.
But he will be put in the way of understanding the essential nature of all
telescopes, motors, and steam-engines so far as they are at present
developed, which I think may be of greater ultimate profit to the uninitiated.

While careful to avoid puzzling the reader by the use of mysterious
phraseology I consider that the parts of a machine should be given their
technical names wherever possible. To prevent misconception, many of the
diagrams accompanying the letterpress have words as well as letters written
on them. This course also obviates the wearisome reference from text to
diagram necessitated by the use of solitary letters or figures.

I may add, with regard to the diagrams of this book, that they are
purposely somewhat unconventional, not being drawn to scale nor

conforming to the canons of professional draughtsmanship. Where
advisable, a part of a machine has been exaggerated to show its details. As a
rule solid black has been preferred to fine shading in sectional drawings,
and all unnecessary lines are omitted. I would here acknowledge my
indebtedness to my draughtsman, Mr. Frank Hodgson, for his care and
industry in preparing the two hundred or more diagrams for which he was
responsible.

Four organs of the body—the eye, the ear, the larynx, and the heart—are
noticed in appropriate places. The eye is compared with the camera, the
larynx with a reed pipe, the heart with a pump, while the ear fitly opens the
chapter on acoustics. The reader who is unacquainted with physiology will
thus be enabled to appreciate the better these marvellous devices, far more
marvellous, by reason of their absolutely automatic action, than any
creation of human hands.

A.W.

Uplands, Stoke Poges, Bucks.

CONTENTS.

Chapter I.—THE STEAM-ENGINE.
What is steam?—The mechanical energy
of steam—The boiler—The circulation
of water in a boiler—The enclosed
furnace—The multitubular boiler—
Fire-tube boilers—Other types of
boilers—Aids to combustion—Boiler
fittings—The safety-valve—The
water-gauge—The steam-gauge—The
water supply to a boiler 13

Chapter II.—THE CONVERSION OF
HEAT ENERGY
INTO MECHANICAL MOTION.
Reciprocating engines—Double-cylinder
engines—The function of the fly-
wheel—The cylinder—The slide-valve
—The eccentric—"Lap" of the valve:
expansion of steam—How the cut-off
is managed—Limit of expansive
working—Compound engines—
Arrangement of expansion engines—
Compound locomotives—Reversing
gears—"Linking-up"—Piston-valves
—Speed governors—Marine-speed
governors—The condenser 44

Chapter III.—THE STEAM TURBINE.
How a turbine works—The De Laval
turbine—The Parsons turbine—
Description of the Parsons turbine—
The expansive action of steam in a
Parsons turbine—Balancing the thrust
—Advantages of the marine turbine 74

Chapter IV.—THE INTERNAL-
COMBUSTION ENGINE.
The meaning of the term—Action of the
internal-combustion engine—The
motor car—The starting-handle—The
engine—The carburetter—Ignition of
the charge—Advancing the spark—
Governing the engine—The clutch—
The gear-box—The compensating gear
—The silencer—The brakes—Speed
of cars 87

Chapter V.—ELECTRICAL
APPARATUS.
What is electricity?—Forms of electricity
—Magnetism—The permanent magnet
—Lines of force—Electro-magnets—
The electric bell—The induction coil
—The condenser—Transformation of
current—Uses of the induction coil 112

Chapter VI.—THE ELECTRIC
TELEGRAPH.
Needle instruments—Influence of current 127
on the magnetic needle—Method of
reversing the current—Sounding

instruments—Telegraphic relays—
Recording telegraphs—High-speed
telegraphy

Chapter VII.—WIRELESS
TELEGRAPHY.
The transmitting apparatus—The receiving
apparatus—Syntonic
transmission—The advance of wireless
telegraphy 137

Chapter VIII.—THE TELEPHONE.
The Bell telephone—The Edison
transmitter—The granular carbon
transmitter—General arrangement of a
telephone circuit—Double-line circuits
—Telephone exchanges—Submarine
telephony 147

Chapter IX.—DYNAMOS AND
ELECTRIC MOTORS.
A simple dynamo--Continuous-current
dynamos--Multipolar dynamos--
Exciting the field magnets--Alternating
current dynamos--The transmission of
power--The electric motor--Electric
lighting--The incandescent lamp--Arc
lamps--"Series" and "parallel"
arrangement of lamps--Current for
electric lamps--Electroplating 159

Chapter X.—RAILWAY BRAKES.
The Vacuum Automatic brake—The
Westinghouse air-brake 187

Chapter XI.—RAILWAY SIGNALLING.
The block system—Position of signals—
Interlocking the signals—Locking gear
—Points—Points and signals in
combination—Working the block
system—Series of signalling
operations—Single line signals—The
train staff—Train staff and ticket—
Electric train staff system—
Interlocking—Signalling operations—
Power signalling—Pneumatic
signalling—Automatic signalling 200

Chapter XII.—OPTICS.
Lenses—The image cast by a convex lens
—Focus—Relative position of object
and lens—Correction of lenses for
colour—Spherical aberration—
Distortion of image—The human eye
—The use of spectacles—The blind
spot 230

Chapter XIII.—THE MICROSCOPE,
THE TELESCOPE,
AND THE MAGIC-LANTERN.
The simple microscope—Use of the
simple microscope in the telescope—
The terrestrial telescope—The
Galilean telescope—The prismatic
telescope—The reflecting telescope—
The parabolic mirror—The compound
microscope—The magic-lantern—The
bioscope—The plane mirror 253

Chapter XIV.—SOUND AND MUSICAL
INSTRUMENTS.
Nature of sound—The ear—Musical
instruments—The vibration of strings
—The sounding-board and the frame
of a piano—The strings—The striking
mechanism—The quality of a note 270

Chapter XV.—WIND INSTRUMENTS.
Longitudinal vibration—Columns of air—
Resonance of columns of air—Length
and tone—The open pipe—The
overtones of an open pipe—Where
overtones are used—The arrangement
of the pipes and pedals—Separate
sound-boards—Varieties of stops—
Tuning pipes and reeds—The bellows
—Electric and pneumatic actions—
The largest organ in the world—
Human reeds 287

Chapter XVI.—TALKING-MACHINES.
The phonograph—The recorder—The
reproducer—The gramophone—The
making of records—Cylinder records
—Gramophone records 310

Chapter XVII.—WHY THE WIND
BLOWS.
Why the wind blows—Land and sea 322
breezes—Light air and moisture—The
barometer—The column barometer—
The wheel barometer—A very simple
barometer—The aneroid barometer—

Barometers and weather—The diving-
bell—The diving-dress—Air-pumps—
Pneumatic tyres—The air-gun—The
self-closing door-stop—The action of
wind on oblique surfaces—The
balloon—The flying-machine

Chapter XVIII.—HYDRAULIC
MACHINERY.
The siphon—The bucket pump—The
force-pump—The most marvellous
pump—The blood channels—The
course of the blood—The hydraulic
press—Household water-supply
fittings—The ball-cock—The water-
meter—Water-supply systems—The
household filter—Gas traps—Water
engines—The cream separator—The
"hydro" 350

Chapter XIX.—HEATING AND
LIGHTING.
The hot-water supply—The tank system—
The cylinder system—How a lamp
works—Gas and gasworks—
Automatic stoking—A gas governor—
The gas meter—Incandescent gas
lighting 386

Chapter XX.—VARIOUS
MECHANISMS.
Clocks and Watches:—A short history of 410
timepieces—The construction of
timepieces—The driving power—The
escapement—Compensating
pendulums—The spring balance—The

cylinder escapement—The lever
escapement—Compensated balance-
wheels—Keyless winding mechanism
for watches—The hour hand train.
Locks:—The Chubb lock—The Yale
lock. The Cycle:—The gearing of a
cycle—The free wheel—The change-
speed gear. Agricultural Machines:—
The threshing-machine—Mowing-
machines. Some Natural Phenomena:
—Why sun-heat varies in intensity—
The tides—Why high tide varies daily

HOW IT WORKS.

Chapter I.

THE STEAM-ENGINE.

What is steam?—The mechanical energy of steam—The boiler—The circulation of
water in a boiler—The enclosed furnace—The multitubular boiler—Fire-tube
boilers—Other types of boilers—Aids to combustion—Boiler fittings—The
safety-valve—The water-gauge—The steam-gauge—The water supply to a
boiler.

WHAT IS STEAM?

I F ice be heated above 32° Fahrenheit, its molecules lose their cohesion,
and move freely round one another—the ice is turned into water. Heat
water above 212° Fahrenheit, and the molecules exhibit a violent mutual
repulsion, and, like dormant bees revived by spring sunshine, separate and
dart to and fro. If confined in an air-tight vessel, the molecules have their
flights curtailed, and beat more and more violently against their prison
walls, so that every square inch of the vessel is subjected to a rising
pressure. We may compare the action of the steam molecules to that of
bullets fired from a machine-gun at a plate mounted on a spring. The faster
the bullets came, the greater would be the continuous compression of the
spring.

THE MECHANICAL ENERGY OF STEAM.

If steam is let into one end of a cylinder behind an air-tight but freely-
moving piston, it will bombard the walls of the cylinder and the piston; and
if the united push of the molecules on the one side of the latter is greater

than the resistance on the other side opposing its motion, the piston must
move. Having thus partly got their liberty, the molecules become less
active, and do not rush about so vigorously. The pressure on the piston
decreases as it moves. But if the piston were driven back to its original
position against the force of the steam, the molecular activity—that is,
pressure—would be restored. We are here assuming that no heat has passed
through the cylinder or piston and been radiated into the air; for any loss of
heat means loss of energy, since heat is energy.

THE BOILER.

The combustion of fuel in a furnace causes the walls of the furnace to
become hot, which means that the molecules of the substance forming the
walls are thrown into violent agitation. If the walls are what are called
"good conductors" of heat, they will transmit the agitation through them to
any surrounding substance. In the case of the ordinary house stove this is
the air, which itself is agitated, or grows warm. A steam-boiler has the
furnace walls surrounded by water, and its function is to transmit molecular
movement (heat, or energy) through the furnace plates to the water until the
point is reached when steam generates. At atmospheric pressure—that is, if
not confined in any way—steam would fill 1,610 times the space which its
molecules occupied in their watery formation. If we seal up the boiler so
that no escape is possible for the steam molecules, their motion becomes
more and more rapid, and pressure is developed by their beating on the
walls of the boiler. There is theoretically no limit to which the pressure may
be raised, provided that sufficient fuel-combustion energy is transmitted to
the vaporizing water.

To raise steam in large quantities we must employ a fuel which develops
great heat in proportion to its weight, is readily procured, and cheap. Coal
fulfils all these conditions. Of the 800 million tons mined annually
throughout the world, 400 million tons are burnt in the furnaces of steam-
boilers.

A good boiler must be—(1) Strong enough to withstand much higher
pressures than that at which it is worked; (2) so designed as to burn its fuel
to the greatest advantage.

Even in the best-designed boilers a large part of the combustion heat
passes through the chimney, while a further proportion is radiated from the
boiler. Professor John Perry[1] considers that this waste amounts, under the
best conditions at present obtainable, to eleven-twelfths of the whole. We
have to burn a shillingsworth of coal to capture the energy stored in a
pennyworth. Yet the steam-engine of to-day is three or four times as
efficient as the engine of fifty years ago. This is due to radical
improvements in the design of boilers and of the machinery which converts
the heat energy of steam into mechanical motion.

CIRCULATION OF WATER IN A BOILER.

If you place a pot filled with water on an open fire, and watch it when it
boils, you will notice that the water heaves up at the sides and plunges
down at the centre. This is due to the water being heated most at the sides,
and therefore being lightest there. The rising steam-bubbles also carry it up.
On reaching the surface, the bubbles burst, the steam escapes, and the water
loses some of its heat, and rushes down again to take the place of steam-
laden water rising.

Fig. 1. Fig. 2.

If the fire is very fierce, steam-bubbles may rise from all points at the
bottom, and impede downward currents (Fig. 1). The pot then "boils over."

Fig. 2 shows a method of preventing this trouble. We lower into our pot
a vessel of somewhat smaller diameter, with a hole in the bottom, arranged
in such a manner as to leave a space between it and the pot all round. The
upward currents are then separated entirely from the downward, and the fire
can be forced to a very much greater extent than before without the water
boiling over. This very simple arrangement is the basis of many devices for
producing free circulation of the water in steam-boilers.

We can easily follow out the process of development. In Fig. 3 we see a
simple U-tube depending from a vessel of water. Heat is applied to the left
leg, and a steady circulation at once commences. In order to increase the
heating surface we can extend the heated leg into a long incline (Fig. 4),

beneath which three lamps instead of only one are placed. The direction of
the circulation is the same, but its rate is increased.

Fig. 3.
A further improvement results from increasing the number of tubes (Fig.
5), keeping them all on the slant, so that the heated water and steam may
rise freely.

THE ENCLOSED FURNACE.

Fig. 4. Fig. 5.

Still, a lot of the heat gets away. In a steam-boiler the burning fuel is
enclosed either by fire-brick or a "water-jacket," forming part of the boiler.
A water-jacket signifies a double coating of metal plates with a space
between, which is filled with water (see Fig. 6). The fire is now enclosed
much as it is in a kitchen range. But our boiler must not be so wasteful of
the heat as is that useful household fixture. On their way to the funnel the
flames and hot gases should act on a very large metal or other surface in
contact with the water of the boiler, in order to give up a due proportion of
their heat.

Fig. 6.—Diagrammatic sketch of a locomotive type of boiler. Water
indicated by dotted lines. The arrows show the direction taken by the
air and hot gases from the air-door to the funnel.

THE MULTITUBULAR BOILER.

Fig. 7.—The Babcock and Wilcox water-tube
boiler. One side of the brick seating has been
removed to show the arrangement of the
water-tubes and furnace.

To save room, boilers which have to make steam very quickly and at
high pressures are largely composed of pipes. Such boilers we call
multitubular. They are of two kinds—(1) Water-tube boilers; in which the
water circulates through tubes exposed to the furnace heat. The Babcock
and Wilcox boiler (Fig. 7) is typical of this variety. (2) Fire-tube boilers; in
which the hot gases pass through tubes surrounded by water. The ordinary
locomotive boiler (Fig. 6) illustrates this form.

The Babcock and Wilcox boiler is widely used in mines, power stations,
and, in a modified form, on shipboard. It consists of two main parts—(1) A
drum, h, in the upper part of which the steam collects; (2) a group of pipes
arranged on the principle illustrated by Fig. 5. The boiler is seated on a
rectangular frame of fire-bricks. At one end is the furnace door; at the other
the exit to the chimney. From the furnace f the flames and hot gases rise
round the upper end of the sloping tubes tt into the space a, where they
play upon the under surface of h before plunging downward again among
the tubes into the space b. Here the temperature is lower. The arrows
indicate further journeys upwards into the space c on the right of a fire-
brick division, and past the down tubes ss into d, whence the hot gases find
an escape into the chimney through the opening e. It will be noticed that the
greatest heat is brought to bear on tt near their junction with uu, the
"uptake" tubes; and that every succeeding passage of the pipes brings the
gradually cooling gases nearer to the "downtake" tubes ss.

The pipes tt are easily brushed and scraped after the removal of plugs
from the "headers" into which the tube ends are expanded.

Other well-known water-tube boilers are the Yarrow, Belleville, Stirling,
and Thorneycroft, all used for driving marine engines.

FIRE-TUBE BOILERS.

Fig. 6 shows a locomotive boiler in section. To the right is the fire-box,
surrounded on all sides by a water-jacket in direct communication with the
barrel of the boiler. The inner shell of the fire-box is often made of copper,

which withstands the fierce heat better than steel; the outer, like the rest of
the boiler, is of steel plates from ½ to ¾ inch thick. The shells of the jacket
are braced together by a large number of rivets, rr; and the top, or crown, is
strengthened by heavy longitudinal girders riveted to it, or is braced to the
top of the boiler by long bolts. A large number of fire-tubes (only three are
shown in the diagram for the sake of simplicity) extend from the fire-box to
the smoke-box. The most powerful "mammoth" American locomotives
have 350 or more tubes, which, with the fire-box, give 4,000 square feet of
surface for the furnace heat to act upon. These tubes are expanded at their
ends by a special tool into the tube-plates of the fire-box and boiler front.
George Stephenson and his predecessors experienced great difficulty in
rendering the tube-end joints quite water-tight, but the invention of the
"expander" has removed this trouble.

The fire-brick arch shown (Fig. 6) in the fire-box is used to deflect the
flames towards the back of the fire-box, so that the hot gases may be
retarded somewhat, and their combustion rendered more perfect. It also
helps to distribute the heat more evenly over the whole of the inside of the
box, and prevents cold air from flying directly from the firing door to the
tubes. In some American and Continental locomotives the fire-brick arch is
replaced by a "water bridge," which serves the same purpose, while giving
additional heating surface.

The water circulation in a locomotive boiler is—upwards at the fire-box
end, where the heat is most intense; forward along the surface; downwards
at the smoke-box end; backwards along the bottom of the barrel.

OTHER TYPES OF BOILERS.

For small stationary land engines the vertical boiler is much used. In Fig.
8 we have three forms of this type—a and b with cross water-tubes; c with
vertical fire-tubes. The furnace in every case is surrounded by water, and
fed through a door at one side.

Fig. 8.—Diagrammatic representation of three types of
vertical boilers.
The Lancashire boiler is of large size. It has a cylindrical shell,
measuring up to 30 feet in length and 7 feet in diameter, traversed from end
to end by two large flues, in the rear part of which are situated the furnaces.
The boiler is fixed on a seating of fire-bricks, so built up as to form three
flues, a and bb, shown in cross section in Fig. 9. The furnace gases, after
leaving the two furnace flues, are deflected downwards into the channel a,
by which they pass underneath the boiler to a point almost under the
furnace, where they divide right and left and travel through cross passages
into the side channels bb, to be led along the boiler's flanks to the chimney
exit c. By this arrangement the effective heating surface is greatly
increased; and the passages being large, natural draught generally suffices
to maintain proper combustion. The Lancashire boiler is much used in
factories and (in a modified form) on ships, since it is a steady steamer and
is easily kept in order.

Fig. 9.—Cross and longitudinal sections of a Lancashire
boiler.

In marine boilers of cylindrical shape cross water-tubes and fire-tubes
are often employed to increase the heating surface. Return tubes are also led
through the water to the funnels, situated at the same end as the furnace.

AIDS TO COMBUSTION.

We may now turn our attention more particularly to the chemical process
called combustion, upon which a boiler depends for its heat. Ordinary steam
coal contains about 85 per cent. of carbon, 7 per cent. of oxygen, and 4 per
cent. of hydrogen, besides traces of nitrogen and sulphur and a small
incombustible residue. When the coal burns, the nitrogen is released and
passes away without combining with any of the other elements. The sulphur
unites with hydrogen and forms sulphuretted hydrogen (also named
sulphurous acid), which is injurious to steel plates, and is largely
responsible for the decay of tubes and funnels. More of the hydrogen unites
with the oxygen as steam.

The most important element in coal is the carbon (known chemically by
the symbol C). Its combination with oxygen, called combustion, is the act
which heats the boiler. Only when the carbon present has combined with the
greatest possible amount of oxygen that it will take into partnership is the
combustion complete and the full heat-value (fixed by scientific experiment
at 14,500 thermal units per pound of carbon) developed.

Now, carbon may unite with oxygen, atom for atom, and form carbon
monoxide (CO); or in the proportion of one atom of carbon to two of

oxygen, and form carbon dioxide (CO2). The former gas is combustible—
that is, will admit another atom of carbon to the molecule—but the latter is
saturated with oxygen, and will not burn, or, to put it otherwise, is the
product of perfect combustion. A properly designed furnace, supplied with a
due amount of air, will cause nearly all the carbon in the coal burnt to
combine with the full amount of oxygen. On the other hand, if the oxygen
supply is inefficient, CO as well as CO2 will form, and there will be a heat
loss, equal in extreme cases to two-thirds of the whole. It is therefore
necessary that a furnace which has to eat up fuel at a great pace should be
artificially fed with air in the proportion of from 12 to 20 pounds of air for
every pound of fuel. There are two methods of creating a violent draught
through the furnace. The first is—

The forced draught; very simply exemplified by the ordinary bellows
used in every house. On a ship (Fig. 10) the principle is developed as
follows:—The boilers are situated in a compartment or compartments
having no communication with the outer air, except for the passages down
which air is forced by powerful fans at a pressure considerably greater than
that of the atmosphere. There is only one "way out"—namely, through the
furnace and tubes (or gas-ways) of the boiler, and the funnel. So through
these it rushes, raising the fuel to white heat. As may easily be imagined,
the temperature of a stokehold, especially in the tropics, is far from
pleasant. In the Red Sea the thermometer sometimes rises to 170°
Fahrenheit or more, and the poor stokers have a very bad time of it.

Fig. 10.—Sketch showing how the "forced draught" is
produced in a stokehold and how it affects the furnaces.

SCENE IN THE STOKEHOLD OF A BATTLE-SHIP.

The second system is that of the induced draught. Here air is sucked
through the furnace by creating a vacuum in the funnel and in a chamber
opening into it. Turning to Fig. 6, we see a pipe through which the exhaust
steam from the locomotive's cylinders is shot upwards into the funnel, in
which, and in the smoke-box beneath it, a strong vacuum is formed while
the engine is running. Now, "nature abhors a vacuum," so air will get into

the smoke-box if there be a way open. There is—through the air-doors at
the bottom of the furnace, the furnace itself, and the fire-tubes; and on the
way oxygen combines with the carbon of the fuel, to form carbon dioxide.
The power of the draught is so great that, as one often notices when a train
passes during the night, red-hot cinders, plucked from the fire-box, and
dragged through the tubes, are hurled far into the air. It might be mentioned
in parenthesis that the so-called "smoke" which pours from the funnel of a
moving engine is mainly condensing steam. A steamship, on the other hand,
belches smoke only from its funnels, as fresh water is far too precious to
waste as steam. We shall refer to this later on (p. 72).

BOILER FITTINGS.

The most important fittings on a boiler are:—(1) the safety-valve; (2) the
water-gauge; (3) the steam-gauge; (4) the mechanisms for feeding it with
water.

THE SAFETY-VALVE.

Professor Thurston, an eminent authority on the steam-engine, has
estimated that a plain cylindrical boiler carrying 100 lbs. pressure to the
square inch contains sufficient stored energy to project it into the air a
vertical distance of 3½ miles. In the case of a Lancashire boiler at equal
pressure the distance would be 2½ miles; of a locomotive boiler, at 125 lbs.,
1½ miles; of a steam tubular boiler, at 75 lbs., 1 mile. According to the
same writer, a cubic foot of heated water under a pressure of from 60 to 70
lbs. per square inch has about the same energy as one pound of gunpowder.

Steam is a good servant, but a terrible master. It must be kept under strict
control. However strong a boiler may be, it will burst if the steam pressure
in it be raised to a certain point; and some device must therefore be fitted on
it which will give the steam free egress before that point is reached. A
device of this kind is called a safety-valve. It usually blows off at less than
half the greatest pressure that the boiler has been proved by experiment to
be capable of withstanding.

In principle the safety-valve denotes an orifice closed by an accurately-
fitting plug, which is pressed against its seat on the boiler top by a weighted
lever, or by a spring. As soon as the steam pressure on the face of the plug
exceeds the counteracting force of the weight or spring, the plug rises, and
steam escapes until equilibrium of the opposing forces is restored.

On stationary engines a lever safety-valve is commonly employed (Fig.
11). The blowing-off point can be varied by shifting the weight along the
arm so as to give it a greater or less leverage. On locomotive and marine
boilers, where shocks and movements have to be reckoned with, weights
are replaced by springs, set to a certain tension, and locked up so that they
cannot be tampered with.

Fig. 11.—A Lever Safety-Valve. v, valve; s, seating; p,
pin; l, lever; f, fulcrum; w, weight. The figures indicate
the positions at which the weight should be placed for the
valve to act when the pressure rises to that number of
pounds per square inch.

Boilers are tested by filling the boilers quite full and (1) by heating the
water, which expands slightly, but with great pressure; (2) by forcing in
additional water with a powerful pump. In either case a rupture would not
be attended by an explosion, as water is very inelastic.

The days when an engineer could "sit on the valves"—that is, screw
them down—to obtain greater pressure, are now past, and with them a
considerable proportion of the dangers of high-pressure steam. The Factory
Act of 1895, in force throughout the British Isles, provides that every boiler

for generating steam in a factory or workshop where the Act applies must
have a proper safety-valve, steam-gauge, and water-gauge; and that boilers
and fittings must be examined by a competent person at least once in every
fourteen months. Neglect of these provisions renders the owner of a boiler
liable to heavy penalties if an explosion occurs.

One of the most disastrous explosions on record took place at the Redcar
Iron Works, Yorkshire, in June 1895. In this case, twelve out of fifteen
boilers ranged side by side burst, through one proving too weak for its
work. The flying fragments of this boiler, striking the sides of other boilers,
exploded them, and so the damage was transmitted down the line. Twenty
men were killed and injured; while masses of metal, weighing several tons
each, were hurled 250 yards, and caused widespread damage.

The following is taken from a journal, dated December 22, 1895:
"Providence (Rhode Island).—A recent prophecy that a boiler would
explode between December 16 and 24 in a store has seriously affected the
Christmas trade. Shoppers are incredibly nervous. One store advertises, 'No
boilers are being used; lifts running electrically.' All stores have had their
boilers inspected."

THE WATER-GAUGE.

No fitting of a boiler is more important than the water-gauge, which
shows the level at which the water stands. The engineer must continually
consult his gauge, for if the water gets too low, pipes and other surfaces
exposed to the furnace flames may burn through, with disastrous results;
while, on the other hand, too much water will cause bad steaming. A section
of an ordinary gauge is seen in Fig. 12. It consists of two parts, each
furnished with a gland, g, to make a steam-tight joint round the glass tube,
which is inserted through the hole covered by the plug p1. The cocks t1 t2
are normally open, allowing the ingress of steam and water respectively to
the tube. Cock t3 is kept closed unless for any reason it is necessary to blow

steam or water through the gauge. The holes c c can be cleaned out if the
plugs p2 p3 are removed.

Fig. 12.—Section of a water-gauge.

Most gauges on high-pressure boilers have a thick glass screen in front,
so that in the event of the tube breaking, the steam and water may not blow
directly on to the attendants. A further precaution is to include two ball-
valves near the ends of the gauge-glass. Under ordinary conditions the balls
lie in depressions clear of the ways; but when a rush of steam or water
occurs they are sucked into their seatings and block all egress.

On many boilers two water-gauges are fitted, since any gauge may work
badly at times. The glasses are tested to a pressure of 3,000 lbs. or more to
the square inch before use.

THE STEAM-GAUGE.

It is of the utmost importance that a person in charge of a boiler should
know what pressure the steam has reached. Every boiler is therefore fitted
with one steam-gauge; many with two, lest one might be unreliable. There
are two principal types of steam-gauge:—(1) The Bourdon; (2) the
Schäffer-Budenberg. The principle of the Bourdon is illustrated by Fig. 13,
in which a is a piece of rubber tubing closed at one end, and at the other
drawn over the nozzle of a cycle tyre inflator. If bent in a curve, as shown,
the section of the tube is an oval. When air is pumped in, the rubber walls
endeavour to assume a circular section, because this shape encloses a larger
area than an oval of equal circumference, and therefore makes room for a
larger volume of air. In doing so the tube straightens itself, and assumes the
position indicated by the dotted lines. Hang an empty "inner tube" of a
pneumatic tyre over a nail and inflate it, and you will get a good illustration
of the principle.

Fig. 13.—Showing the principle of the steam-gauge.

Fig. 14.—Bourdon steam-gauge. Part of dial removed to
show mechanism.
In Fig. 14 we have a Bourdon gauge, with part of the dial face broken
away to show the internal mechanism. t is a flattened metal tube soldered at
one end into a hollow casting, into which screws a tap connected with the
boiler. The other end (closed) is attached to a link, l, which works an arm of
a quadrant rack, r, engaging with a small pinion, p, actuating the pointer. As
the steam pressure rises, the tube t moves its free end outwards towards the
position shown by the dotted lines, and traverses the arm of the rack, so

shifting the pointer round the scale. As the pressure falls, the tube gradually
returns to its zero position.

The Schäffer-Budenberg gauge depends for its action on the elasticity of
a thin corrugated metal plate, on one side of which steam presses. As the
plate bulges upwards it pushes up a small rod resting on it, which operates a
quadrant and rack similar to that of the Bourdon gauge. The principle is
employed in another form for the aneroid barometer (p. 329).

THE WATER SUPPLY TO A BOILER.

The water inside a boiler is kept at a proper level by (1) pumps or (2)
injectors. The former are most commonly used on stationary and marine
boilers. As their mechanism is much the same as that of ordinary force
pumps, which will be described in a later chapter, we may pass at once to
the injector, now almost universally used on locomotive, and sometimes on
stationary boilers. At first sight the injector is a mechanical paradox, since it
employs the steam from a boiler to blow water into the boiler. In Fig. 15 we
have an illustration of the principle of an injector. Steam is led from the
boiler through pipe a, which terminates in a nozzle surrounded by a cone, e,
connected by the pipe b with the water tank. When steam is turned on it
rushes with immense velocity from the nozzle, and creates a partial vacuum
in cone e, which soon fills with water. On meeting the water the steam
condenses, but not before it has imparted some of its velocity to the water,
which thus gains sufficient momentum to force down the valve and find its
way to the boiler. The overflow space o o between e and c allows steam and
water to escape until the water has gathered the requisite momentum.

Fig. 15.—Diagram illustrating the principle of a steam-
injector.

Fig. 16.—The Giffard injector.

A form of injector very commonly used is Giffard's (Fig. 16). Steam is
allowed to enter by screwing up the valve v. As it rushes through the nozzle
of the cone a it takes up water and projects it into the "mixing cone" b,
which can be raised or lowered by the pinion d (worked by the hand-wheel
wheel shown) so as to regulate the amount of water admitted to b. At the
centre of b is an aperture, o, communicating with the overflow. The water

passes to the boiler through the valve on the left. It will be noticed that the
cone a and the part of b above the orifice o contract downward. This is to
convert the pressure of the steam into velocity. Below o is a cone, the
diameter of which increases downwards. Here the velocity of the water is
converted back into pressure in obedience to a well-known hydromechanic
law.

An injector does not work well if the feed-water be too hot to condense
the steam quickly; and it may be taken as a rule that the warmer the water,
the smaller is the amount of it injected by a given weight of steam.[2] Some
injectors have flap-valves covering the overflow orifice, to prevent air being
sucked in and carried to the boiler.

When an injector receives a sudden shock, such as that produced by the
passing of a locomotive over points, it is liable to "fly off"—that is, stop
momentarily—and then send the steam and water through the overflow. If
this happens, both steam and water must be turned off, and the injector be
restarted; unless it be of the self-starting variety, which automatically
controls the admission of water to the "mixing-cone," and allows the
injector to "pick up" of itself.

For economy's sake part of the steam expelled from the cylinders of a
locomotive is sometimes used to work an injector, which passes the water
on, at a pressure of 70 lbs. to the square inch, to a second injector operated
by high-pressure steam coming direct from the boiler, which increases its
velocity sufficiently to overcome the boiler pressure. In this case only a
fraction of the weight of high-pressure steam is required to inject a given
weight of water, as compared with that used in a single-stage injector.
[1] "The Steam-Engine," p. 3.
[2] By "weight of steam" is meant the steam produced by boiling a certain
weight of water. A pound of steam, if condensed, would form a pound of water.

Chapter II.

THE CONVERSION OF HEAT ENERGY INTO MECHANICAL
MOTION.

Reciprocating engines—Double-cylinder engines—The function of the fly-wheel—
The cylinder—The slide-valve—The eccentric—"Lap" of the valve: expansion
of steam—How the cut-off is managed—Limit of expansive working—
Compound engines—Arrangement of expansion engines—Compound
locomotives—Reversing gears—"Linking-up"—Piston-valves—Speed
governors—Marine-speed governors—The condenser.

H AVING treated at some length the apparatus used for converting water
into high-pressure steam, we may pass at once to a consideration of
the mechanisms which convert the energy of steam into mechanical motion,
or work.

Steam-engines are of two kinds:—(1) reciprocating, employing
cylinders and cranks; (2) rotary, called turbines.

RECIPROCATING ENGINES.

Fig. 17.—Sketch showing parts of a horizontal steam-
engine.

Fig. 17 is a skeleton diagram of the simplest form of reciprocating
engine. c is a cylinder to which steam is admitted through the steam-ways[3]
w w, first on one side of the piston p, then on the other. The pressure on the
piston pushes it along the cylinder, and the force is transmitted through the
piston rod p r to the connecting rod c r, which causes the crank k to
revolve. At the point where the two rods meet there is a "crosshead," h,
running to and fro in a guide to prevent the piston rod being broken or bent
by the oblique thrusts and pulls which it imparts through c r to the crank k.
The latter is keyed to a shaft s carrying the fly-wheel, or, in the case of a
locomotive, the driving-wheels. The crank shaft revolves in bearings. The
internal diameter of a cylinder is called its bore. The travel of the piston is
called its stroke. The distance from the centre of the shaft to the centre of
the crank pin is called the crank's throw, which is half of the piston's stroke.
An engine of this type is called double-acting, as the piston is pushed
alternately backwards and forwards by the steam. When piston rod,
connecting rod, and crank lie in a straight line—that is, when the piston is
fully out, or fully in—the crank is said to be at a "dead point;" for, were the
crank turned to such a position, the admission of steam would not produce
motion, since the thrust or pull would be entirely absorbed by the bearings.

Fig. 18.—Sectional plan of a horizontal engine.

DOUBLE-CYLINDER ENGINES.

Fig. 19.

Fig. 20.
Locomotive, marine, and all other engines which must be started in any
position have at least two cylinders, and as many cranks set at an angle to
one another. Fig. 19 demonstrates that when one crank, c1, of a double-

cylinder engine is at a "dead point," the other, c2, has reached a position at
which the piston exerts the maximum of turning power. In Fig. 20 each
crank is at 45° with the horizontal, and both pistons are able to do work.
The power of one piston is constantly increasing while that of the other is
decreasing. If single-action cylinders are used, at least three of these are
needed to produce a perpetual turning movement, independently of a fly-
wheel.

THE FUNCTION OF THE FLY-WHEEL.

A fly-wheel acts as a reservoir of energy, to carry the crank of a single-
cylinder engine past the "dead points." It is useful in all reciprocating
engines to produce steady running, as a heavy wheel acts as a drag on the
effects of a sudden increase or decrease of steam pressure. In a pump,
mangold-slicer, cake-crusher, or chaff-cutter, the fly-wheel helps the
operator to pass his dead points—that is, those parts of the circle described
by the handle in which he can do little work.

THE CYLINDER.

Fig. 21.—Diagrammatic section of a cylinder and its slide-
valve.
The cylinders of an engine take the place of the muscular system of the
human body. In Fig. 21 we have a cylinder and its slide-valve shown in
section. First of all, look at p, the piston. Round it are white grooves, r r, in
which rings are fitted to prevent the passage of steam past the piston. The
rings are cut through at one point in their circumference, and slightly
opened, so that when in position they press all round against the walls of the
cylinder. After a little use they "settle down to their work"—that is, wear to
a true fit in the cylinder. Each end of the cylinder is closed by a cover, one
of which has a boss cast on it, pierced by a hole for the piston rod to work
through. To prevent the escape of steam the boss is hollowed out true to
accommodate a gland, g1, which is threaded on the rod and screwed up
against the boss; the internal space between them being filled with packing.
Steam from the boiler enters the steam-chest, and would have access to both

sides of the piston simultaneously through the steam-ways, w w, were it not
for the

SLIDE-VALVE,

a hollow box open at the bottom, and long enough for its edges to cover
both steam-ways at once. Between w w is e, the passage for the exhaust
steam to escape by. The edges of the slide-valve are perfectly flat, as is the
face over which the valve moves, so that no steam may pass under the
edges. In our illustration the piston has just begun to move towards the
right. Steam enters by the left steam-way, which the valve is just
commencing to uncover. As the piston moves, the valve moves in the same
direction until the port is fully uncovered, when it begins to move back
again; and just before the piston has finished its stroke the steam-way on the
right begins to open. The steam-way on the left is now in communication
with the exhaust port e, so that the steam that has done its duty is released
and pressed from the cylinder by the piston. Reciprocation is this backward
and forward motion of the piston: hence the term "reciprocating" engines.
The linear motion of the piston rod is converted into rotatory motion by the
connecting rod and crank.

Fig. 22.—Perspective section of cylinder.
The use of a crank appears to be so obvious a method of producing this
conversion that it is interesting to learn that, when James Watt produced his
"rotative engine" in 1780 he was unable to use the crank because it had
already been patented by one Matthew Wasborough. Watt was not easily
daunted, however, and within a twelvemonth had himself patented five
other devices for obtaining rotatory motion from a piston rod. Before
passing on, it may be mentioned that Watt was the father of the modern—
that is, the high-pressure—steam-engine; and that, owing to the
imperfection of the existing machinery, the difficulties he had to overcome
were enormous. On one occasion he congratulated himself because one of
his steam-cylinders was only three-eighths of an inch out of truth in the
bore. Nowadays a good firm would reject a cylinder 1⁄500 of an inch out of
truth; and in small petrol-engines 1⁄5000 of an inch is sometimes the greatest
"limit of error" allowed.

Fig. 23.—The eccentric and its rod.

THE ECCENTRIC

is used to move the slide-valve to and fro over the steam ports (Fig. 23). It
consists of three main parts—the sheave, or circular plate s, mounted on the
crank shaft; and the two straps which encircle it, and in which it revolves.
To one strap is bolted the "big end" of the eccentric rod, which engages at
its other end with the valve rod. The straps are semicircular and held
together by strong bolts, b b, passing through lugs, or thickenings at the
ends of the semicircles. The sheave has a deep groove all round the edges,
in which the straps ride. The "eccentricity" or "throw" of an eccentric is the
distance between c2, the centre of the shaft, and c1, the centre of the sheave.
The throw must equal half of the distance which the slide-valve has to
travel over the steam ports. A tapering steel wedge or key, k, sunk half in
the eccentric and half in a slot in the shaft, holds the eccentric steady and
prevents it slipping. Some eccentric sheaves are made in two parts, bolted
together, so that they may be removed easily without dismounting the shaft.

The eccentric is in principle nothing more than a crank pin so
exaggerated as to be larger than the shaft of the crank. Its convenience lies
in the fact that it may be mounted at any point on a shaft, whereas a crank
can be situated at an end only, if it is not actually a V-shaped bend in the
shaft itself—in which case its position is of course permanent.

SETTING OF THE SLIDE-VALVE AND ECCENTRIC.

The subject of valve-setting is so extensive that a full exposition might
weary the reader, even if space permitted its inclusion. But inasmuch as the

effectiveness of a reciprocating engine depends largely on the nature and
arrangement of the valves, we will glance at some of the more elementary
principles.

Fig. 24.

Fig. 25.

In Fig. 24 we see in section the slide-valve, the ports of the cylinder, and
part of the piston. To the right are two lines at right angles—the thicker, c,
representing the position of the crank; the thinner, e, that of the eccentric.
(The position of an eccentric is denoted diagrammatically by a line drawn
from the centre of the crank shaft through the centre of the sheave.) The
edges of the valve are in this case only broad enough to just cover the ports
—that is, they have no lap. The piston is about to commence its stroke
towards the left; and the eccentric, which is set at an angle of 90° in
advance of the crank, is about to begin opening the left-hand port. By the
time that c has got to the position originally occupied by e, e will be
horizontal (Fig. 25)—that is, the eccentric will have finished its stroke

towards the left; and while c passes through the next right angle the valve
will be closing the left port, which will cease to admit steam when the
piston has come to the end of its travel. The operation is repeated on the
right-hand side while the piston returns.

Fig. 26.
It must be noticed here—(1) that steam is admitted at full pressure all
through the stroke; (2) that admission begins and ends simultaneously with
the stroke. Now, in actual practice it is necessary to admit steam before the
piston has ended its travel, so as to cushion the violence of the sudden
change of direction of the piston, its rod, and other moving parts. To effect
this, the eccentric is set more than 90° in advance—that is, more than what
the engineers call square. Fig. 26 shows such an arrangement. The angle
between e and e1 is called the angle of advance. Referring to the valve, you
will see that it has opened an appreciable amount, though the piston has not
yet started on its rightwards journey.

"LAP" OF THE VALVE—EXPANSION OF STEAM.

In the simple form of valve that appears in Fig. 24, the valve faces are
just wide enough to cover the steam ports. If the eccentric is not square
with the crank, the admission of steam lasts until the very end of the stroke;
if set a little in advance—that is, given lead—the steam is cut off before the
piston has travelled quite along the cylinder, and readmitted before the back
stroke is accomplished. Even with this lead the working is very
uneconomical, as the steam goes to the exhaust at practically the same

pressure as that at which it entered the cylinder. Its property of expansion
has been neglected. But supposing that steam at 100 lbs. pressure were
admitted till half-stroke, and then suddenly cut off, the expansive nature of
the steam would then continue to push the piston out until the pressure had
decreased to 50 lbs. per square inch, at which pressure it would go to the
exhaust. Now, observe that all the work done by the steam after the cut-off
is so much power saved. The average pressure on the piston is not so high
as in the first case; still, from a given volume of 100 lbs. pressure steam we
get much more work.

HOW THE CUT-OFF IS MANAGED.

Fig. 27.—A slide-valve with "lap."

Fig. 28.

Look at Fig. 27. Here we have a slide-valve, with faces much wider than
the steam ports. The parts marked black, p p, are those corresponding to the
faces of the valves shown in previous diagrams (p. 54). The shaded parts, l
l, are called the lap. By increasing the length of the lap we increase the

range of expansive working. Fig. 28 shows the piston full to the left; the
valve is just on the point of opening to admit steam behind the piston. The
eccentric has a throw equal to the breadth of a port + the lap of the valve.
That this must be so is obvious from a consideration of Fig. 27, where the
valve is at its central position. Hence the very simple formula:—Travel of
valve = 2 × (lap + breadth of port). The path of the eccentric's centre round
the centre of the shaft is indicated by the usual dotted line (Fig. 28). You
will notice that the "angle of advance," denoted by the arrow a, is now very
considerable. By the time that the crank c has assumed the position of the
line s, the eccentric has passed its dead point, and the valve begins to travel
backwards, eventually returning to the position shown in Fig. 28, and
cutting off the steam supply while the piston has still a considerable part of
its stroke to make. The steam then begins to work expansively, and
continues to do so until the valve assumes the position shown in Fig. 27.

If the valve has to have "lead" to admit steam before the end of the
stroke to the other side of the piston, the angle of advance must be
increased, and the eccentric centre line would lie on the line e2. Therefore—
total angle of advance = angle for lap and angle for lead.

LIMIT OF EXPANSIVE WORKING.

Theoretically, by increasing the lap and cutting off the steam earlier and
earlier in the stroke, we should economize our power more and more. But in
practice a great difficulty is met with—namely, that as the steam expands
its temperature falls. If the cut-off occurs early, say at one-third stroke, the
great expansion will reduce the temperature of the metal walls of the
cylinder to such an extent, that when the next spirt of steam enters from the
other end a considerable proportion of the steam's energy will be lost by
cooling. In such a case, the difference in temperature between admitted
steam and exhausted steam is too great for economy. Yet we want to utilize
as much energy as possible. How are we to do it?

COMPOUND ENGINES.

In the year 1853, John Elder, founder of the shipping firm of Elder and
Co., Glasgow, introduced the compound engine for use on ships. The steam,
when exhausted from the high-pressure cylinder, passed into another
cylinder of equal stroke but larger diameter, where the expansion continued.
In modern engines the expansion is extended to three and even four stages,
according to the boiler pressure; for it is a rule that the higher the initial
pressure is, the larger is the number of stages of expansion consistent with
economical working.

Fig. 29.—Sketch of the arrangement of a triple-expansion marine
engine. No valve gear or supports, etc., shown.
In Fig. 29 we have a triple-expansion marine engine. Steam enters the
high-pressure cylinder[4] at, say, 200 lbs. per square inch. It exhausts at 75
lbs. into the large pipe 2, and passes to the intermediate cylinder, whence it
is exhausted at 25 lbs. or so through pipe 3 to the low-pressure cylinder.
Finally, it is ejected at about 8 lbs. per square inch to the condenser, and is
suddenly converted into water; an act which produces a vacuum, and

diminishes the back-pressure of the exhaust from cylinder c. In fact, the
condenser exerts a sucking power on the exhaust side of c's piston.

ARRANGEMENT OF EXPANSION ENGINES.

In the illustration the cranks are set at angles of 120°, or a third of a
circle, so that one or other is always at or near the position of maximum
turning power. Where only two stages are used the cylinders are often
arranged tandem, both pistons having a common piston rod and crank. In
order to get a constant turning movement they must be mounted separately,
and work cranks set at right angles to one another.

COMPOUND LOCOMOTIVES.

In 1876 Mr. A. Mallet introduced compounding in locomotives; and the
practice has been largely adopted. The various types of "compounds" may
be classified as follows:—(1) One low-pressure and one high-pressure
cylinder; (2) one high-pressure and two low-pressure; (3) one low-pressure
and two high-pressure; (4) two high-pressure and two low-pressure. The
last class is very widely used in France, America, and Russia, and seems to
give the best results. Where only two cylinders are used (and sometimes in
the case of three and four), a valve arrangement permits the admission of
high-pressure steam to both high and low-pressure cylinders for starting a
train, or moving it up heavy grades.

REVERSING GEARS.

Figs. 30, 31, 32.—Showing how a reversing gear alters the
position of the slide-valve.
The engines of a locomotive or steamship must be reversible—that is,
when steam is admitted to the cylinders, the engineer must be able to so
direct it through the steam-ways that the cranks may turn in the desired
direction. The commonest form of reversing device (invented by George
Stephenson) is known as Stephenson's Link Gear. In Fig. 30 we have a
diagrammatic presentment of this gear. e1 and e2 are two eccentrics set
square with the crank at opposite ends of a diameter. Their rods are
connected to the ends of a link, l, which can be raised and lowered by
means of levers (not shown). b is a block which can partly revolve on a pin
projecting from the valve rod, working through a guide, g. In Fig. 31 the
link is half raised, or in "mid-gear," as drivers say. Eccentric e1 has pushed
the lower end of the link fully back; e2 has pulled it fully forward; and since
any movement of the one eccentric is counterbalanced by the opposite
movement of the other, rotation of the eccentrics would not cause the valve
to move at all, and no steam could be admitted to the cylinder.

Let us suppose that Fig. 30 denotes one cylinder, crank, rods, etc., of a
locomotive. The crank has come to rest at its half-stroke; the reversing lever
is at the mid-gear notch. If the engineer desires to turn his cranks in an anti-
clockwise direction, he raises the link, which brings the rod of e1 into line
with the valve rod and presses the block backwards till the right-hand port
is uncovered (Fig. 31). If steam be now admitted, the piston will be pushed
towards the left, and the engine will continue to run in an anti-clockwise
direction. If, on the other hand, he wants to run the engine the other way, he
would drop the link, bringing the rod of e2 into line with the valve rod, and
drawing v forward to uncover the rear port (Fig. 32). In either case the
eccentric working the end of the link remote from b has no effect, since it
merely causes that end to describe arcs of circles of which b is the centre.

"LINKING UP."

If the link is only partly lowered or raised from the central position it
still causes the engine to run accordingly, but the movement of the valve is
decreased. When running at high speed the engineer "links up" his reversing
gear, causing his valves to cut off early in the stroke, and the steam to work
more expansively than it could with the lever at full, or end, gear; so that
this device not only renders an engine reversible, but also gives the engineer
an absolute command over the expansion ratio of the steam admitted to the
cylinder, and furnishes a method of cutting off the steam altogether. In Figs.
30, 31, 32, the valve has no lap and the eccentrics are set square. In actual
practice the valve faces would have "lap" and the eccentric "lead" to
correspond; but for the sake of simplicity neither is shown.

OTHER GEARS.

In the Gooch gear for reversing locomotives the link does not shift, but
the valve rod and its block is raised or lowered. The Allan gear is so
arranged that when the link is raised the block is lowered, and vice versâ.
These are really only modifications of Stephenson's principle—namely, the
employment of two eccentrics set at equal angles to and on opposite sides of

the crank. There are three other forms of link-reversing gear, and nearly a
dozen types of radial reversing devices; but as we have already described
the three most commonly used on locomotives and ships, there is no need to
give particulars of these.

Before the introduction of Stephenson's gear a single eccentric was used
for each cylinder, and to reverse the engine this eccentric had to be loose on
the axle. "A lever and gear worked by a treadle on the footplate controlled
the position of the eccentrics. When starting the engine, the driver put the
eccentrics out of gear by the treadle; then, by means of a lever he raised the
small-ends[5] of the eccentric rods, and, noting the position of the cranks, or,
if more convenient, the balance weight in the wheels, he, by means of
another handle, moved the valves to open the necessary ports to steam and
worked them by hand until the engine was moving; then, with the treadle,
he threw the eccentrics over to engage the studs, at the same time dropping
the small-ends of the rods to engage pins upon the valve spindles, so that
they continued to keep up the movement of the valve."[6] One would
imagine that in modern shunting yards such a device would somewhat delay
operations!

PISTON VALVES.

In marine engines, and on many locomotives and some stationary
engines, the D-valve (shown in Figs. 30–32) is replaced by a piston valve,
or circular valve, working up and down in a tubular seating. It may best be
described as a rod carrying two pistons which correspond to the faces of a
D-valve. Instead of rectangular ports there are openings in the tube in
which the piston valve moves, communicating with the steam-ways into the
cylinder and with the exhaust pipe. In the case of the D-valve the pressure
above it is much greater than that below, and considerable friction arises if
the rubbing faces are not kept well lubricated. The piston valve gets over
this difficulty, since such steam as may leak past it presses on its
circumference at all points equally.

SPEED GOVERNORS.

Fig. 33.—A speed governor.
Practically all engines except locomotives and those known as "donkey-
engines"—used on cranes—are fitted with some device for keeping the
rotatory speed of the crank constant within very narrow limits. Perhaps you
have seen a pair of balls moving round on a seating over the boiler of a
threshing-engine. They form part of the "governor," or speed-controller,
shown in principle in Fig. 33. A belt driven by a pulley on the crank shaft
turns a small pulley, p, at the foot of the governor. This transmits motion
through two bevel-wheels, g, to a vertical shaft, from the top of which hang
two heavy balls on links, k k. Two more links, l l, connect the balls with a
weight, w, which has a deep groove cut round it at the bottom. When the

shaft revolves, the balls fly outwards by centrifugal force, and as their
velocity increases the quadrilateral figure contained by the four links
expands laterally and shortens vertically. The angles between k k and l l
become less and less obtuse, and the weight w is drawn upwards, bringing
with it the fork c of the rod a, which has ends engaging with the groove. As
c rises, the other end of the rod is depressed, and the rod b depresses rod o,
which is attached to the spindle operating a sort of shutter in the steam-pipe.
Consequently the supply of steam is throttled more and more as the speed
increases, until it has been so reduced that the engine slows, and the balls
fall, opening the valve again. Fig. 34 shows the valve fully closed. This
form of governor was invented by James Watt. A spring is often used
instead of a weight, and the governor is arranged horizontally so that it may
be driven direct from the crank shaft without the intervention of bevel
gearing.

The Hartwell governor employs
a link motion. You must here
picture the balls raising and
lowering the free end of the valve
rod, which carries a block moving
in a link connected with the
eccentric rod. The link is pivoted at
the upper end, and the eccentric rod
is attached to the lower. When the
engine is at rest the end of the valve
rod and its block are dropped till in
a line with the eccentric rod; but
when the machinery begins to work
the block is gradually drawn up by Fig. 34.
the governor, diminishing the
movement of the valve, and so shortening the period of steam admission to
the cylinder.

Governors are of special importance where the load of an engine is
constantly varying, as in the case of a sawmill. A good governor will limit
variation of speed within two per cent.—that is, if the engine is set to run at
100 revolutions a minute, it will not allow it to exceed 101 or fall below 99.
In very high-speed engines the governing will prevent variation of less than
one per cent., even when the load is at one instant full on, and the next
taken completely off.

MARINE GOVERNORS.

These must be more quick-acting than those used on engines provided
with fly-wheels, which prevent very sudden variations of speed. The screw
is light in proportion to the engine power, and when it is suddenly raised
from the water by the pitching of the vessel, the engine would race till the
screw took the water again, unless some regulating mechanism were
provided. Many types of marine governors have been tried. The most
successful seems to be one in which water is being constantly forced by a
pump driven off the engine shaft into a cylinder controlling a throttle-valve
in the main steam-pipe. The water escapes through a leak, which is
adjustable. As long as the speed of the engine is normal, the water escapes
from the cylinder as fast as it is pumped in, and no movement of the piston
results; but when the screw begins to race, the pump overcomes the leak,
and the piston is driven out, causing a throttling of the steam supply.

CONDENSERS.

The condenser serves two purposes:—(1) It makes it possible to use the
same water over and over again in the boilers. On the sea, where fresh
water is not obtainable in large quantities, this is a matter of the greatest
importance. (2) It adds to the power of a compound engine by exerting a
back pull on the piston of the low-pressure cylinder while the steam is being
exhausted.

Fig. 35.—The marine condenser.
Fig. 35 is a sectional illustration of a marine condenser. Steam enters the
condenser through the large pipe e, and passes among a number of very thin
copper tubes, through which sea-water is kept circulating by a pump. The
path of the water is shown by the featherless arrows. It comes from the
pump through pipe a into the lower part of a large cap covering one end of
the condenser and divided transversely by a diaphragm, d. Passing through
the pipes, it reaches the cap attached to the other end, and flows back
through the upper tubes to the outlet c. This arrangement ensures that, as the
steam condenses, it shall meet colder and colder tubes, and finally be turned
to water, which passes to the well through the outlet f. In some condensers
the positions of steam and water are reversed, steam going through the
tubes outside which cold water circulates.
[3] Also called ports.

[4] The bores of the cylinders are in the proportion of 4: 6: 9. The stroke of all
three is the same.

[5] The ends furthest from the eccentric.

[6] "The Locomotive of To-day," p. 87.

Chapter III.

THE STEAM TURBINE.

How a turbine works—The De Laval turbine—The Parsons turbine—Description of
the Parsons turbine—The expansive action of steam in a Parsons turbine—
Balancing the thrust—Advantages of the marine turbine.

M ORE than two thousand years ago Hero of Alexandria produced the
first apparatus to which the name of steam-engine could rightly be
given. Its principle was practically the same as that of the revolving jet used
to sprinkle lawns during dry weather, steam being used in the place of
water. From the top of a closed cauldron rose two vertical pipes, which at
their upper ends had short, right-angle bends. Between them was hung a
hollow globe, pivoted on two short tubes projecting from its sides into the
upright tubes. Two little L-shaped pipes projected from opposite sides of
the globe, at the ends of a diameter, in a plane perpendicular to the axis. On
fire being applied to the cauldron, steam was generated. It passed up
through the upright, through the pivots, and into the globe, from which it
escaped by the two L-shaped nozzles, causing rapid revolution of the ball.
In short, the first steam-engine was a turbine. Curiously enough, we have
reverted to this primitive type (scientifically developed, of course) in the
most modern engineering practice.

HOW A TURBINE WORKS.

In reciprocating—that is, cylinder—engines steam is admitted into a
chamber and the door shut behind it, as it were. As it struggles to expand, it
forces out one of the confining walls—that is, the piston—and presently the
door opens again, and allows it to escape when it has done its work. In
Hero's toy the impact of the issuing molecules against other molecules that
have already emerged from the pipes was used. One may compare the
reaction to that exerted by a thrown stone on the thrower. If the thrower is
standing on skates, the reaction of the stone will cause him to glide
backwards, just as if he had pushed off from some fixed object. In the case

of the reaction—namely, the Hero-type—turbine the nozzle from which the
steam or water issues moves, along with bodies to which it may be attached.
In action turbines steam is led through fixed nozzles or steam-ways, and the
momentum of the steam is brought to bear on the surfaces of movable
bodies connected with the shaft.

THE DE LAVAL TURBINE.

In its earliest form this turbine was a modification of Hero's. The wheel
was merely a pipe bent in S form, attached at its centre to a hollow vertical
shaft supplied with steam through a stuffing-box at one extremity. The
steam blew out tangentially from the ends of the S, causing the shaft to
revolve rapidly and work the machinery (usually a cream separator)
mounted on it. This motor proved very suitable for dairy work, but was too
wasteful of steam to be useful where high power was needed.

In the De Laval
turbine as now
constructed the steam
is blown from
stationary nozzles
against vanes mounted
on a revolving wheel.
Fig. 36 shows the
nozzles and a turbine
wheel. The wheel is
made as a solid disc, to
the circumference of
which the vanes are
dovetailed separately in
a single row. Each vane
Fig. 36.—The wheel and nozzles of a De Laval
turbine. is of curved section, the
concave side directed
towards the nozzles,

which, as will be gathered from the "transparent" specimen on the right of
our illustration, gradually expand towards the mouth. This is to allow the
expansion of the steam, and a consequent gain of velocity. As it issues, each
molecule strikes against the concave face of a vane, and, while changing its
direction, is robbed of its kinetic energy, which passes to the wheel. To turn
once more to a stone-throwing comparison, it is as if a boy were pelting the
wheel with an enormous number of tiny stones. Now, escaping high-
pressure steam moves very fast indeed. To give figures, if it enters the small
end of a De Laval nozzle at 200 lbs. per square inch, it will leave the big
end at a velocity of 48 miles per minute—that is, at a speed which would
take it right round the world in 8½ hours! The wheel itself would not move
at more than about one-third of this speed as a maximum.[7] But even so, it
may make as many as 30,000 revolutions per minute. A mechanical
difficulty is now encountered—namely, that arising from vibration. No
matter how carefully the turbine wheel may be balanced, it is practically
impossible to make its centre of gravity coincide exactly with the central
point of the shaft; in other words, the wheel will be a bit—perhaps only a
tiny fraction of an ounce—heavier on one side than the other. This want of
truth causes vibration, which, at the high speed mentioned, would cause the
shaft to knock the bearings in which it revolves to pieces, if—and this is the
point—those bearings were close to the wheel M. de Laval mounted the
wheel on a shaft long enough between the bearings to "whip," or bend a
little, and the difficulty was surmounted.

The normal speed of the turbine wheel is too high for direct driving of
some machinery, so it is reduced by means of gearing. To dynamos, pumps,
and air-fans it is often coupled direct.

THE PARSONS TURBINE.

At the grand naval review held in 1897 in honour of Queen Victoria's
diamond jubilee, one of the most noteworthy sights was the little Turbinia
of 44½ tons burthen, which darted about among the floating forts at a speed
much surpassing that of the fastest "destroyer." Inside the nimble little craft

were engines developing 2,000 horse power, without any of the clank and
vibration which usually reigns in the engine-room of a high-speed vessel.
The Turbinia was the first turbine-driven boat, and as such, even apart from
her extraordinary pace, she attracted great attention. Since 1897 the Parsons
turbine has been installed on many ships, including several men-of-war, and
it seems probable that the time is not far distant when reciprocating engines
will be abandoned on all high-speed craft.

DESCRIPTION OF THE PARSONS TURBINE.

Fig. 37.—Section of a Parsons turbine.
The essential parts of a Parsons turbine are:—(1) The shaft, on which is
mounted (2) the drum; (3) the cylindrical casing inside which the drum
revolves; (4) the vanes on the drum and casing; (5) the balance pistons. Fig.
37 shows a diagrammatic turbine in section. The drum, it will be noticed,
increases its diameter in three stages, d1, d2, d3, towards the right. From end
to end it is studded with little vanes, m m, set in parallel rings small
distances apart. Each vane has a curved section (see Fig. 38), the hollow
side facing towards the left. The vanes stick out from the drum like short
spokes, and their outer ends almost touch the casing. To the latter are
attached equally-spaced rings of fixed vanes, f f, pointing inwards towards
the drum, and occupying the intervals between the rings of moving vanes.
Their concave sides also face towards the left, but, as seen in Fig. 38, their
line of curve lies the reverse way to that of m m. Steam enters the casing at
a, and at once rushes through the vanes towards the outlet at b. It meets the

first row of fixed vanes, and has its path so deflected that it strikes the ring
of moving (or drum) vanes at the most effective angle, and pushes them
round. It then has its direction changed by the ring of f f, so that it may treat
the next row of m m in a similar fashion.

Fig. 38.—Blades or vanes of a Parsons turbine.

One of the low-pressure turbines of the Carmania, in casing.
Its size will be inferred from comparison with the man
standing near the end of the casing.

THE EXPANSIVE ACTION OF STEAM IN A TURBINE.

On reaching the end of d1 it enters the second, or intermediate, set of
vanes. The drum here is of a greater diameter, and the blades are longer and
set somewhat farther apart, to give a freer passage to the now partly
expanded steam, which has lost pressure but gained velocity. The process of
movement is repeated through this stage; and again in d3, the low-pressure
drum. The steam then escapes to the condenser through b, having by this
time expanded very many times; and it is found advisable, for reasons
explained in connection with compound steam-engines, to have a separate
turbine in an independent casing for the extreme stages of expansion.

The vanes are made of brass. In the turbines of the Carmania, the huge
Cunard liner, 1,115,000 vanes are used. The largest diameter of the drums is
11 feet, and each low-pressure turbine weighs 350 tons.

BALANCING OF THRUST.

The push exerted by the steam on the blades not only turns the drum, but
presses it in the direction in which the steam flows. This end thrust is
counterbalanced by means of the "dummy" pistons, p1, p2, p3. Each dummy
consists of a number of discs revolving between rings projecting from the
casing, the distance between discs and rings being so small that but little
steam can pass. In the high-pressure compartment the steam pushes p1 to the
left with the same pressure as it pushes the blades of d1 to the right. After
completing the first stage it fills the passage c, which communicates with
the second piston, p2, and the pressure on that piston negatives the thrust on
2 3
d . Similarly, the passage e causes the steam to press equally on p and the
vanes of d3. So that the bearings in which the shaft revolves have but little
thrust to take. This form of compensation is necessary in marine as well as
in stationary turbines. In the former the dummy pistons are so proportioned
that the forward thrust given by them and the screw combined is almost
equal to the thrust aft of the moving vanes.

One of the turbine drums of the Carmania. Note the rows of
vanes. The drum is here being tested for perfect balance on
two absolutely level supports.

ADVANTAGES OF THE MARINE TURBINE.

(1.) Absence of vibration. Reciprocating engines, however well
balanced, cause a shaking of the whole ship which is very unpleasant to
passengers. The turbine, on the other hand, being almost perfectly balanced,
runs so smoothly at the highest speeds that, if the hand be laid on the
covering, it is sometimes almost impossible to tell whether the machinery is
in motion. As a consequence of this smooth running there is little noise in
the engine-room—a pleasant contrast to the deafening roar of reciprocating
engines. (2.) Turbines occupy less room. (3.) They are more easily tended.
(4.) They require fewer repairs, since the rubbing surfaces are very small as
compared to those of reciprocating engines. (5.) They are more economical
at high speeds. It must be remembered that a turbine is essentially meant for
high speeds. If run slowly, the steam will escape through the many passages
without doing much work.

Owing to its construction, a turbine cannot be reversed like a cylinder
engine. It therefore becomes necessary to fit special astern turbines to one
or more of the screw shafts, for use when the ship has to be stopped or
moved astern. Under ordinary conditions these turbines revolve idly in their
cases.

The highest speed ever attained on the sea was the forty-two miles per
hour of the unfortunate Viper, a turbine destroyer which developed 11,500
horse power, though displacing only 370 tons. This velocity would compare
favourably with that of a good many expresses on certain railways that we
could name. In the future thirty miles an hour will certainly be attained by
turbine-driven liners.
[7] Even at this speed the wheel has a circumferential velocity of two-thirds
that of a bullet shot from a Lee-Metford rifle. A vane weighing only 250 grains
(about ½ oz.) exerts under these conditions a centrifugal pull of 15 cwt. on the
wheel!

Chapter IV.

THE INTERNAL-COMBUSTION ENGINE.

The meaning of the term—Action of the internal-combustion engine—The motor car
—The starting-handle—The engine—The carburetter—Ignition of the charge—
Advancing the spark—Governing the engine—The clutch—The gear-box—The
compensating gear—The silencer—The brakes—Speed of cars.

THE MEANING OF THE TERM "INTERNAL-COMBUSTION ENGINE."

I N the case of a steam-boiler the energy of combustion is transmitted to
water inside an air-tight vessel. The fuel does not actually touch the
"working fluid." In the gas or oil engine the fuel is brought into contact and
mixed with the working fluid, which is air. It combines suddenly with it in
the cylinder, and heat energy is developed so rapidly that the act is called an

explosion. Coal gas, mineral oils, alcohol, petrol, etc., all contain hydrogen
and carbon. If air, which contributes oxygen, be added to any of these in
due proportion, the mixture becomes highly explosive. On a light being
applied, oxygen and carbon unite, also hydrogen and oxygen, and violent
heat is generated, causing a violent molecular bombardment of the sides of
the vessel containing the mixture. Now, if the mixture be compressed it
becomes hotter and hotter, until a point is reached at which it ignites
spontaneously. Early gas-engines did not compress the charge before
ignition. Alphonse Beau de Rochas, a Frenchman, first thought of making
the piston of the engine squeeze the mixture before ignition; and from the
year 1862, when he proposed this innovation, the success of the internal-
combustion engine may be said to date.

Fig. 39.—Showing the four strokes that the piston of a
gas-engine makes during one "cycle."

ACTION OF THE ENGINE.

The gas-engine, the oil-engine, and the motor-car engine are similar in
general principles. The cylinder has, instead of a slide-valve, two, or
sometimes three, "mushroom" valves, which may be described as small and
thick round plates, with bevelled edges, mounted on the ends of short rods,
called stems. These valves open into the cylinder, upwards, downwards, or
horizontally, as the case may be; being pushed in by cams projecting from a
shaft rotated by the engine. For the present we will confine our attention to
the series of operations which causes the engine to work. This series is
called the Beau de Rochas, or Otto, cycle, and includes four movements of
the piston. Reference to Fig. 39 will show exactly what happens in a gas-
engine—(1) The piston moves from left to right, and just as the movement
commences valves g (gas) and a (air) open to admit the explosive mixture.
By the time that p has reached the end of its travel these valves have closed
again. (2) The piston returns to the left, compressing the mixture, which has
no way of escape open to it. At the end of the stroke the charge is ignited by
an incandescent tube i (in motor car and some stationary engines by an
electric spark), and (3) the piston flies out again on the "explosion" stroke.
Before it reaches the limit position, valve e (exhaust) opens, and (4) the
piston flies back under the momentum of the fly-wheel, driving out the
burnt gases through the still open e. The "cycle" is now complete. There has
been suction, compression (including ignition), combustion, and
exhaustion. It is evident that a heavy fly-wheel must be attached to the
crank shaft, because the energy of one stroke (the explosion) has to serve
for the whole cycle; in other words, for two complete revolutions of the
crank. A single-cylinder steam-engine develops an impulse every half-turn
—that is, four times as often. In order to get a more constant turning effect,
motor cars have two, three, four, six, and even eight cylinders. Four-
cylinder engines are at present the most popular type for powerful cars.

THE MOTOR CAR.

Fig. 40.—Plan of the chassis of a motor car.
We will now proceed to an examination of the motor car, which, in
addition to mechanical apparatus for the transmission of motion to the
driving-wheels, includes all the fundamental adjuncts of the internal-
combustion engine.[8] Fig. 40 is a bird's-eye view of the chassis (or "works"
and wheels) of a car, from which the body has been removed. Starting at the
left, we have the handle for setting the engine in motion; the engine (a two-
cylinder in this case); the fly-wheel, inside which is the clutch; the gear-
box, containing the cogs for altering the speed of revolution of the driving-
wheels relatively to that of the engine; the propeller shaft; the silencer, for
deadening the noise of the exhaust; and the bevel-gear, for turning the
driving-wheels. In the particular type of car here considered you will notice
that a "direct," or shaft, drive is used. The shaft has at each end a flexible, or
"universal," joint, which allows the shaft to turn freely, even though it may
not be in a line with the shaft projecting from the gear-box. It must be
remembered that the engine and gear-box are mounted on the frame,
between which and the axles are springs, so that when the car bumps up and
down, the shaft describes part of a circle, of which the gear-box end is the
centre.

An alternative method of driving is by means of chains, which run round
sprocket (cog) wheels on the ends of a shaft crossing the frame just behind

the gear-box, and round larger sprockets attached to the hubs of the driving-
wheels. In such a case the axles of the driving-wheel are fixed to the
springs, and the wheels revolve round them. Where a Cardan (shaft) drive is
used the axles are attached rigidly to the wheels at one end, and extend,
through tubes fixed to the springs, to bevel-wheels in a central
compensating-gear box (of which more presently).

Several parts—the carburetter, tanks, governor, and pump—are not
shown in the general plan. These will be referred to in the more detailed
account that follows.

THE STARTING-HANDLE.

Fig. 41.—The starting-handle.

Fig. 41 gives the starting-handle in part section. The handle h is attached
to a tube which terminates in a clutch, c. A powerful spring keeps c
normally apart from a second clutch, c1, keyed to the engine shaft. When
the driver wishes to start the engine he presses the handle towards the right,
brings the clutches together, and turns the handle in a clockwise direction.
As soon as the engine begins to fire, the faces of the clutches slip over one
another.

THE ENGINE.

Fig. 42.—End and cross sections of a two-cylinder motor.
We next examine the two-cylinder engine (Fig. 42). Each cylinder is
surrounded by a water-jacket, through which water is circulated by a
pump[9] (Fig. 43). The heat generated by combustion is so great that the
walls of the cylinder would soon become red-hot unless some of the heat
were quickly carried away. The pistons are of "trunk" form—that is, long
enough to act as guides and absorb the oblique thrust of the piston rods.
Three or more piston rings lying in slots (not shown) prevent the escape of
gas past the piston. It is interesting to notice that the efficiency of an
internal-combustion engine depends so largely on the good fit of these
moving parts, that cylinders, pistons, and rings must be exceedingly true. A
good firm will turn out standard parts which are well within 1⁄5000 of an inch
of perfect truth. It is also a wonderful testimony to the quality of the
materials used that, if properly looked after, an engine which has made
many millions of revolutions, at the rate of 1,000 to 2,000 per minute, often
shows no appreciable signs of wear. In one particular test an engine was run
continuously for several months, and at the end of the trial was in absolutely
perfect condition.

The cranks revolve in an oil-tight case (generally made of aluminium),
and dip in oil, which they splash up into the cylinder to keep the piston well
lubricated. The plate, p p, through a slot in which the piston rod works,
prevents an excess of oil being flung up. Channels are provided for leading
oil into the bearings. The cranks are 180° apart. While one piston is being
driven out by an explosion, the other is compressing its charge prior to
ignition, so that the one action deadens the other. Therefore two explosions
occur in one revolution of the cranks, and none during the next revolution.
If both cranks were in line, the pistons would move together, giving one
explosion each revolution.

Fig. 43.—Showing how the water which cools the
cylinders is circulated.
The valve seats, and the inlet and exhaust pipes, are seen in section. The
inlet valve here works automatically, being pulled in by suction; but on
many engines—on all powerful engines—the inlet, like the exhaust valve,
is lifted by a cam, lest it should stick or work irregularly. Three dotted
circles show a, a cog on the crank shaft; b, a "lay" cog, which transmits
motion to c, on a short shaft rotating the cam that lifts the exhaust valve. c,
having twice as many teeth as a, revolves at half its rate. This ensures that
the valve shall be lifted only once in two revolutions of the crank shaft to
which it is geared. The cogs are timed, or arranged, so that the cam begins
to lift the valve when the piston has made about seven-eighths of its
explosion stroke, and closes the valve at the end of the exhaust stroke.

THE CARBURETTER.

A motor car generally uses petrol as its fuel. Petrol is one of the more
volatile products of petroleum, and has a specific gravity of about 680—
that is, volume for volume, its weight is to that of water in the proportion of
680 to 1,000. It is extremely dangerous, as it gives off an inflammable gas
at ordinary temperatures. Benzine, which we use to clean clothes, is
practically the same as petrol, and should be treated with equal care. The
function of a carburetter is to reduce petrol to a very fine spray and mix it
with a due quantity of air. The device consists of two main parts (Fig. 44)—
the float chamber and the jet chamber. In the former is a contrivance for
regulating the petrol supply. A float—a cork, or air-tight metal box—is
arranged to move freely up and down the stem of a needle-valve, which
closes the inlet from the tank. At the bottom of the chamber are two pivoted
levers, w w, which, when the float rests on them, tip up and lift the valve.
Petrol flows in and raises the float. This allows the valve to sink and cut off
the supply. If the valve is a good fit and the float is of the correct weight,
the petrol will never rise higher than the tip of the jet g.

Fig. 44.—Section of a carburetter.

The suction of the engine makes petrol spirt through the jet (which has a
very small hole in its end) and atomize itself against a spraying-cone, a. It
then passes to the engine inlet pipe through a number of openings, after
mixing with air entering from below. An extra air inlet, controllable by the
driver, is generally added, unless the carburetter be of a type which
automatically maintains constant proportions of air and vapour. The jet
chamber is often surrounded by a jacket, through which part of the hot
exhaust gases circulate. In cold weather especially this is a valuable aid to
vaporization.

Fig. 45.—Sketch of the electrical ignition arrangements on
a motor car.

IGNITION OF THE CHARGE.

All petrol-cars now use electrical ignition. There are two main systems
—(1) by an accumulator and induction coil; (2) magneto ignition, by means

of a small dynamo driven by the engine. A general arrangement of the first
is shown in Fig. 45. A disc, d, of some insulating material—fibre or
vulcanite—is mounted on the cam, or half-speed, shaft. Into the
circumference is let a piece of brass, called the contact-piece, through
which a screw passes to the cam shaft. A movable plate, m p, which can be
rotated concentrically with d through part of a circle, carries a "wipe" block
at the end of a spring, which presses it against d. The spring itself is
attached to an insulated plate. When the revolution of d brings the wipe and
contact together, current flows from the accumulator through switch s to the
wipe; through the contact-piece to c; from c to m p and the induction coil;
and back to the accumulator. This is the primary, or low-tension, circuit. A
high-tension current is induced by the coil in the secondary circuit,
indicated by dotted lines.[10] In this circuit is the sparking-plug (see Fig.
46), having a central insulated rod in connection with one terminal of the
secondary coil. Between it and a bent wire projecting from the iron casing
of the plug (in contact with the other terminal of the secondary coil through
the metal of the engine, to which one wire of the circuit is attached) is a
small gap, across which the secondary current leaps when the primary
current is broken by the wipe and contact parting company. The spark is
intensely hot, and suffices to ignite the compressed charge in the cylinder.

Fig. 46.—Section of a sparking-plug.

ADVANCING THE SPARK.

We will assume that the position of w (in Fig. 45) is such that the contact
touches w at the moment when the piston has just completed the
compression stroke. Now, the actual combustion of the charge occupies an
appreciable time, and with the engine running at high speed the piston
would have travelled some way down the cylinder before the full force of
the explosion was developed. But by raising lever l, the position of w may
be so altered that contact is made slightly before the compression stroke is
complete, so that the charge is fairly alight by the time the piston has altered
its direction. This is called advancing the spark.

GOVERNING THE ENGINE.

There are several methods of controlling the speed of internal-
combustion engines. The operating mechanism in most cases is a
centrifugal ball-governor. When the speed has reached the fixed limit it
either (1) raises the exhaust valve, so that no fresh charges are drawn in; (2)

prevents the opening of the inlet valve; or (3) throttles the gas supply. The
last is now most commonly used on motor cars, in conjunction with some
device for putting it out of action when the driver wishes to exceed the
highest speed that it normally permits.

Fig. 47.—One form of governor used on motor cars.
A sketch of a neat governor, with regulating attachment, is given in Fig.
47. The governor shaft is driven from the engine. As the balls, b b, increase
their velocity, they fly away from the shaft and move the arms, a a, and a
sliding tube, c, towards the right. This rocks the lever r, and allows the
valves in the inlet pipe to close and reduce the supply of air and gas. A
wedge, w, which can be raised or lowered by lever l, intervenes between
the end of r and the valve stem. If this lever be lifted to its highest position,
the governing commences at a lower speed, as the valve then has but a short
distance to travel before closing completely. For high speeds the driver
depresses l, forces the wedge down, and so minimizes the effect of the
governor.

THE CLUTCH.

The engine shaft has on its rear end the fly-wheel, which has a broad and
heavy rim, turned to a conical shape inside. Close to this, revolving loosely
on the shaft, is the clutch plate, a heavy disc with a broad edge so shaped as
to fit the inside of a fly-wheel. It is generally faced with leather. A very
strong spring presses the plate into the fly-wheel, and the resulting friction
is sufficient to prevent any slip. Projections on the rear of the clutch engage
with the gear-box shaft. The driver throws out the clutch by depressing a
lever with his foot. Some clutches dispense with the leather lining. These
are termed metal to metal clutches.

THE GEAR-BOX.

We now come to a very interesting detail of the motor car, the gear-box.
The steam-engine has its speed increased by admitting more steam to the
cylinders. But an explosion engine must be run at a high speed to develop
its full power, and when heavier work has to be done on a hill it becomes
necessary to alter the speed ratio of engine to driving-wheels. Our
illustration (Fig. 48) gives a section of a gear-box, which will serve as a
typical example. It provides three forward speeds and one reverse. To
understand how it works, we must study the illustration carefully. Pinion 1
is mounted on a hollow shaft turned by the clutch. Into the hollow shaft
projects the end of another shaft carrying pinions 6 and 4. Pinion 6 slides up
and down this shaft, which is square at this point, but round inside the loose
pinion 4. Pinions 2 and 3 are keyed to a square secondary shaft, and are
respectively always in gear with 1 and 4; but 5 can be slid backwards and
forwards so as to engage or disengage with 6. In the illustration no gear is
"in." If the engine is working, 1 revolves 2, 2 turns 3, and 3 revolves 4 idly
on its shaft.

Fig. 48.—The gear-box of a motor car.

To get the lowest, or "first," speed the driver moves his lever and slides 5
into gear with 6. The transmission then is: 1 turns 2, 2 turns 5, 5 turns 6, 6
turns the propeller shaft through the universal joint. For the second speed, 5
and 6 are disengaged, and 6 is moved up the page, as it were, till projections
on it interlock with slots in 4; thus driving 1, 2, 3, 4, shaft. For the third, or
"solid," speed, 6 is pulled down into connection with 1, and couples the
engine shaft direct to the propeller shaft.

The "reverse" is accomplished by raising a long pinion, 7, which lies in
the gear-box under 5 and 6. The drive then is 1, 2, 5, 7, 6. There being an
odd number of pinions now engaged, the propeller shaft turns in the reverse
direction to that of the engine shaft.

Fig. 49.

THE COMPENSATING GEAR.

Every axle of a railway train carries a wheel at each end, rigidly attached to it.
When rounding a corner the outside wheel has further to travel than the other, and
consequently one or both wheels must slip. The curves are made so gentle, however,
that the amount of slip is very small. But with a traction-engine, motor car, or
tricycle the case is different, for all have to describe circles of very small diameter in
proportion to the length of the vehicle. Therefore in every case a compensating gear
is fitted, to allow the wheels to turn at different speeds, while permitting them both
to drive. Fig. 49 is an exaggerated sketch of the gear. The axles of the moving
wheels turn inside tubes attached to the springs and a central casing (not shown), and
terminate in large bevel-wheels, c and d. Between these are small bevels mounted on
a shaft supported by the driving drum. If the latter be rotated, the bevels would turn
c and d at equal speeds, assuming that both axles revolve without friction in their
bearings. We will suppose that the drum is turned 50 times a minute. Now, if one
wheel be held, the other will revolve 100 times a minute; or, if one be slowed, the
other will increase its speed by a corresponding amount. The average speed remains
50. It should be mentioned that drum a has incorporated with it on the outside a
bevel-wheel (not shown) rotated by a smaller bevel on the end of the propeller shaft.

THE SILENCER.

The petrol-engine, as now used, emits the products of combustion at a high
pressure. If unchecked, they expand violently, and cause a partial vacuum in the
exhaust pipe, into which the air rushes back with such violence as to cause a loud
noise. Devices called silencers are therefore fitted, to render the escape more
gradual, and split it up among a number of small apertures. The simplest form of
silencer is a cylindrical box, with a number of finely perforated tubes passing from
end to end of it. The exhaust gases pouring into the box maintain a constant pressure
somewhat higher than that of the atmosphere, but as the gases are escaping from it in
a fairly steady stream the noise becomes a gentle hiss rather than a "pop." There are
numerous types of silencers, but all employ this principle in one form or another.

THE BRAKES.

Every car carries at least two brakes of band pattern—one, usually worked by a
side hand-lever, acting on the axle or hubs of the driving-wheel; the other, operated
by the foot, acting on the transmission gear (see Fig. 48). The latter brake is
generally arranged to withdraw the clutch simultaneously. Tests have proved that
even heavy cars can be pulled up in astonishingly short distances, considering their
rate of travel. Trials made in the United States with a touring car and a four-in-hand
coach gave 25⅓ and 70 feet respectively for the distance in which the speed could
be reduced from sixteen miles per hour to zero.

SPEED OF CARS.

As regards speed, motor cars can rival the fastest express trains, even on long
journeys. In fact, feats performed during the Gordon-Bennett and other races have
equalled railway performances over equal distances. When we come to record
speeds, we find a car, specially built for the purpose, covering a mile in less than half
a minute. A speed of over 120 miles an hour has actually been reached. Engines of
150 h.p. can now be packed into a vehicle scaling less than 1½ tons. Even on touring
cars are often found engines developing 40 to 60 h.p., which force the car up steep
hills at a pace nothing less than astonishing. In the future the motor car will
revolutionize our modes of life to an extent comparable to the changes effected by
the advent of the steam-engine. Even since 1896, when the "man-with-the-flag" law
was abolished in the British Isles, the motor has reduced distances, opened up
country districts, and generally quickened the pulses of the community in a manner
which makes it hazardous to prophesy how the next generation will live.

Note.—The author is much indebted to Mr. Wilfrid J. Lineham, M. Inst. C.E., for several of the illustrations
which appear in the above chapter.

[8] Steam-driven cars are not considered in this chapter, as their principle is much the
same as that of the ordinary locomotive.
[9] On some cars natural circulation is used, the hot water flowing from the top of the
cylinder to the tank, from which it returns, after being cooled, to the bottom of the
cylinder.

[10] For explanation of the induction coil, see p. 122

Chapter V.

ELECTRICAL APPARATUS.

What is electricity?—Forms of electricity—Magnetism—The permanent magnet—Lines of
force—Electro-magnets—The electric bell—The induction coil—The condenser—
Transformation of current—Uses of the induction coil.

WHAT IS ELECTRICITY?

O F the ultimate nature of electricity, as of that of heat and light, we are at
present ignorant. But it has been clearly established that all three phenomena
are but manifestations of the energy pervading the universe. By means of suitable
apparatus one form can be converted into another form. The heat of fuel burnt in a
boiler furnace develops mechanical energy in the engine which the boiler feeds with
steam. The engine revolves a dynamo, and the electric current thereby generated can
be passed through wires to produce mechanical motion, heat, or light. We must
remain content, therefore, with assuming that electricity is energy or motion
transmitted through the ether from molecule to molecule, or from atom to atom, of
matter. Scientific investigation has taught us how to produce it at will, how to
harness it to our uses, and how to measure it; but not what it is. That question may,
perhaps, remain unanswered till the end of human history. A great difficulty
attending the explanation of electrical action is this—that, except in one or two
cases, no comparison can be established between it and the operation of gases and
fluids. When dealing with the steam-engine, any ordinary intelligence soon grasps
the principles which govern the use of steam in cylinders or turbines. The diagrams
show, it is hoped, quite plainly "how it works." But electricity is elusive, invisible;

and the greatest authorities cannot say what goes on at the poles of a magnet or on
the surface of an electrified body. Even the existence of "negative" and "positive"
electricity is problematical. However, we see the effects, and we know that if one
thing is done another thing happens; so that we are at least able to use terms which,
while convenient, are not at present controverted by scientific progress.

FORMS OF ELECTRICITY.

Rub a vulcanite rod and hold one end near some tiny pieces of paper. They fly to
it, stick to it for a time, and then fall off. The rod was electrified—that is, its surface
was affected in such a way as to be in a state of molecular strain which the contact of
the paper fragments alleviated. By rubbing large surfaces and collecting the
electricity in suitable receivers the strain can be made to relieve itself in the form of
a violent discharge accompanied by a bright flash. This form of electricity is known
as static.

Next, place a copper plate and a zinc plate into a jar full of diluted sulphuric acid.
If a wire be attached to them a current of electricity is said to flow along the wire.
We must not, however, imagine that anything actually moves along inside the wire,
as water, steam, or air, passes through a pipe. Professor Trowbridge says,[11] "No
other agency for transmitting power can be stopped by such slight obstacles as
electricity. A thin sheet of paper placed across a tube conveying compressed air
would be instantly ruptured. It would take a wall of steel at least an inch thick to
stand the pressure of steam which is driving a 10,000 horse-power engine. A thin
layer of dirt beneath the wheels of an electric car can prevent the current which
propels the car from passing to the rail, and then back to the power-house." There
would, indeed, be a puncture of the paper if the current had a sufficient voltage, or
pressure; yet the fact remains that current electricity can be very easily confined to
its conductor by means of some insulating or nonconducting envelope.

MAGNETISM.

The most familiar form of electricity is that known as magnetism. When a bar of
steel or iron is magnetized, it is supposed that the molecules in it turn and arrange
themselves with all their north-seeking poles towards the one end of the bar, and
their south-seeking poles towards the other. If the bar is balanced freely on a pivot, it
comes to rest pointing north and south; for, the earth being a huge magnet, its north
pole attracts all the north-seeking poles of the molecules, and its south poles the

south-seeking poles. (The north-seeking pole of a magnet is marked N., though it is
in reality the south pole; for unlike poles are mutually attractive, and like poles
repellent.)

There are two forms of magnet—permanent and temporary. If steel is
magnetized, it remains so; but soft iron loses practically all its magnetism as soon as
the cause of magnetization is withdrawn. This is what we should expect; for steel is
more closely compacted than iron, and the molecules therefore would be able to turn
about more easily.[12] It is fortunate for us that this is so, since on the rapid
magnetization and demagnetization of soft iron depends the action of many of our
electrical mechanisms.

THE PERMANENT MAGNET.

Magnets are either (1) straight, in which case they are called bar magnets; or (2)
of horseshoe form, as in Figs. 50 and 51. By bending the magnet the two poles are
brought close together, and the attraction of both may be exercised simultaneously
on a bar of steel or iron.

LINES OF FORCE.

In Fig. 50 are seen a number of dotted lines. These are called lines of magnetic
force. If you lay a sheet of paper on a horseshoe magnet and sprinkle it with iron
dust, you will at once notice how the particles arrange themselves in curves similar
in shape to those shown in the illustration. It is supposed (it cannot be proved) that
magnetic force streams away from the N. pole and describes a circular course
through the air back to the S. pole. The same remark applies to the bar magnet.

ELECTRICAL MAGNETS.

If an insulated wire is wound round and round a steel or iron bar from end to end,
and has its ends connected to the terminals of an electric battery, current rotates
round the bar, and the bar is magnetized. By increasing the strength and volume of
the current, and multiplying the number of turns of wire, the attractive force of the
magnet is increased. Now disconnect the wires from the battery. If of iron, the
magnet at once loses its attractive force; but if of steel, it retains it in part. Instead of
a simple horseshoe-shaped bar, two shorter bars riveted into a plate are generally
used for electromagnets of this type. Coils of wire are wound round each bar, and
connected so as to form one continuous whole; but the wire of one coil is wound in

the direction opposite to that of the other.
The free end of each goes to a battery
terminal.

In Fig. 51 you will notice that some of
the "lines of force" are deflected through the
iron bar a. They pass more easily through
iron than through air; and will choose iron
by preference. The attraction exercised by a
magnet on iron may be due to the effort of
the lines of force to shorten their paths. It is
evident that the closer a comes to the poles
of the magnet the less will be the distance to
be travelled from one pole to the bar, along
it, and back to the other pole.
Fig. 50.—Permanent magnet, and
the "lines of force" emanating H
from it. avin
g
now considered electricity in three of its
forms—static, current, and rotatory—we
will pass to some of its applications.

THE ELECTRIC BELL.

A fit device to begin with is the Electric
Bell, which has so largely replaced wire-
pulled bells. These last cause a great deal of
trouble sometimes, since if a wire snaps it
may be necessary to take up carpets and
floor-boards to put things right. Their
installation is not simple, for at every corner Fig. 51.—Electro-magnet: a,
must be put a crank to alter the direction of armature; b, battery.
the pull, and the cranks mean increased
friction. But when electric wires have once been properly installed, there should be
no need for touching them for an indefinite period. They can be taken round as many
corners as you wish without losing any of their conductivity, and be placed wherever
is most convenient for examination. One bell may serve a large number of rooms if

an indicator be used to show where the call was made from, by a card appearing in
one of a number of small windows. Before answering a call, the attendant presses in
a button to return the card to its normal position.

In Fig. 52 we have a diagrammatic view of an electric bell and current. When the
bell-push is pressed in, current flows from the battery to terminal t1, round the
electro-magnet m, through the pillar p and flat steel springs s and b, through the
platinum-pointed screw, and back to the battery through the push. The circulation of
current magnetizes m, which attracts the iron armature a attached to the spring s, and
draws the hammer h towards the gong. Just before the stroke occurs, the spring b
leaves the tip of the screw, and the circuit is broken, so that the magnet no longer
attracts. h is carried by its momentum against the gong, and is withdrawn by the
spring, until b once more makes contact, and the magnet is re-excited. The hammer
vibrations recur many times a second as long as the push is pressed in.

Fig. 52.—Sketch of an electric-bell circuit.
The electric bell is used for so many purposes that they cannot all be noted. It
plays an especially important part in telephonic installations to draw the attention of
the subscribers, forms an item in automatic fire and burglar alarms, and is a
necessary adjunct of railway signalling cabins.

THE INDUCTION OR RUHMKORFF COIL.

Reference was made in connection with the electrical ignition of internal-
combustion engines (p. 101) to the induction coil. This is a device for increasing the
voltage, or pressure, of a current. The two-cell accumulator carried in a motor car
gives a voltage (otherwise called electro-motive force = E.M.F.) of 4·4 volts. If you
attach a wire to one terminal of the accumulator and brush the loose end rapidly
across the other terminal, you will notice that a bright spark passes between the wire
and the terminal. In reality there are two sparks, one when they touch, and another
when they separate, but they occur so closely together that the eye cannot separate
the two impressions. A spark of this kind would not be sufficiently hot to ignite a
charge in a motor cylinder, and a spark from the induction coil is therefore used.

Fig. 53.—Sketch of an induction coil.
We give a sketch of the induction coil in Fig. 53. It consists of a core of soft iron
wires round which is wound a layer of coarse insulated wire, denoted by the thick
line. One end of the winding of this primary coil is attached to the battery, the other
to the base of a hammer, h, vibrating between the end of the core and a screw, s,
passing through an upright, t, connected with the other terminal of the battery. The
action of the hammer is precisely the same as that of the armature of an electric bell.
Outside the primary coil are wound many turns of a much finer wire completely
insulated from the primary coil. The ends of this secondary coil are attached to the
objects (in the case of a motor car, the insulated wire of the sparking-plug and a wire

projecting from its outer iron casing) between which a spark has to pass. As soon as
h touches s the circuit is completed. The core becomes a powerful magnet with
external lines of force passing from one pole to the other over and among the turns
of the secondary coil. h is almost instantaneously attracted by the core, and the break
occurs. The lines of force now (at least so it is supposed) sink into the core, cutting
through the turns of the "secondary," and causing a powerful current to flow through
them. The greater the number of turns, the greater the number of times the lines of
force are cut, and the stronger is the current. If sufficiently intense, it jumps any gap
in the secondary circuit, heating the intermediate air to a state of incandescence.

THE CONDENSER.

The sudden parting of h and s would produce strong sparking across the gap
between them if it were not for the condenser, which consists of a number of tinfoil
sheets separated by layers of paraffined paper. All the "odd" sheets are connected
with t, all the "even" with t1. Now, the more rapid the extinction of magnetism in
the core after "break" of the primary circuit, the more rapidly will the lines of force
collapse, and the more intense will be the induced current in the secondary coil. The
condenser diminishes the period of extinction very greatly, while lengthening the
period of magnetization after the "make" of the primary current, and so decreasing
the strength of the reverse current.

TRANSFORMATION OF CURRENT.

The difference in the voltage of the primary and secondary currents depends on
the length of the windings. If there are 100 turns of wire in the primary, and 100,000
turns in the secondary, the voltage will be increased 1,000 times; so that a 4-volt
current is "stepped up" to 4,000 volts. In the largest induction coils the secondary
winding absorbs 200–300 miles of wire, and the spark given may be anything up to
four feet in length. Such a spark would pierce a glass plate two inches thick.

It must not be supposed that an induction coil increases the amount of current
given off by a battery. It merely increases its pressure at the expense of its volume—
stores up its energy, as it were, until there is enough to do what a low-tension flow
could not effect. A fair comparison would be to picture the energy of the low-tension
current as the momentum of a number of small pebbles thrown in succession at a
door, say 100 a minute. If you went on pelting the door for hours you might make no

impression on it, but if you could knead every 100 pebbles into a single stone, and
throw these stones one per minute, you would soon break the door in.

Any intermittent current can be transformed as regards its intensity. You may
either increase its pressure while decreasing its rate of flow, or amperage; or
decrease its pressure and increase its flow. In the case that we have considered, a
continuous battery current is rendered intermittent by a mechanical contrivance. But
if the current comes from an "alternating" dynamo—that is, is already intermittent—
the contact-breaker is not needed. There will be more to say about transformation of
current in later paragraphs.

USES OF THE INDUCTION COIL.

The induction coil is used—(1.) For passing currents through glass tubes almost
exhausted of air or containing highly rarefied gases. The luminous effects of these
"Geissler" tubes are very beautiful. (2.) For producing the now famous X or Röntgen
rays. These rays accompany the light rays given off at the negative terminal
(cathode) of a vacuum tube, and are invisible to the eye unless caught on a
fluorescent screen, which reduces their rate of vibration sufficiently for the eye to be
sensitive to them. The Röntgen rays have the peculiar property of penetrating many
substances quite opaque to light, such as metals, stone, wood, etc., and as a
consequence have proved of great use to the surgeon in localizing or determining the
nature of an internal injury. They also have a deterrent effect upon cancerous
growths. (3.) In wireless telegraphy, to cause powerful electric oscillations in the
ether. (4.) On motor cars, for igniting the cylinder charges. (5.) For electrical
massage of the body.
[11] "What is Electricity?" p. 46.

[12] If a magnetized bar be heated to white heat and tapped with a hammer it loses its
magnetism, because the distance between the molecules has increased, and the molecules
can easily return to their original positions.

Chapter VI.

THE ELECTRIC TELEGRAPH.

Needle instruments—Influence of current on the magnetic needle—Method of reversing the
current—Sounding instruments—Telegraphic relays—Recording telegraphs—High-speed
telegraphy.

T AKE a small pocket compass and wind several turns of fine insulated wire
round the case, over the top and under the bottom. Now lay the compass on a
table, and turn it about until the coil is on a line with the needle—in fact, covers it.
Next touch the terminals of a battery with the ends of the wire. The needle at once
shifts either to right or left, and remains in that position as long as the current flows.
If you change the wires over, so reversing the direction of the current, the needle at
once points in the other direction. It is to this conduct on the part of a magnetic
needle when in a "magnetic field" that we owe the existence of the needle telegraph
instrument.

NEEDLE INSTRUMENTS.

Probably the best-known needle
instrument is the Cooke-Wheatstone, largely
used in signal-boxes and in some post-
offices. A vertical section of it is shown in
Fig. 54. It consists of a base, b, and an
upright front, a, to the back of which are
attached two hollow coils on either side of a
magnetic needle mounted on the same shaft
as a second dial needle, n, outside the front.
The wires w w are connected to the
telegraph line and to the commutator, a
device which, when the operator moves the
handle h to right and left, keeps reversing
the direction of the current. The needles on
both receiving and transmitting instruments
wag in accordance with the movements of
the handle. One or more movements form an
alphabetical letter of the Morse code. Thus,
if the needle points first to left, and then to
Fig. 54.—Sketch of the side right, and comes to rest in a normal position
elevation of a Wheatstone needle for a moment, the letter A is signified; right-
instrument. left-left-left in quick succession = B; right-

left-right-left = C, and so on. Where a marking instrument is used, a dot signifies a
"left," and a dash a right; and if a "sounder" is employed, the operator judges by the
length of the intervals between the clicks.

INFLUENCE OF CURRENT ON A MAGNETIC NEEDLE.

Figs. 55 and 56 are two views of the coils
and magnetic needle of the Wheatstone
instrument as they appear from behind. In
Fig. 55 the current enters the left-hand coil
from the left, and travels round and round it
in a clockwise direction to the other end,
whence it passes to the other coil and away
to the battery. Now, a coil through which a
current passes becomes a magnet. Its
polarity depends on the direction in which
the current flows. Suppose that you are
looking through the coil, and that the current
enters it from your end. If the wire is wound
in a clockwise direction, the S. pole will be
nearest you; if in an anti-clockwise
direction, the N. pole. In Fig. 55 the N. poles Figs. 55, 56.—The coils of a needle
are at the right end of the coils, the S. poles instrument. The arrows show the
direction taken by the current.
at the left end; so the N. pole of the needle is
attracted to the right, and the S. pole to the
left. When the current is reversed, as in Fig. 56, the needle moves over. If no current
passes, it remains vertical.

METHOD OF REVERSING THE CURRENT.

Fig. 57.—General arrangement of needle-instrument
circuit. The shaded plates on the left (b and r) are in
contact.
A simple method of changing the direction of the current in a two-instrument
circuit is shown diagrammatically in Fig. 57. The principle is used in the
Wheatstone needle instrument. The battery terminals at each station are attached to
two brass plates, a b, a1 b1. Crossing these at right angles (under a a1 and over b b1)
are the flat brass springs, l r, l1 r1, having buttons at their lower ends, and fixed at
their upper ends to baseboards. When at rest they all press upwards against the plates
1 1
a and a respectively. r and l are connected with the line circuit, in which are the
coils of dials 1 and 2, one at each station. l and r1 are connected with the earth-
plates e e1. An operator at station 1 depresses r so as to touch b. Current now flows
from the battery to b, thence through r to the line circuit, round the coils of both
dials through l1 a1 and r to earth-plate e1, through the earth to e, and then back to
the battery through l and a. The needles assume the position shown. To reverse the
current the operator allows r to rise into contact with a, and depresses l to touch b.
The course can be traced out easily.

In the Wheatstone "drop-handle" instrument (Fig. 54) the commutator may be
described as an insulated core on which are two short lengths of brass tubing. One of
these has rubbing against it a spring connected with the + terminal of the battery; the
other has similar communication with the – terminal. Projecting from each tube is a
spike, and rising from the baseboard are four upright brass strips not quite touching
the commutator. Those on one side lead to the line circuit, those on the other to the
earth-plate. When the handle is turned one way, the spikes touch the forward line
strip and the rear earth strip, and vice versâ when moved in the opposite direction.

SOUNDING INSTRUMENTS.

Sometimes little brass strips are attached to the dial plate of a needle instrument
for the needle to strike against. As these give different notes, the operator can
comprehend the message by ear alone. But the most widely used sounding
instrument is the Morse sounder, named after its inventor. For this a reversible
current is not needed. The receiver is merely an electro-magnet (connected with the
line circuit and an earth-plate) which, when a current passes, attracts a little iron bar
attached to the middle of a pivoted lever. The free end of the lever works between
two stops. Every time the circuit is closed by the transmitting key at the sending

station the lever flies down against the lower stop, to rise again when the circuit is
broken. The duration of its stay decides whether a "long" or "short" is meant.

TELEGRAPHIC RELAYS.

When an electric current has travelled for
a long distance through a wire its strength is
much reduced on account of the resistance
of the wire, and may be insufficient to cause
the electro-magnet of the sounder to move
the heavy lever. Instead, therefore, of the
current acting directly on the sounder
magnet, it is used to energize a small
magnet, or relay, which pulls down a light
bar and closes a second "local" circuit—that
is, one at the receiver end—worked by a
separate battery, which has sufficient power
to operate the sounder.

RECORDING TELEGRAPHS.

By attaching a small wheel to the end of
a Morse-sounder lever, by arranging an ink-
well for the wheel to dip into when the end
falls, and by moving a paper ribbon slowly
along for the wheel to press against when it
rises, a self-recording Morse inker is Fig. 58.—Section of a telegraph
produced. The ribbon-feeding apparatus is wire insulator on its arm. The
set in motion automatically by the current, shaded circle is the line wire, the
and continues to pull the ribbon along until two blank circles indicate the wire
the message is completed. which ties the line wire to the
insulator.
The Hughes type-printer covers a sheet of
paper with printed characters in bold Roman type. The transmitter has a keyboard,
on which are marked letters, signs, and numbers; also a type-wheel, with the
characters on its circumference, rotated by electricity. The receiver contains
mechanisms for rotating another type-wheel synchronously—that is, in time—with
the first; for shifting the wheel across the paper; for pressing the paper against the
wheel; and for moving the paper when a fresh line is needed. These are too

complicated to be described here in detail. By means of relays one transmitter may
be made to work five hundred receivers. In London a single operator, controlling a
keyboard in the central dispatching office, causes typewritten messages to spell
themselves out simultaneously in machines distributed all over the metropolis.

The tape machine resembles that just described in many details. The main
difference is that it prints on a continuous ribbon instead of on sheets.

Automatic electric printers of some kind or other are to be found in the vestibules
of all the principal hotels and clubs of our large cities, and in the offices of bankers,
stockbrokers, and newspaper editors. In London alone over 500 million words are
printed by the receivers in a year.

HIGH-SPEED TELEGRAPHY.

At certain seasons, or when important political events are taking place, the
telegraph service would become congested with news were there not some means of
transmitting messages at a much greater speed than is possible by hand signalling.
Fifty words a minute is about the limit speed that a good operator can maintain. By
means of Wheatstone's automatic transmitter the rate can be increased to 400 words
per minute. Paper ribbons are punched in special machines by a number of clerks
with a series of holes which by their position indicate a dot or a dash. The ribbons
are passed through a special transmitter, over little electric brushes, which make
contact through the holes with surfaces connected to the line circuit. At the receiver
end the message is printed by a Morse inker.

It has been found possible to send several messages simultaneously over a single
line. To effect this a distributer is used to put a number of transmitters at one end of
the line in communication with an equal number of receivers at the other end, fed by
a second distributer keeping perfect time with the first. Instead of a signal coming as
a whole to any one instrument it arrives in little bits, but these follow one another so
closely as to be practically continuous. By working a number of automatic
transmitters through a distributer, a thousand words or more per minute are easily
dispatched over a single wire.

The Pollak Virag system employs a punched ribbon, and the receiver traces out
the message in alphabetical characters on a moving strip of sensitized photographic
paper. A mirror attached to a vibrating diaphragm reflects light from a lamp on to the
strip, which is automatically developed and fixed in chemical baths. The method of

moving the mirror so as to make the rays trace out words is extremely ingenious.
Messages have been transmitted by this system at the rate of 180,000 words per
hour.

Chapter VII.

WIRELESS TELEGRAPHY.

The transmitting apparatus—The receiving apparatus—Syntonic transmission—The advance
of wireless telegraphy.

I N our last chapter we reviewed briefly some systems of sending telegraphic
messages from one point of the earth's surface to another through a circuit
consisting partly of an insulated wire and partly of the earth itself. The metallic
portion of a long circuit, especially if it be a submarine cable, is costly to install, so
that in quite the early days of telegraphy efforts were made to use the ether in the
place of wire as one conductor.

When a hammer strikes an anvil the air around is violently disturbed. This
disturbance spreads through the molecules of the air in much the same way as
ripples spread from the splash of a stone thrown into a pond. When the sound waves
reach the ear they agitate the tympanum, or drum membrane, and we "hear a noise."
The hammer is here the transmitter, the air the conductor, the ear the receiver.

In wireless telegraphy we use the ether as the conductor of electrical
disturbances.[13] Marconi, Slaby, Branly, Lodge, De Forest, Popoff, and others have
invented apparatus for causing disturbances of the requisite kind, and for detecting
their presence.

The main features of a wireless telegraphy outfit are shown in Figs. 59 and 61.

THE TRANSMITTER APPARATUS.

We will first consider the transmitting outfit (Fig. 59). It includes a battery,
dispatching key, and an induction coil having its secondary circuit terminals
connected with two wires, the one leading to an earth-plate, the other carried aloft on
poles or suspended from a kite. In the large station at Poldhu, Cornwall, for

transatlantic signalling, there are special wooden towers 215 feet high, between
which the aërial wires hang. At their upper and lower ends respectively the earth and
aërial wires terminate in brass balls separated by a gap. When the operator depresses
the key the induction coil charges these balls and the wires attached thereto with
high-tension electricity. As soon as the quantity collected exceeds the resistance of
the air-gap, a discharge takes place between the balls, and the ether round the aërial
wire is violently disturbed, and waves of electrical energy are propagated through it.
The rapidity with which the discharges follow one another, and their travelling
power, depends on the strength of the induction coil, the length of the air-gap, and
the capacity of the wires.[14]

Fig. 59.—Sketch of the transmitter of a wireless
telegraphy outfit.

Fig. 60.—A Marconi coherer.

RECEIVING APPARATUS.

The human body is quite insensitive to these etheric waves. We cannot feel, hear,
or see them. But at the receiving station there is what may be called an "electric
eye." Technically it is named a coherer. A Marconi coherer is seen in Fig. 60. Inside
a small glass tube exhausted of air are two silver plugs, p p, carrying terminals, t t,

projecting through the glass at both ends. A small gap separates the plugs at the
centre, and this gap is partly filled with nickel-silver powder. If the terminals of the
coherer are attached to those of a battery, practically no current will pass under
ordinary conditions, as the particles of nickel-silver touch each other very lightly and
make a "bad contact." But if the coherer is also attached to wires leading into the
earth and air, and ether waves strike those wires, at every impact the particles will
cohere—that is, pack tightly together—and allow battery current to pass. The
property of cohesion of small conductive bodies when influenced by Hertzian waves
was first noticed in 1874 by Professor D.E. Hughes while experimenting with a
telephone.

Fig. 61.—Sketch of the receiving apparatus in a wireless
telegraphy outfit.
We are now in a position to examine the apparatus of which a coherer forms part
(Fig. 61). First, we notice the aërial and earth wires, to which are attached other
wires from battery a. This battery circuit passes round the relay magnet r and
through two choking coils, whose function is to prevent the Hertzian waves entering
the battery. The relay, when energized, brings contact d against e and closes the
circuit of battery b, which is much more powerful than battery a, and operates the
magnet m as well as the tapper, which is practically an electric bell minus the gong.
(The tapper circuit is indicated by the dotted lines.)

We will suppose the transmitter of a distant station to be at work. The electric
waves strike the aërial wire of the receiving station, and cause the coherer to cohere

and pass current. The relay is closed, and both tapper and Morse inker begin to
work. The tapper keeps striking the coherer and shakes the particles loose after every
cohesion. If this were not done the current of a would pass continuously after
cohesion had once taken place. When the key of the transmitter is pressed down, the
waves follow one another very quickly, and the acquired conductivity of the coherer
is only momentarily destroyed by the tap of the hammer. During the impression of a
dot by the Morse inker, contact is made and broken repeatedly; but as the armature
of the inker is heavy and slow to move it does not vibrate in time with the relay and
tapper. Therefore the Morse instrument reproduces in dots and dashes the short and
long depressions of the key at the transmitting station, while the tapper works
rapidly in time with the relay. The Morse inker is shown diagrammatically. While
current passes through m the armature is pulled towards it, the end p, carrying an
inked wheel, rises, and a mark is made on the tape w, which is moved continuously
being drawn forward off reel r by the clockwork—or electrically-driven rollers r1
2
r .

SYNTONIC TRANSMISSION.

If a number of transmitting stations are sending out messages simultaneously, a
jumble of signals would affect all the receivers round, unless some method were
employed for rendering a receiver sensitive only to the waves intended to influence
it. Also, if distinction were impossible, even with one transmitter in action its
message might go to undesired stations.

There are various ways of "tuning" receivers and transmitters, but the principle
underlying them all is analogous to that of mechanical vibration. If a weight is
suspended from the end of a spiral spring, and given an upward blow, it bobs up and
down a certain number of times per minute, every movement from start to finish
having exactly the same duration as the rest. The resistance of the air and the internal
friction of the spring gradually lessen the amplitude of the movements, and the
weight finally comes to rest. Suppose that the weight scales 30 lbs., and that it
naturally bobs twenty times a minute. If you now take a feather and give it a push
every three seconds you can coax it into vigorous motion, assuming that every push
catches it exactly on the rebound. The same effect would be produced more slowly if
6 or 9 second intervals were substituted. But if you strike it at 4, 5, or 7 second
intervals it will gradually cease to oscillate, as the effect of one blow neutralizes that
of another. The same phenomenon is witnessed when two tuning-forks of equal pitch

are mounted near one another, and one is struck. The other soon picks up the note.
But a fork of unequal pitch would remain dumb.

Now, every electrical circuit has a "natural period of oscillation" in which its
electric charge vibrates. It is found possible to "tune," or "syntonize," the aërial rod
or wire of a receiving station with a transmitter. A vertical wire about 200 feet in
length, says Professor J.A. Fleming,[15] has a natural time period of electrical
oscillation of about one-millionth of a second. Therefore if waves strike this wire a
million times a second they will reinforce one another and influence the coherer;
whereas a less or greater frequency will leave it practically unaffected. By adjusting
the receiving circuit to the transmitter, or vice versâ, selective wireless telegraphy
becomes possible.

ADVANCE OF WIRELESS TELEGRAPHY.

The history of wireless telegraphy may be summed up as follows:—

1842.—Professor Morse sent aërial messages across the Susquehanna River. A
line containing a battery and transmitter was carried on posts along one bank and
"earthed" in the river at each end. On the other bank was a second wire attached to a
receiver and similarly earthed. Whenever contact was made and broken on the
battery side, the receiver on the other was affected. Distance about 1 mile.

1859.—James Bowman Lindsay transmitted messages across the Tay at
Glencarse in a somewhat similar way. Distance about ½ mile.

1885.—Sir William Preece signalled from Lavernock Point, near Cardiff, to
Steep Holm, an island in the Bristol Channel. Distance about 5½ miles.

In all these electrical induction of current was employed.

1886.—Hertzian waves discovered.

1895.—Professor A. Popoff sent Hertzian wave messages over a distance of 3
miles.

1897.—Marconi signalled from the Needles Hotel, Isle of Wight, to Swanage;
17½ miles.

1901.—Messages sent at sea for 380 miles.

1901, Dec. 17.—Messages transmitted from Poldhu, Cornwall, to Hospital Point,
Newfoundland; 2,099 miles.

Mr. Marconi has so perfected tuning devices that his transatlantic messages do
not affect receivers placed on board ships crossing the ocean, unless they are
purposely tuned. Atlantic liners now publish daily small newspapers containing the
latest news, flashed through space from land stations. In the United States the De
Forest and Fessenden systems are being rapidly extended to embrace the most out-
of-the-way districts. Every navy of importance has adopted wireless telegraphy,
which, as was proved during the Russo-Japanese War, can be of the greatest help in
directing operations.
[13] Named after their first discoverer, Dr. Hertz of Carlsruhe, "Hertzian waves."

[14] For long-distance transmission powerful dynamos take the place of the induction
coil and battery.

[15] "Technics," vol. ii. p. 566.

Chapter VIII.

THE TELEPHONE.

The Bell telephone—The Edison transmitter—The granular carbon transmitter—General
arrangement of a telephone circuit—Double-line circuits—Telephone exchanges—
Submarine telephony.

F OR the purposes of everyday life the telephone is even more useful than the
telegraph. Telephones now connect one room of a building with another, house
with house, town with town, country with country. An infinitely greater number of
words pass over the telephonic circuits of the world in a year than are transmitted by
telegraph operators. The telephone has become an important adjunct to the
transaction of business of all sorts. Its wires penetrate everywhere. Without moving
from his desk, the London citizen may hold easy converse with a Parisian, a New
Yorker with a dweller in Chicago.

Wonderful as the transmission of signals over great distances is, the transmission
of human speech so clearly that individual voices may be distinguished hundreds of

miles away is even more so. Yet the instrument which works the miracle is
essentially simple in its principles.

THE BELL TELEPHONE.

Fig. 62.—Section of a Bell telephone.

The first telephone that came into general use was that of Bell, shown in Fig. 62.
In a central hole of an ebonite casing is fixed a permanent magnet, m. The casing
expands at one end to accommodate a coil of insulated wire wound about one
extremity of a magnet. The coil ends are attached to wires passing through small
channels to terminals at the rear. A circular diaphragm, d, of very thin iron plate,
clamped between the concave mouthpiece and the casing, almost touches the end of
the magnet.

We will suppose that two Bell telephones, A and B, are connected up by wires, so
that the wires and the coils form a complete circuit. Words are spoken into A. The air
vibrations, passing through the central hole in the cover, make the diaphragm vibrate
towards and away from the magnet. The distances through which the diaphragm
moves have been measured, and found not to exceed in some cases more than
1⁄10,000,000 of an inch! Its movements distort the shape of the "lines of force" (see p.

118) emanating from the magnet, and these, cutting through the turns of the coil,
induce a current in the line circuit. As the diaphragm approaches the magnet a circuit
is sent in one direction; as it leaves it, in the other. Consequently speech produces
rapidly alternating currents in the circuit, their duration and intensity depending on
the nature of the sound.

Now consider telephone B. The currents passing through its coil increase or
diminish the magnetism of the magnet, and cause it to attract its diaphragm with
varying force. The vibration of the diaphragm disturbs the air in exact accordance
with the vibrations of A's diaphragm, and speech is reproduced.

THE EDISON TRANSMITTER.

The Bell telephone may be used both as a transmitter and a receiver, and the
permanent magnetism of the cores renders it independent of an electric battery. But
currents generated by it are so minute that they cannot overcome the resistance of a
long circuit; therefore a battery is now always used, and with it a special device as
transmitter.

If in a circuit containing a telephone and a battery there be a loose contact, and
this be shaken, the varying resistance of the contact will cause electrical currents of
varying force to pass through the circuit. Edison introduced the first successful
microphone transmitter, in which a small platinum disc connected to the diaphragm
pressed with varying force against a disc of carbon, each disc forming part of the
circuit. Vibrations of the diaphragm caused current to flow in a series of rapid
pulsations.

Fig. 63.—Section of a granular carbon transmitter.

THE GRANULAR CARBON TRANSMITTER.

In Fig. 63 we have a section of a microphone transmitter now very widely used.
It was invented, in its original form, by an English clergyman named Hunnings.
Resting in a central cavity of an ebonite seating is a carbon block, c, with a face
moulded into a number of pyramidal projections, p p. The space between c and a
carbon diaphragm, d, is packed with carbon granules, g g. c has direct contact with
line terminal t, which screws into it; d with t1 through the brass casing, screw s, and
a small plate at the back of the transmitter. Voice vibrations compress g g, and allow
current to pass more freely from d to c. This form of microphone is very delicate,
and unequalled for long-distance transmission.

Fig. 64.—A diagrammatic representation of a telephonic circuit.

GENERAL ARRANGEMENT OF A TELEPHONE CIRCUIT.

In many forms of subscriber's instruments both receiver and transmitter are
mounted on a single handle in such a way as to be conveniently placed for ear and
mouth. For the sake of clearness the diagrammatic sketch of a complete installation
(Fig. 64) shows them separated. The transmitters, it will be noticed, are located in
battery circuits, including the primary windings p p2 of induction coils. The
transmitters are in the line circuit, which includes the secondary windings s s2 of the
coils.

We will assume that the transmitters are, in the first instance, both hung on the
hooks of the metallic switches, which their weight depresses to the position
indicated by the dotted lines. The handle of the magneto-generator at the left-end
station is turned, and current passes through the closed circuit:—Line a, e b2, contact
10, the switch 9; line b, 4, the other switch, contact 5, and e b. Both bells ring. Both
parties now lift their receivers from the switch hooks. The switches rise against
contacts 1, 2, 3 and 6, 7, 8 respectively. Both primary and both secondary circuits

are now completed, while the bells are disconnected from the line wires. The
pulsations set up by transmitter t in primary coil p are magnified by secondary coil s
for transmission through the line circuit, and affect both receivers. The same thing
happens when t2 is used. At the end of the conversation the receivers are hung on
their hooks again, and the bell circuit is remade, ready for the next call.

A TELEPHONE EXCHANGE.

DOUBLE-LINE CIRCUITS.

The currents used in telephones pulsate very rapidly, but are very feeble. Electric
disturbances caused by the proximity of telegraph or tram wires would much
interfere with them if the earth were used for the return circuit. It has been found that
a complete metallic circuit (two wires) is practically free from interference, though
where a number of wires are hung on the same poles, speech-sounds may be faintly
induced in one circuit from another. This defect is, however, minimized by crossing
the wires about among themselves, so that any one line does not pass round the
corresponding insulator on every pole.

TELEPHONE EXCHANGES.

In a district where a number of telephones are used the subscribers are put into
connection with one another through an "exchange," to which all the wires lead. One
wire of each subscriber runs to a common "earth;" the other terminates at a
switchboard presided over by an operator. In an exchange used by many subscribers

the terminals are distributed over a number of switchboards, each containing 80 to
100 terminals, and attended to by an operator, usually a girl.

When a subscriber wishes to be connected to another subscriber, he either turns
the handle of a magneto generator, which causes a shutter to fall and expose his
number at the exchange, or simply depresses a key which works a relay at the
exchange and lights a tiny electric lamp. The operator, seeing the signal, connects
her telephone with the subscriber's circuit and asks the number wanted. This given,
she rings up the other subscriber, and connects the two circuits by means of an
insulated wire cord having a spike at each end to fit the "jack" sockets of the
switchboard terminals. The two subscribers are now in communication.

If a number on switchboard
A calls for a number on
switchboard C, the operator at A
connects her subscriber by a
jack cord to a trunk line running
to C, where the operator
similarly connects the trunk line
with the number asked for, after
ringing up the subscriber. The
central exchange of one town is
connected with that of another
by one or more trunk lines, so
that a subscriber may speak
through an indefinite number of
exchanges. So perfect is the
modern telephone that the
writer remembers on one
occasion hearing the door-bell
ring in a house more than a
hundred miles away, with which
he was at the moment in
Fig. 65.—The headdress of an operator at a telephonic connection, though
telephone exchange. The receiver is fastened three exchanges were in the
over one ear, and the transmitter to the chest. circuit.

SUBMARINE TELEPHONY.

Though telegraphic messages are transmitted easily through thousands of miles
of cable,[16] submarine telephony is at present restricted to comparatively short
distances. When a current passes through a cable, electricity of opposite polarity
induced on the outside of the cable damps the vibration in the conductor. In the
Atlantic cable, strong currents of electricity are poured periodically into one end,
and though much enfeebled when they reach the other they are sufficiently strong to
work a very delicate "mirror galvanometer" (invented by Lord Kelvin), which moves
a reflected ray up and down a screen, the direction of the movements indicating a
dot or a dash. Reversible currents are used in transmarine telegraphy. The
galvanometer is affected like the coils and small magnet in Wheatstone's needle
instrument (p. 128).

Telephonic currents are too feeble to penetrate many miles of cable. There is
telephonic communication between England and France, and England and Ireland.
But transatlantic telephony is still a thing of the future. It is hoped, however, that by
inserting induction coils at intervals along the cables the currents may be "stepped
up" from point to point, and so get across. Turning to Fig. 64, we may suppose s to
be on shore at the English end, and s2 to be the primary winding of an induction coil
a hundred miles away in the sea, which magnifies the enfeebled vibrations for a
journey to s3, where they are again revived; and so on, till the New World is reached.
The difficulty is to devise induction coils of great power though of small size. Yet
science advances nowadays so fast that we may live to hear words spoken at the
Antipodes.
[16] In 1896 the late Li Hung Chang sent a cablegram from China to England (12,608
miles), and received a reply, in seven minutes.

Chapter IX.

DYNAMOS AND ELECTRIC MOTORS.

A simple dynamo—Continuous-current dynamos—Multipolar dynamos—Exciting the field
magnets—Alternating current dynamos—The transmission of power—The electric motor
—Electric lighting—The incandescent lamp—Arc lamps—"Series" and "parallel"
arrangement of lamps—Current for electric lamps—Electroplating.

I N previous chapters we have incidentally referred to the conversion of
mechanical work into electrical energy. In this we shall examine how it is done—
how the silently spinning dynamo develops power, and why the motor spins when
current is passed through it.

We must begin by returning to our first electrical diagram (Fig. 50), and calling to
mind the invisible "lines of force" which permeate the ether in the immediate
neighbourhood of a magnet's poles, called the magnetic field of the magnet.

Many years ago (1831) the great Michael Faraday discovered that if a loop of
wire were moved up and down between the poles of an electro-magnet (Fig. 66) a
current was induced in the loop, its direction depending upon that in which the loop
was moved. The energy required to cut the lines of force passed in some mysterious
way into the wire. Why this is so we cannot say, but, taking advantage of the fact,
electricians have gradually developed the enormous machines which now send
vehicles spinning over metal tracks, light our streets and houses, and supply energy
to innumerable factories.

Fig. 66.
The strength of the current induced in a circuit cutting the lines of force of a
magnet is called its pressure, voltage, or electro-motive force (expressed shortly
E.M.F.). It may be compared with the pounds-to-the-square-inch of steam. In order
to produce an E.M.F. of one volt it is calculated that 100,000,000 lines of force must
be cut every second.

The voltage depends on three things:—(1.) The strength of the magnet: the
stronger it is, the greater the number of lines of force coming from it. (2.) The length
of the conductor cutting the lines of force: the longer it is, the more lines it will cut.
(3.) The speed at which the conductor moves: the faster it travels, the more lines it
will cut in a given time. It follows that a powerful dynamo, or mechanical producer
of current, must have strong magnets and a long conductor; and the latter must be
moved at a high speed across the lines of force.

A SIMPLE DYNAMO.

In Fig. 67 we have the simplest possible form of dynamo—a single turn of wire,
w x y z, mounted on a spindle, and having one end attached to an insulated ring c,
the other to an insulated ring c1. Two small brushes, b b1, of wire gauze or carbon,
rubbing continuously against these collecting rings, connect them with a wire which
completes the circuit. The armature, as the revolving coil is called, is mounted
between the poles of a magnet, where the lines of force are thickest. These lines are
supposed to stream from the N. to the S. pole.

In Fig. 67 the armature has reached a position in which y z and w x are cutting no,
or very few, lines of force, as they move practically parallel to the lines. This is
called the zero position.

Fig. 67.

Fig. 68.
In Fig. 68 the armature, moving at right angles to the lines of force, cuts a
maximum number in a given time, and the current induced in the coil is therefore
now most intense. Here we must stop a moment to consider how to decide in which
direction the current flows. The armature is revolving in a clockwise direction, and y
z, therefore, is moving downwards. Now, suppose that you rest your left hand on the
N. pole of the magnet so that the arm lies in a line with the magnet. Point your
forefinger towards the S. pole. It will indicate the direction of the lines of force.
Bend your other three fingers downwards over the edge of the N. pole. They will
indicate the direction in which the conductor is moving across the magnetic field.
Stick out the thumb at right angles to the forefinger. It points in the direction in
which the induced current is moving through the nearer half of the coil. Therefore
lines of force, conductor, and induced current travel in planes which, like the top and
two adjacent sides of a box, are at right angles to one another.

While current travels from z to y—that is, from the ring c1 to y—it also travels
from x to w, because w x rises while y z descends. So that a current circulates
through the coil and the exterior part of the circuit, including the lamp. After z y has
passed the lowest possible point of the circle it begins to ascend, w x to descend. The
direction of the current is therefore reversed; and as the change is repeated every
half-revolution this form of dynamo is called an alternator or creator of alternating
currents. A well-known type of alternator is the magneto machine which sends

shocks through any one who completes the external circuit by holding the brass
handles connected by wires to the brushes. The faster the handle of the machine is
turned the more frequent is the alternation, and the stronger the current.

Fig. 69.

CONTINUOUS-CURRENT DYNAMOS.

An alternating current is not so convenient for some purposes as a continuous
current. It is therefore sometimes desirable (even necessary) to convert the
alternating into a uni-directional or continuous current. How this is done is shown in
Figs. 69 and 70. In place of the two collecting rings c c1, we now have a single ring
split longitudinally into two portions, one of which is connected to each end of the
coil w x y z. In Fig. 69 brush b has just passed the gap on to segment c, brush b1 on
to segment c1. For half a revolution these remain respectively in contact; then, just
as y z begins to rise and w x to descend, the brushes cross the gaps again and
exchange segments, so that the current is perpetually flowing one way through the
circuit. The effect of the commutator[17] is, in fact, equivalent to transposing the
brushes of the collecting rings of the alternator every time the coil reaches a zero
position.

Figs. 71 and 72 give end views in section of the coil and the commutator, with
the coil in the position of minimum and maximum efficiency. The arrow denotes the
direction of movement; the double dotted lines the commutator end of the revolving
coil.

Fig. 70.

PRACTICAL CONTINUOUS-CURRENT DYNAMOS.

The electrical output of our simple dynamo would be increased if, instead of a
single turn of wire, we used a coil of many turns. A further improvement would
result from mounting on the shaft, inside the coil, a core or drum of iron, to entice
the lines of force within reach of the revolving coil. It is evident that any lines which
pass through the air outside the circle described by the coil cannot be cut, and are
wasted.

Fig. 71. Fig. 72.

The core is not a solid mass of iron, but built up of a number of very thin iron
discs threaded on the shaft and insulated from one another to prevent electric eddies,
which would interfere with the induced current in the conductor.[18] Sometimes there
are openings through the core from end to end to ventilate and cool it.

Fig. 73.
We have already noticed that in the case of a single coil the current rises and falls
in a series of pulsations. Such a form of armature would be unsuitable for large
dynamos, which accordingly have a number of coils wound over their drums, at
equal distances round the circumference, and a commutator divided into an equal
number of segments. The subject of drum winding is too complicated for brief
treatment, and we must therefore be content with noticing that the coils are so
connected to their respective commutator segments and to one another that they
mutually assist one another. A glance at Fig. 73 will help to explain this. Here we
have in section a number of conductors on the right of the drum (marked with a
cross to show that current is moving, as it were, into the page), connected with
conductors on the left (marked with a dot to signify current coming out of the page).
If the "crossed" and "dotted" conductors were respectively the "up" and "down"
turns of a single coil terminating in a simple split commutator (Fig. 69), when the
coil had been revolved through an angle of 90° some of the up turns would be
ascending and some descending, so that conflicting currents would arise. Yet we
want to utilize the whole surface of the drum; and by winding a number of coils in
the manner hinted at, each coil, as it passes the zero point, top or bottom, at once
generates a current in the desired direction and reinforces that in all the other turns
of its own and of other coils on the same side of a line drawn vertically through the
centre. There is thus practically no fluctuation in the pressure of the current
generated.

The action of single and multiple coil windings may be compared to that of single
and multiple pumps. Water is ejected by a single pump in gulps; whereas the flow
from a pipe fed by several pumps arranged to deliver consecutively is much more
constant.

MULTIPOLAR DYNAMOS.

Hitherto we have considered the magnetic field produced by one bi-polar magnet
only. Large dynamos have four, six, eight, or more field magnets set inside a casing,
from which their cores project towards the armature so as almost to touch it (Fig.
74). The magnet coils are wound to give N. and S. poles alternately at their armature
ends round the field; and the lines of force from each N. pole stream each way to the
two adjacent S. poles across the path of the armature coils. In dynamos of this kind
several pairs of collecting brushes pick current off the commutator at equidistant
points on its circumference.

Fig. 74.—A Holmes continuous
current dynamo: a, armature; c,
commutator; m, field magnets.

EXCITING THE FIELD MAGNETS.

Until current passes through the field magnet coils, no magnetic field can be
created. How are the coils supplied with current? A dynamo, starting for the first
time, is excited by a current from an outside source; but when it has once begun to
generate current it feeds its magnets itself, and ever afterwards will be self-exciting,
[19] owing to the residual magnetism left in the magnet cores.

Fig. 75.—Partly finished commutator.
Look carefully at Figs. 77 and 78. In the first of these you will observe that part
of the wire forming the external circuit is wound round the arms of the field magnet.
This is called a series winding. In this case all the current generated helps to excite
the dynamo. At the start the residual magnetism of the magnet cores gives a weak
field. The armature coils cut this and pass a current through the circuit. The magnets
are further excited, and the field becomes stronger; and so on till the dynamo is
developing full power. Series winding is used where the current in the external
circuit is required to be very constant.

Fig. 76.—The brushes of a
Holmes dynamo.

Fig. 78 shows another method of winding—the shunt. Most of the current
generated passes through the external circuit 2, 2; but a part is switched through a
separate winding for the magnets, denoted by the fine wire 1, 1. Here the strength of
the magnetism does not vary directly with the current, as only a small part of the
current serves the magnets. The shunt winding is therefore used where the voltage
(or pressure) must be constant.

Fig. 77.—Sketch showing a "series"
winding. Fig. 78.—"Shunt" winding.

A third method is a combination of the two already named. A winding of fine
wire passes from brush to brush round the magnets; and there is also a series
winding as in Fig. 77. This compound method is adapted more especially for electric
traction.

ALTERNATING DYNAMOS.

These have their field magnets excited by a separate continuous current dynamo
of small size. The field magnets usually revolve inside a fixed armature (the reverse
of the arrangement in a direct-current generator); or there may be a fixed central
armature and field magnets revolving outside it. This latter arrangement is found in

the great power stations at Niagara Falls, where the enormous field-rings are
mounted on the top ends of vertical shafts, driven by water-turbines at the bottom of
pits 178 feet deep, down which water is led to the turbines through great pipes, or
penstocks. The weight of each shaft and the field-ring attached totals about thirty-
five tons. This mass revolves 250 times a minute, and 5,000 horse power is
constantly developed by the dynamo. Similar dynamos of 10,000 horse power each
have been installed on the Canadian side of the Falls.

Fig. 79.

TRANSMISSION OF POWER.

Alternating current is used where power has to be transmitted for long distances,
because such a current can be intensified, or stepped up, by a transformer somewhat
similar in principle to a Ruhmkorff coil minus a contact-breaker (see p. 122). A
typical example of transformation is seen in Fig. 79. Alternating current of 5,000
volts pressure is produced in the generating station and sent through conductors to a
distant station, where a transformer, b, reduces the pressure to 500 volts to drive an
alternating motor, c, which in turn operates a direct current dynamo, d. This dynamo
has its + terminal connected with the insulated or "live" rail of an electric railway,
and its – terminal with the wheel rails, which are metallically united at the joints to
act as a "return." On its way from the live rail to the return the current passes
through the motors. In the case of trams the conductor is either a cable carried
overhead on standards, from which it passes to the motor through a trolley arm, or a
rail laid underground in a conduit between the rails. In the top of the conduit is a slit
through which an arm carrying a contact shoe on the end projects from the car. The
shoe rubs continuously on the live rail as the car moves.

To return for a moment to the question of transformation of current. "Why," it
may be asked, "should we not send low-pressure direct current to a distant station
straight from the dynamo, instead of altering its nature and pressure? Or, at any rate,
why not use high-pressure direct current, and transform that?" The answer is, that to
transmit a large amount of electrical energy at low pressure (or voltage) would
necessitate large volume (or amperage) and a big and expensive copper conductor to
carry it. High-pressure direct current is not easily generated, since the sparking at the
collecting brushes as they pass over the commutator segments gives trouble. So
engineers prefer high-pressure alternating current, which is easily produced, and can
be sent through a small and inexpensive conductor with little loss. Also its voltage
can be transformed by apparatus having no revolving parts.

THE ELECTRIC MOTOR.

Anybody who understands the dynamo will also be able to understand the electric
motor, which is merely a reversed dynamo.

Imagine in Fig. 70 a dynamo taking the place of the lamp and passing current
through the brushes and commutator into the coil w x y z. Now, any coil through
which current passes becomes a magnet with N. and S. poles at either end. (In Fig.
70 we will assume that the N. pole is below and the S. pole above the coil.) The coil
poles therefore try to seek the contrary poles of the permanent magnet, and the coil
revolves until its S. pole faces the N. of the magnet, and vice versâ. The lines of
force of the coil and the magnet are now parallel. But the momentum of revolution
carries the coil on, and suddenly the commutator reverses its polarity, and a further
half-revolution takes place. Then comes a further reversal, and so on ad infinitum.
The rotation of the motor is therefore merely a question of repulsion and attraction
of like and unlike poles. An ordinary compass needle may be converted into a tiny
motor by presenting the N. and S. poles of a magnet to its S. and N. poles alternately
every half-revolution.

In construction and winding a motor is practically the same as a dynamo. In fact,
either machine can perform either function, though perhaps not equally well adapted
for both. Motors may be run with direct or alternating current, according to their
construction.

On electric cars the motor is generally suspended from the wheel truck, and a
small pinion on the armature shaft gears with a large pinion on a wheel axle. One
great advantage of electric traction is that every vehicle of a train can carry its own

motor, so that the whole weight of the train may be used to get a grip on the rails
when starting. Where a single steam locomotive is used, the adhesion of its driving-
wheels only is available for overcoming the inertia of the load; and the whole strain
of starting is thrown on to the foremost couplings. Other advantages may be
summed up as follows:—(1) Ease of starting and rapid acceleration; (2) absence of
waste of energy (in the shape of burning fuel) when the vehicles are at rest; (3)
absence of smoke and smell.

ELECTRIC LIGHTING.

Dynamos are used to generate current for two main purposes—(1) To supply
power to motors of all kinds; (2) to light our houses, factories, and streets. In private
houses and theatres incandescent lamps are generally used; in the open air, in shops,
and in larger buildings, such as railway stations, the arc lamp is more often found.

INCANDESCENT LAMP.

If you take a piece of very fine iron wire and lay it across the terminals of an
accumulator, it becomes white hot and melts, owing to the heat generated by its
resistance to the current. A piece of fine platinum wire would become white hot
without melting, and would give out an intense light. Here we have the principle of
the glow or incandescent lamp—namely, the interposition in an electric circuit of a
conductor which at once offers a high resistance to the current, but is not destroyed
by the resulting heat.

In Fig. 80 is shown a fan propelling liquid constantly through a pipe. Let us
assume that the liquid is one which develops great friction on the inside of the pipe.
At the contraction, where the speed of travel is much greater than elsewhere in the
circuit, most heat will be produced.

Fig. 80.—Diagram to show
circulation of water through a
pipe.
In quite the early days of the glow-lamp platinum wire was found to be unreliable
as regards melting, and filaments of carbon are now used. To prevent the wasting
away of the carbon by combination with oxygen the filament is enclosed in a glass
bulb from which practically all air has been sucked by a mercury pump before
sealing.

Fig. 81.—The electrical
counterpart of Fig. 80. The
filament takes the place of the
contraction in the pipe.
The manufacture of glow-lamps is now an important industry. One brand of
lamp[20] is made as follows:—First, cotton-wool is dissolved in chloride of zinc, and
forms a treacly solution, which is squirted through a fine nozzle into a settling
solution which hardens it and makes it coil up like a very fine violin string. After
being washed and dried, it is wound on a plumbago rod and baked in a furnace until
only the carbon element remains. This is the filament in the rough. It is next
removed from the rod and tipped with two short pieces of fine platinum wire. To
make the junction electrically perfect the filament is plunged in benzine and heated
to whiteness by the passage of a strong current, which deposits the carbon of the
benzine on the joints. The filament is now placed under the glass receiver of an air-
pump, the air is exhausted, hydro-carbon vapour is introduced, and the filament has
a current passed through it to make it white hot. Carbon from the vapour is deposited
all over the filament until the required electrical resistance is attained. The filament
is now ready for enclosure in the bulb. When the bulb has been exhausted and
sealed, the lamp is tested, and, if passed, goes to the finishing department, where the
two platinum wires (projecting through the glass) are soldered to a couple of brass

plates, which make contact with two terminals in a lamp socket. Finally, brass caps
are affixed with a special water-tight and hard cement.

ARC LAMPS.

In arc lighting, instead of a contraction at a point in the circuit, there is an actual
break of very small extent. Suppose that to the ends of the wires leading from a
dynamo's terminals we attach two carbon rods, and touch the end of the rods
together. The tips become white hot, and if they are separated slightly, atoms of
incandescent carbon leap from the positive to the negative rod in a continuous and
intensely luminous stream, which is called an arc because the path of the particles is
curved. No arc would be formed unless the carbons were first touched to start
incandescence. If they are separated too far for the strength of the current to bridge
the gap the light will flicker or go out. The arc lamp is therefore provided with a
mechanism which, when the current is cut off, causes the carbons to fall together,
gradually separates them when it is turned on, and keeps them apart. The principle
employed is the effort of a coil through which a current passes to draw an iron rod
into its centre. Some of the current feeding the lamp is shunted through a coil, into
which projects one end of an iron bar connected with one carbon point. A spring
normally presses the points together when no current flows. As soon as current
circulates through the coil the bar is drawn upwards against the spring.

SERIES AND PARALLEL ARRANGEMENT OF LAMPS.

When current passes from one lamp to another, as in Fig. 82, the lamps are said
to be in series. Should one lamp fail, all in the circuit would go out. But where arc
lamps are thus arranged a special mechanism on each lamp "short-circuits" it in case
of failure, so that current may pass uninterruptedly to the next.

Fig. 82.—Incandescent lamps connected in "series."

Fig. 83 shows a number of lamps set in parallel. One terminal of each is attached
to the positive conductor, the other to the negative conductor. Each lamp therefore
forms an independent bridge, and does not affect the efficiency of the rest. Parallel
series signifies a combination of the two systems, and would be illustrated if, in Fig.
83, two or more lamps were connected in series groups from one conductor to the
other. This arrangement is often used in arc lighting.

Fig. 83.—Incandescent lamps connected in "parallel."

CURRENT FOR ELECTRIC LAMPS.

This may be either direct or alternating. The former is commonly used for arc
lamps, the latter for incandescent, as it is easily stepped-down from the high-
pressure mains for use in a house. Glow-lamps usually take current of 110 or 250
volts pressure.

In arc lamps fed with direct current the tip of the positive carbon has a bowl-
shaped depression worn in it, while the negative tip is pointed. Most of the
illumination comes from the inner surface of the bowl, and the positive carbon is
therefore placed uppermost to throw the light downwards. An alternating current, of
course, affects both carbons in the same manner, and there is no bowl.

The carbons need frequent renewal. A powerful lamp uses about 70 feet of rod in
1,000 hours if the arc is exposed to the air. Some lamps have partly enclosed arcs—
that is, are surrounded by globes perforated by a single small hole, which renders
combustion very slow, though preventing a vacuum.

ELECTROPLATING.

Electroplating is the art of coating metals with metals by means of electricity.
Silver, copper, and nickel are the metals most generally deposited. The article to be
coated is suspended in a chemical solution of the metal to be deposited. Fig. 84
shows a very simple plating outfit. a is a battery; b a vessel containing, say, an
acidulated solution of sulphate of copper. A spoon, s, hanging in this from a glass
rod, r, is connected with the zinc or negative element, z, of the battery, and a plate of
copper, p, with the positive element, c. Current flows in the direction shown by the
arrows, from z to c, c to p, p to s, s to z. The copper deposited from the solution on
the spoon is replaced by gradual dissolution of the plate, so that the latter serves a
double purpose.

Fig. 84.—An electroplating outfit.
In silver plating, p is of silver, and the solution one of cyanide of potassium and
silver salts. Where nickel or silver has to be deposited on iron, the article is often

given a preliminary coating of copper, as iron does not make a good junction with
either of the first two metals, but has an affinity for copper.
[17] From the Latin commuto, "I exchange."

[18] Only the "drum" type of armature is treated here.

[19] This refers to continuous-current dynamos only.
[20] The Robertson.

Chapter X.

RAILWAY BRAKES.

The Vacuum Automatic brake—The Westinghouse air-brake.

I N the early days of the railway, the pulling up of a train necessitated the shutting
off of steam while the stopping-place was still a great distance away. The train
gradually lost its velocity, the process being hastened to a comparatively small
degree by the screw-down brakes on the engine and guard's van. The goods train of
to-day in many cases still observes this practice, long obsolete in passenger traffic.

An advance was made when a chain, running along the entire length of the train,
was arranged so as to pull on subsidiary chains branching off under each carriage
and operating levers connected with brake blocks pressing on every pair of wheels.
The guard strained the main chain by means of a wheel gear in his van. This system
was, however, radically defective, since, if any one branch chain was shorter than
the rest, it alone would get the strain. Furthermore, it is obvious that the snapping of
the main chain would render the whole arrangement powerless. Accordingly, brakes
operated by steam were tried. Under every carriage was placed a cylinder, in
connection with a main steam-pipe running under the train. When the engineer
wished to apply the brakes, he turned high-pressure steam into the train pipe, and the
steam, passing into the brake cylinders, drove out in each a piston operating the
brake gear. Unfortunately, the steam, during its passage along the pipe, was
condensed, and in cold weather failed to reach the rear carriages. Water formed in
the pipes, and this was liable to freeze. If the train parted accidentally, the apparatus
of course broke down.

Hydraulic brakes have been tried; but these are open to several objections; and
railway engineers now make use of air-pressure as the most suitable form of power.
Whatever air system be adopted, experience has shown that three features are
essential:—(1.) The brakes must be kept "off" artificially. (2.) In case of the train
parting accidentally, the brakes must be applied automatically, and quickly bring all
the vehicles of the train to a standstill. (3.) It must be possible to apply the brakes
with greater or less force, according to the needs of the case.

At the present day one or other of two systems is used on practically all
automatically-braked cars and coaches. These are known as—(1) The vacuum
automatic, using the pressure of the atmosphere on a piston from the other side of
which air has been mechanically exhausted; and (2) the Westinghouse automatic,
using compressed air. The action of these brakes will now be explained as simply as
possible.

THE VACUUM AUTOMATIC BRAKE.

Under each carriage is a vacuum chamber (Fig. 85) riding on trunnions, e e, so
that it may swing a little when the brakes are applied. Inside the chamber is a
cylinder, the piston of which is rendered air-tight by a rubber ring rolling between it
and the cylinder walls. The piston rod works through an air-tight stuffing-box in the
bottom of the casing, and when it rises operates the brake rods. It is obvious that if
air is exhausted from both sides of the piston at once, the piston will sink by reason
of its own weight and that of its attachments. If air is now admitted below the piston,
the latter will be pushed upwards with a maximum pressure of 15 lbs. to the square
inch. The ball-valve ensures that while air can be sucked from both sides of the
piston, it can be admitted to the lower side only.

Fig. 85.—Vacuum brake "off."

Fig. 86.—Vacuum brake "on."
Let us imagine that a train has been standing in a siding, and that air has
gradually filled the vacuum chamber by leakage. The engine is coupled on, and the
driver at once turns on the steam ejector,[21] which sucks all the air out of the pipes
and chambers throughout the train. The air is sucked directly from the under side of
the piston through pipe d; and from the space a a and the cylinder (open at the top)
through the channel c, lifting the ball, which, as soon as exhaustion is complete, or
when the pressure on both sides of the piston is equal, falls back on its seat. On air
being admitted to the train pipe, it rushes through d and into the space b (Fig. 86)
below the piston, but is unable to pass the ball, so that a strong upward pressure is
exerted on the piston, and the brakes go on. To throw them off, the space below the
piston must be exhausted. This is to be noted: If there is a leak, as in the case of the
train parting, the brakes go on at once, since the vacuum below the piston is
automatically broken.

For ordinary stops the vacuum is only partially broken—that is, an air-pressure of
but from 5 to 10 lbs. per square inch is admitted. For emergency stops full
atmospheric pressure is used. In this case it is advisable that air should enter at both

ends of the train; so in the guard's van there
is installed an ingenious automatic valve,
which can at any time be opened by the
guard pressing down a lever, but which
opens of itself when the train-pipe vacuum is
rapidly destroyed. Fig. 87 shows this device
in section. Seated on the top of an upright
pipe is a valve, A, connected by a bolt, b, to
an elastic diaphragm, c, sealing the bottom
of the chamber d. The bolt b has a very small
hole bored through it from end to end. When
the vacuum is broken slowly, the pressure
falls in d as fast as in the pipe; but a sudden
inrush of air causes the valve a to be pulled
off its seat by the diaphragm c, as the
vacuum in d has not been broken to any
appreciable extent. Air then rushes into the
Fig. 87.—Guard's valve for train pipe through the valve. It is thus
applying the Vacuum brake. evident that the driver controls this valve as
effectively as if it were on the engine. These
"emergency" valves are sometimes fitted to every vehicle of a train.

When a carriage is slipped, taps on each side of the coupling joint of the train
pipe are turned off by the guard in the "slip;" and when he wishes to stop he merely
depresses the lever e, gradually opening the valve. Under the van is an auxiliary
vacuum chamber, from which the air is exhausted by the train pipe. If the guard,
after the slip has parted from the train, finds that he has applied his brakes too hard,
he can put this chamber into communication with the brake cylinder, and restore the
vacuum sufficiently to pull the brakes off again.

When a train has come to rest, the brakes must be sucked off by the ejector. Until
this has been done the train cannot be moved, so that it is impossible for it to leave
the station unprepared to make a sudden stop if necessary.

THE WESTINGHOUSE AIR-BRAKE.

This system is somewhat more complicated than the vacuum, though equally
reliable and powerful. Owing to the complexity of certain parts, such as the steam

air-pump and the triple-valve, it is impossible to explain the system in detail; we
therefore have recourse to simple diagrammatic sketches, which will help to make
clear the general principles employed.

The air-brake, as first evolved by Mr. George Westinghouse, was a very simple
affair—an air-pump and reservoir on the engine; a long pipe running along the train;
and a cylinder under every vehicle to work the brakes. To stop the train, the high-
pressure air collected in the reservoir was turned into the train pipe to force out the
pistons in the coach cylinders, connected to it by short branch pipes. One defect of
this "straight" system was that the brakes at the rear of a long train did not come into
action until a considerable time after the driver turned on the air; and since, when
danger is imminent, a very few seconds are of great importance, this slowness of
operation was a serious fault. Also, it was found that the brakes on coaches near the
engine went on long before those more distant, so that during a quick stop there was
a danger of the forward coaches being bumped by those behind. It goes without
saying that any coaches which might break loose were uncontrollable. Mr.
Westinghouse therefore patented his automatic brake, now so largely used all over
the world. The brake ensures practically instantaneous and simultaneous action on
all the vehicles of a train of any length.

Fig. 88.—Diagrammatic sketch of the details of the
Westinghouse air-brake. Brake "off."
The principle of the brake will be gathered from Figs. 88 and 89. p is a steam-
driven air-pump on the engine, which compresses air into a reservoir, a, situated
below the engine or tender, and maintains a pressure of from 80 to 90 lbs. per square
inch. A three-way cock, c, puts the train pipe into communication with a or the open

air at the wish of the driver. Under each coach is a triple-valve, t, an auxiliary
reservoir, b, and a brake cylinder, d. The triple-valve is the most noteworthy feature
of the whole system. The reader must remember that the valve shown in the section
is only diagrammatic.

Now for the operation of the brake. When the engine is coupled to the train, the
compressed air in the main reservoir is turned into the train pipe, from which it
passes through the triple-valve into the auxiliary reservoir, and fills it till it has a
pressure of, say, 80 lbs. per square inch. Until the brakes are required, the pressure in
the train pipe must be maintained. If accidentally, or purposely (by turning the cock
c to the position shown in Fig. 89), the train-pipe pressure is reduced, the triple-
valve at once shifts, putting b in connection with the brake cylinder d, and cutting off
the connection between d and the air, and the brakes go on. To get them off, the
pressure in the train pipe must be made equal to that in b, when the valve will
assume its original position, allowing the air in d to escape.

The force with which the brake is applied depends upon the reduction of pressure
in the train pipe. A slight reduction would admit air very slowly from b to d, whereas
a full escape from the train pipe would open the valve to its utmost. We have not
represented the means whereby the valve is rendered sensitive to these changes, for
the reason given above.

Fig. 89.—Brake "on."
The latest form of triple-valve includes a device which, when air is rapidly
discharged from the train pipe, as in an emergency application of the brake, opens a
port through which compressed air is also admitted from the train pipe directly into
d. It will easily be understood that a double advantage is hereby gained—first, in

utilizing a considerable portion of the air in the train pipe to increase the available
brake force in cases of emergency; and, secondly, in producing a quick reduction of
pressure in the whole length of the pipe, which accelerates the action of the brakes
with extraordinary rapidity.

It may be added that this secondary communication is kept open only until the
pressure in d is equal to that in the train pipe. Then it is cut off, to prevent a return of
air from b to the pipe.

An interesting detail of the system is the automatic regulation of air-pressure in
the main reservoir by the air-pump governor (Fig. 90). The governor is attached to
the steam-pipe leading from the locomotive boiler to the air-pump. Steam from the
boiler, entering at f, flows through valve 14 and passes by d into the pump, which is
thus brought into operation, and continues to work until the pressure in the main
reservoir, acting on the under side of the diaphragm 9, exceeds the tension to which
the regulating spring 7 is set. Any excess of pressure forces the diaphragm upwards,
lifting valve 11, and allowing compressed air from the main reservoir to flow into
the chamber c. The air-pressure forces piston 12 downwards and closes steam-valve
14, thus cutting off the supply of steam to the pump. As soon as the pressure in the
reservoir is reduced (by leakage or use) below the normal, spring 7 returns
diaphragm 9 to the position shown in Fig. 90, and pin-valve 11 closes. The
compressed air previously admitted to the chamber c escapes through the small port
a to the atmosphere. The steam, acting on the lower surface of valve 14, lifts it and
its piston to the position shown, and again flows to the pump, which works until the
required air-pressure is again obtained in the reservoir.

Fig. 90.—Air-pump of
Westinghouse brake.
[21] This resembles the upper part of the rudimentary water injector shown in Fig. 15.
The reader need only imagine pipe b to be connected with the train pipe. A rush of steam
through pipe a creates a partial vacuum in the cone e, causing air from the train pipe to
rush into it and be expelled by the steam blast.

Chapter XI.

RAILWAY SIGNALLING.

The block system—Position of signals—Interlocking the signals—Locking gear—Points—
Points and signals in combination—Working the block system—Series of signalling
operations—Single line signals—The train staff—Train staff and ticket—Electric train
staff system—Interlocking—Signalling operations—Power signalling—Pneumatic
signalling—Automatic signalling.

U NDER certain conditions—namely, at sharp curves or in darkness—the most
powerful brakes might not avail to prevent a train running into the rear of
another, if trains were allowed to follow each other closely over the line. It is
therefore necessary to introduce an effective system of keeping trains running in the
same direction a sufficient distance apart, and this is done by giving visible and
easily understood orders to the driver while a train is in motion.

In the early days of the railway it was customary to allow a time interval between
the passings of trains, a train not being permitted to leave a station until at least five
minutes after the start of a preceding train. This method did not, of course, prevent
collisions, as the first train sometimes broke down soon after leaving the station; and
in the absence of effective brakes, its successor ran into it. The advent of the electric
telegraph, which put stations in rapid communication with one another, proved of
the utmost value to the safe working of railways.

THE BLOCK SYSTEM.

Time limits were abolished and distance limits substituted. A line was divided
into blocks, or lengths, and two trains going in the same direction were never
allowed on any one block at the same time.

The signal-posts carrying the movable arms, or semaphores, by means of which
the signalman communicates with the engine-driver, are well known to us. They are
usually placed on the left-hand side of the line of rails to which they apply, with their
arms pointing away from the rails. The side of the arms which faces the direction
from which a train approaches has a white stripe painted on a red background, the
other side has a black stripe on a white background.

The distant and other signal arms vary slightly in shape (Fig. 91). A distant signal
has a forked end and a V-shaped stripe; the home and starting signals are square-
ended, with straight stripes. When the arm stands horizontally, the signal is "on," or
at "danger"; when dropped, it is "off," and indicates "All right; proceed." At the end
nearest the post it carries a spectacle frame glazed with panes of red and green glass.
When the arm is at danger, the red pane is opposite a lamp attached to the signal
post; when the arm drops, the green pane rises to that position—so that a driver is
kept as fully informed at night as during the day, provided the lamp remains alight.

POSITION OF SIGNALS.

On double lines each set of rails has its
own separate signals, and drivers travelling
on the "up" line take no notice of signals
meant for the "down" line. Each signal-box
usually controls three signals on each set of
rails—the distant, the home, and the starting.
Their respective positions will be gathered
from Fig. 92, which shows a station on a
double line. Between the distant and the
home an interval is allowed of 800 yards on
the level, 1,000 yards on a falling gradient,
and 600 yards on a rising gradient. The
home stands near the approach end of the
station, and the starting at the departure end
of the platform. The last is sometimes Fig. 91.—Distant and home
reinforced by an "advance starting" signal signals.
some distance farther on.

It should be noted that the distant is only a caution signal, whereas both home
and starting are stop signals. This means that when the driver sees the distant "on,"

he does not stop his train, but slackens speed, and prepares to stop at the home
signal. He must, however, on no account pass either home or starting if they are at
danger. In short, the distant merely warns the driver of what he may expect at the
home. To prevent damage if a driver should overrun the home, it has been laid down
that no train shall be allowed to pass the starting signal of one box unless the line is
clear to a point at least a quarter of a mile beyond the home of the next box. That
point is called the standard clearing point.

Technically described, a block is a length of line between the last stop signal
worked from one signal-box and the first stop signal worked from the next signal-
box in advance.

Fig. 92.—Showing position of signals. Those at the top are "off."

INTERLOCKING SIGNALS.

A signalman cannot lower or restore his signals to their normal positions in any
order he likes. He is compelled to lower them as follows:—Starting and home; then
distant. And restore them—distant; then starting and home. If a signalman were
quite independent, he might, after the passage of a train, restore the home or starting,
but forget all about the distant, so that the next train, which he wants to stop, would
dash past the distant without warning and have to pull up suddenly when the home
came in sight. But by a mechanical arrangement he is prevented from restoring the
home or starting until the distant is at danger; and, vice versâ, he cannot lower the
last until the other two are off. This mechanism is called locking gear.

LOOKING GEAR.

There are many different types of locking gear in use. It is impossible to describe
them all, or even to give particulars of an elaborate locking-frame of any one type.
But if we confine ourselves to the simplest combination of a stud-locking apparatus,
such as is used in small boxes on the Great Western Railway, the reader will get an
insight into the general principles of these safety devices, as the same principles
underlie them all.

Fig. 93.—A signal lever and its connections. To move the
lever, c is pressed towards b raising the catch-rod from its
nick in the rack, g g g, guides; r r, anti-friction rollers; s,
sockets for catch-rod to work in.
The levers in the particular type of locking gear which we are considering have
each a tailpiece or "tappet arm" attached to it, which moves backwards and forwards
with the lever (Fig. 93). Running at right angles to this tappet, and close to it, either
under or above, are the lock bars, or stud bars. Refer now to Fig. 94, which shows
the ends of the three tappet arms, d, h, and s, crossed by a bar, b, from which project
these studs. The levers are all forward and the signals all "on." If the signalman tried
to pull the lever attached to d down the page, as it were, he would fail to move it on
account of the stud a, which engages with a notch in d. Before this stud can be got
free of the notch the tappets h and s must be pulled over, so as to bring their notches

in line with studs b and c (Fig. 95). The signalman can now move d, since the notch
easily pushes the stud a to the left (Fig. 96). The signals must be restored to danger.
As h and s are back-locked by d—that is, prevented by d from being put back into
their normal positions—d must be moved first. The interlocking of the three signals
described is merely repeated in the interlocking of a large number of signals.

Fig. 94.

Fig. 95.
On entering a signal-box a visitor will notice that the levers have different
colours:—Green, signifying distant signals; red, signifying home and starting

signals; blue, signifying facing points; black, signifying trailing points; white,
signifying spare levers. These different colours help the signalman to pick out the
right levers easily.

To the front of each lever is attached a small brass tablet bearing certain numbers;
one in large figures on the top, then a line, and other numbers in small figures
beneath. The large number is that of the lever itself; the others, called leads, refer to
levers which must be pulled before that particular lever can be released.

Fig. 96.

Fig. 97.—Model signal equipment in a signalling school. (By
permission of the "G.W.R. Magazine").

POINTS.

Mention was made, in connection with the lever, of points. Before going further
we will glance at the action of these devices for enabling a train to run from one set
of rails to another. Figs. 98 and 99 show the points at a simple junction. It will be
noticed that the rails of the line to the left of the points are continued as the outer
rails of the main and branch lines. The inner rails come to a sharp V-point, and to the
left of this are the two short rails which, by means of shifting portions, decide the
direction of a train's travel. In Fig. 98 the main line is open; in Fig. 99, the branch.
The shifting parts are kept properly spaced by cross bars (or tie-rods), a a.

Fig. 98.—Points open to main line.

Fig. 99.—Points open to branch line.

It might be thought that the wheels would bump badly when they reach the point
b, where there is a gap. This is prevented, however, by the bent ends e e (Fig. 98), on
which the tread of the wheel rests until it has reached some distance along the point
of v. The safety rails s r keep the outer wheel up against its rail until the V has been
passed.

POINTS AND SIGNALS IN COMBINATION.

Let us suppose that a train is approaching the junction shown in Figs. 98 and 99
from the left. It is not enough that the driver should know that the tracks are clear.
He must also be assured that the track, main or branch, as the case may be, along
which he has to go, is open; and on the other hand, if he were approaching from the
right, he would want to be certain that no train on the other line was converging on
his. Danger is avoided and assurance given by interlocking the points and signals. To
the left of the junction the home and distant signals are doubled, there being two
semaphore arms on each post. These are interlocked with the points in such a
manner that the signals referring to either line can be pulled off only when the points
are set to open the way to that line. Moreover, before any shifting of points can be
made, the signals behind must be put to danger. The convergence of trains is
prevented by interlocking, which renders it impossible to have both sets of distant
and home signals at "All right" simultaneously.

WORKING OF BLOCK SYSTEM.

We may now pass to the working of the block system of signalling trains from
station to station on one line of a double track. Each signal-box (except, of course,
those at termini) has electric communication with the next box in both directions.
The instruments used vary on different systems, but the principle is the same; so we
will concentrate our attention on those most commonly employed on the Great

Western Railway. They are:—(1.) Two tapper-bell instruments, connected with
similar instruments in the adjacent boxes on both sides. Each of these rings one beat
in the corresponding box every time its key is depressed. (2.) Two Spagnoletti disc
instruments—one, having two keys, communicating with the box in the rear; and the
other, in connection with the forward box, having no keys. Their respective
functions are to give signals and receive them. In the centre of the face of each is a
square opening, behind which moves a disc carrying two "flags"—"Train on line" in
white letters on red ground, and "Line clear" in black letters on a white ground. The
keyed instrument has a red and a white key. When the red key is depressed, "Train
on line" appears at the opening; also in that of a keyless disc at the adjacent signal-
box. A depression of the white key similarly gives "Line clear." A piece of wire with
the ends turned over and passed through two eyes slides over the keys, and can be
made to hold either down. In addition to these, telephonic and telegraphic
instruments are provided to enable the signalmen to converse.

SERIES OF SIGNALLING OPERATIONS.

Fig. 100.—The signaling instruments in three adjacent cabins. The
featherless arrows show the connection of the instruments.
We may now watch the doings of signalmen in four successive boxes, A,
B, C, and D, during the passage of an express train. Signalman A calls
signalman B's attention by one beat on the tapper-bell. B answers by
repeating it to show that he is attending. A asks, "Is line clear for passenger
express?"—four beats on the bell. B, seeing that the line is clear to his
clearing point, sends back four beats, and pins down the white key of his
instrument. "Line clear" appears on the opening, and also at that of A's
keyless disc. A lowers starting signal. Train moves off. A gives two beats on
the tapper = "Train entering section." B pins indicator at "Train on line,"
which also appears on A's instrument. A places signals at danger. B asks C,
"Is line clear?" C repeats the bell code, and pins indicator at "Line clear,"
shown on B's keyless disc also. B lowers all signals. Train passes. B signals
to C, "Train entering section." B signals to A, "Train out of section," and
releases indicator, which returns to normal position with half of each flag
showing at the window. B signals to C, "Train on line," and sets all his
signals to danger. C pins indicator to "Train on line." C asks, "Is line clear?"
But there is a train at station D, and signalman D therefore gives no reply,
which is equivalent to a negative. The driver, on approaching C's distant,
sees it at danger, and slows down, stopping at the home. C lowers home,
and allows train to proceed to his starting signal. D, when the line is clear to
his clearing point, signals "Line clear," and pins indicator at "Line clear." C
lowers starting signals, and train proceeds. C signals to D, "Train entering
section," and D pins indicator at "Train on line." C signals to B, "Train out
of section," sets indicator at normal, and puts signals at danger. And so the
process is repeated from station to station. Where, however, sections are

short, the signalman is advised one section ahead of the approach of a train
by an additional signal signifying, "Fast train approaching." The block
indicator reminds the signalman of the whereabouts of the train. Unless his
keyless indicator is at normal, he may not ask, "Is line clear?" And until he
signals back "Line clear" to the box behind, a train is not allowed to enter
his section. In this way a section of line with a full complement of signals is
always interposed between any two trains.

THE WORKING OF SINGLE LINES.

We have dealt with the signalling arrangements pertaining to double
lines of railway, showing that a system of signals is necessary to prevent a
train running into the back of its predecessor. Where trains in both
directions pass over a single line, not only has this element of danger to be
dealt with, but also the possibility of a train being allowed to enter a section
of line from each end at the same time. This is effected in several ways, the
essence of each being that the engine-driver shall have in his possession
visible evidence of the permission accorded him by the signalman to enter a
section of single line.

A SINGLE TRAIN STAFF.

The simplest form of working is to allocate to the length of line a "train
staff"—a piece of wood about 14 inches long, bearing the names of the
stations at either end. This is adopted where only one engine is used for
working a section, such as a short branch line. In a case like this there is
obviously no danger of two trains meeting, and the train staff is merely the
authority to the driver to start a journey. No telegraphic communication is
necessary with such a system, and signals are placed only at the ends of the
line.

TRAIN STAFF AND TICKET.

On long lengths of single line where more than one train has to be
considered, the line is divided into blocks in the way already described for

double lines, and a staff is assigned to each, the staffs for the various blocks
differing from each other in shape and colour. The usual signals are
provided at each station, and block telegraph instruments are employed, the
only difference being that one disc, of the key pattern, is used for trains in
both directions. On such a line it is, of course, possible that two or more
trains may require to follow each other without any travelling
intermediately in the opposite direction. This would be impossible if the
staff passed uniformly to and fro in the block section; but it is arranged by
the introduction of a train staff ticket used in conjunction with the staff.

No train is permitted to leave a staff station unless the staff for the
section of line to be traversed is at the station; and the driver has the
strictest possible instructions that he must see the staff. If a second train is
required to follow, the staff is shown to the driver, and a train staff ticket
handed him as his authority to proceed. If, however, the next train over the
section will enter from the opposite end, the staff is handed to the driver.

To render this system as safe as possible, train staff tickets are of the
same colour and shape as the staff for the section to which they apply, and
are kept in a special box at the stations, the key being attached to the staff
and the lock so arranged that the key cannot be withdrawn unless the box
has been locked.

ELECTRIC TRAIN STAFF AND TABLET SYSTEMS.

These systems of working are developments of the last mentioned, by
which are secured greater safety and ease in working the line. On some
sections of single line circumstances often necessitate the running of several
trains in one direction without a return train. For such cases the train staff
ticket was introduced; but even on the best regulated lines it is not always
possible to secure that the staff shall be at the station where it is required at
the right time, and cases have arisen where, no train being available at the
station where the staff was, it had to be taken to the other station by a man
on foot, causing much delay to traffic. The electric train staff and tablet

systems overcome this difficulty. Both work on much the same principle,
and we will therefore describe the former.

Fig. 101.—An electric train staff
holder: s s, staffs in the slot of the
instrument. Leaning against the side
of the cabin is a staff showing the key
k at the end for unlocking a siding
points between two stations. The
engine driver cannot remove the staff
until the points have been locked
again.
At each end of a block section a train staff instrument (Fig. 101) is
provided. In the base of these instruments are a number of train staffs, any
one of which would be accepted by an engine-driver as permission to travel

over the single line. The instruments are electrically connected, the
mechanism securing that a staff can be withdrawn only by the co-operation
of the signalman at each end of the section; that, when all the staffs are in
the instruments, a staff may be withdrawn at either end; that, when a staff
has been withdrawn, another cannot be obtained until the one out has been
restored to one or other of the instruments. The safety of such a system is
obvious, as also the assistance to the working by having a staff available for
a train no matter from which end it is to enter the section.

The mechanism of the instruments is quite simple. A double-poled
electro-magnet is energized by the depression of a key by the signalman at
the further end of the block into which the train is to run, and by the turning
of a handle by the signalman who requires to withdraw a staff. The magnet,
being energized, is able to lift a mechanical lock, and permits the
withdrawal of a staff. In its passage through the instrument the staff
revolves a number of iron discs, which in turn raise or lower a switch
controlling the electrical connections. This causes the electric currents
actuating the electro-magnet to oppose each other, the magnetism to cease,
and the lock to fall back, preventing another staff being withdrawn. It will
naturally be asked, "How is the electrical system restored?" We have said
that there were a number of staffs in each instrument—in other words, a
given number of staffs, usually twenty, is assigned to the section. Assume
that there are ten in each instrument, and that the switch in each is in its
lower position. Now withdraw a staff, and one instrument has an odd, the
other an even, number of staffs, and similarly one switch is raised while the
other remains lowered, therefore the electrical circuit is "out of phase"—
that is, the currents in the magnets of each staff instrument are opposed to
one another, and cannot release the lock. The staff travels through the
section and is placed in the instrument at the other end, bringing the number
of staffs to eleven—an odd number, and, what is more important, raising
the switch. Both switches are now raised, consequently the electric currents
will support each other, so that a staff may be withdrawn. Briefly, then,
when there is an odd number of staffs in one instrument and an even

number in the other, as when a staff is in use, the signalmen are unable to
obtain a staff, and consequently cannot give authority for a train to enter the
section; but when there is either an odd or an even number of staffs in each
instrument a staff may be withdrawn at either end on the co-operation of the
signalmen.

We may add that, where two instruments are in the same signal-box, one
for working to the box in advance, the other to the rear, it is arranged that
the staffs pertaining to one section shall not fit the instrument for the other,
and must be of different colours. This prevents the driver accidentally
accepting a staff belonging to one section as authority to travel over the
other.

INTERLOCKING.

The remarks made on the interlocking of points and signals on double
lines apply also to the working of single lines, with the addition that not
only are the distant, home, and starting signals interlocked with each other,
but with the signals and points governing the approach of a train from the
opposite direction—in other words, the signals for the approach of a train to
a station from one direction cannot be lowered unless those for the
approach to the station of a train from the opposite direction are at danger,
and the points correctly set.

SIGNALLING OPERATIONS.

In the working of single lines, as of double, the signalman at the station
from which a train is to proceed has to obtain the consent of the signalman
ahead, the series of questions to be signalled being very similar to those
detailed for double lines. There is, however, one notable exception. On long
lengths of single line it is necessary to make arrangements for trains to pass
each other. This is done by providing loop lines at intervals, a second pair of
rails being laid for the accommodation of one train while another in the
opposite direction passes it. To secure that more than one train shall not be
on a section of single line between two crossing-places it is laid down that,

when a signalman at a non-crossing station is asked to allow a train to
approach his station, he must not give permission until he has notified the
signalman ahead of him, thus securing that he is not asking permission for
trains to approach from both directions at the same time. Both for single
and double line working a number of rules designed to deal with cases of
emergency are laid down, the guiding principle being safety; but we have
now dealt with all the conditions of everyday working, and must pass to the
consideration of

Fig. 102.—An electric lever-frame in a signalling cabin
at Didcot.

"POWER" SIGNALLING.

In a power system of signalling the signalman is provided with some
auxiliary means—electricity, compressed air, etc.—of moving the signals or
points under his control. It is still necessary to have a locking-frame in the
signal-box, with levers interlocked with each other, and connections
between the box and the various points and signals. But the frame is much
smaller than an ordinary manual frame, and but little force is needed to

move the little levers which make or break an electric circuit, or open an
air-valve, according to the power-agent used.

ELECTRIC SIGNALLING.

Fig. 102 represents the locking-frame of a cabin at Didcot, England,
where an all-electric system has been installed. Wires lead from the cabin to
motors situated at the points and signals, which they operate through worm
gearing. When a lever is moved it closes a circuit and sets the current
flowing through a motor, the direction of the flow (and consequently of the
motor's revolution) depending on whether the lever has been moved
forward or backward. Indicators arranged under the levers tell the
signalman when the desired movements at the points and signals have been
completed. If any motion is not carried through, owing to failure of the
current or obstruction of the working parts, an electric lock prevents him
continuing operations. Thus, suppose he has to open the main line to an
express, he is obliged by the mechanical locking-frame to set all the points
correctly before the signals can be lowered. He might move all the
necessary levers in due order, yet one set of points might remain open, and,
were the signals lowered, an accident would result. But this cannot happen,
as the electric locks worked by the points in question block the signal
levers, and until the failure has been set right, the signals must remain at
"danger."

The point motors are connected direct to the points; but between a signal
motor and its arm there is an "electric slot," consisting of a powerful
electro-magnet which forms a link in the rod work. To lower a signal it is
necessary that the motor shall revolve and a control current pass round the
magnet to give it the requisite attractive force. If no control current flows,
as would happen were any pair of points not in their proper position, the
motor can have no effect on the signal arm to lower it, owing to the magnet
letting go its grip. Furthermore, if the signal had been already lowered when
the control current failed, it would rise to "danger" automatically, as all
signals are weighted to assume the danger position by gravity. The signal

control currents can be broken by the signalman moving a switch, so that in
case of emergency all signals may be thrown simultaneously to danger.

PNEUMATIC SIGNALLING.

In England and the United States compressed air is also used to do the
hard labour of the signalman for him. Instead of closing a circuit, the
signalman, by moving a lever half-way over, admits air to a pipe running
along the track to an air reservoir placed beside the points or signal to
which the lever relates. The air opens a valve and puts the reservoir in
connection with a piston operating the points or signal-arm, as the case may
be. This movement having been performed, another valve in the reservoir is
opened, and air passes back through a second pipe to the signal-box, where
it opens a third valve controlling a piston which completes the movement of
the lever, so showing the signalman that the operation is complete. With
compressed air, as with electricity, a mechanical locking-frame is of course
used.

AUTOMATIC SIGNALLING.

To reduce expense, and increase the running speed on lines where the
sections are short, the train is sometimes made to act as its own signalman.
The rails of each section are all bonded together so as to be in metallic
contact, and each section is insulated from the two neighbouring sections.
At the further end of a section is installed an electric battery, connected to
the rails, which lead the current back to a magnet operating a signal
stationed some distance back on the preceding section. As long as current
flows the signal is held at "All right." When a train enters the section the
wheels and axles short-circuit the current, so that it does not reach the
signal magnet, and the signal rises to "danger," and stays there until the last
pair of wheels has passed out of the section. Should the current fail or a
vehicle break loose and remain on the section, the same thing would
happen.

The human element can thus be practically eliminated from signalling.
To make things absolutely safe, a train should have positive control over a
train following, to prevent the driver overrunning the signals. On electric
railways this has been effected by means of contacts working in
combination with the signals, which either cut the current off from the
section preceding that on which a train may be, or raise a trigger to strike an
arm on the train following and apply its brakes.

Chapter XII.

OPTICS.

Lenses—The image cast by a convex lens—Focus—Relative position of object and
lens—Correction of lenses for colour—Spherical aberration—Distortion of
image—The human eye—The use of spectacles—The blind spot.

L IGHT is a third form of that energy of which we have already treated
two manifestations—heat and electricity. The distinguishing
characteristic of ether light-waves is their extreme rapidity of vibration,
which has been calculated to range from 700 billion movements per second
for violet rays to 400 billion for red rays.

If a beam of white light be passed through a prism it is resolved into the
seven visible colours of the spectrum—violet, indigo, blue, green, yellow,
orange, and red—in this order. The human eye is most sensitive to the
yellow-red rays, a photographic plate to the green-violet rays.

All bodies fall into one of two classes—(1) Luminous—that is, those
which are a source of light, such as the sun, a candle flame, or a red-hot
coal; and (2) non-luminous, which become visible only by virtue of light
which they receive from other bodies and reflect to our eyes.

THE PROPAGATION OF LIGHT.

Light naturally travels in a straight line. It is deflected only when it
passes from one transparent medium into another—for example, from air to
water—and the mediums are of different densities. We may regard the
surface of a visible object as made up of countless points, from each of
which a diverging pencil of rays is sent off through the ether.

LENSES.

If a beam of light encounters a transparent glass body with non-parallel
sides, the rays are deflected. The direction they take depends on the shape
of the body, but it may be laid down as a rule that they are bent toward the
thicker part of the glass. The common burning-glass is well known to us.
We hold it up facing the sun to concentrate all the heat rays that fall upon it
into one intensely brilliant spot, which speedily ignites any inflammable
substance on which it may fall (Fig. 103). We may imagine that one ray
passes from the centre of the sun through the centre of the glass. This is
undeflected; but all the others are bent towards it, as they pass through the
thinner parts of the lens.

Fig. 103.—Showing how a burning-glass concentrates the
heat rays which fall upon it.

It should be noted here that sunlight, as we call it, is accompanied by
heat. A burning-glass is used to concentrate the heat rays, not the light rays,
which, though they are collected too, have no igniting effect.

In photography we use a lens to concentrate light rays only. Such heat
rays as may pass through the lens with them are not wanted, and as they
have no practical effect are not taken any notice of. To be of real value, a

lens must be quite symmetrical—that is, the curve from the centre to the
circumference must be the same in all directions.

There are six forms of simple lenses, as given in Fig. 104. Nos. 1 and 2
have one flat and one spherical surface. Nos. 3, 4, 5, 6 have two spherical
surfaces. When a lens is thicker at the middle than at the sides it is called a
convex lens; when thinner, a concave lens. The names of the various shapes
are as follows:—No. 1, plano-convex; No. 2, plano-concave; No. 3, double
convex; No. 4, double concave; No. 5, meniscus; No. 6, concavo-convex.
The thick-centre lenses, as we may term them (Nos. 1, 3, 5), concentrate a
pencil of rays passing through them; while the thin-centre lenses (Nos. 2, 4,
6) scatter the rays (see Fig. 105).

Fig. 104.—Six forms of lenses.

THE CAMERA.

Fig. 105.

Fig. 106.

We said above that light is propagated in straight lines. To prove this is
easy. Get a piece of cardboard and prick a hole in it. Set this up some
distance away from a candle flame, and hold behind it a piece of tissue
paper. You will at once perceive a faint, upside-down image of the flame on
the tissue. Why is this? Turn for a moment to Fig. 106, which shows a
"pinhole" camera in section. At the rear is a ground-glass screen, b, to catch
the image. Suppose that a is the lowest point of the flame. A pencil of rays
diverging from it strikes the front of the camera, which stops them all
except the one which passes through the hole and makes a tiny luminous
spot on b, above the centre of the screen, though a is below the axis of the
camera. Similarly the tip of the flame (above the axis) would be represented
by a dot on the screen below its centre. And so on for all the millions of
points of the flame. If we were to enlarge the hole we should get a brighter
image, but it would have less sharp outlines, because a number of rays from
every point of the candle would reach the screen and be jumbled up with
the rays of neighbouring pencils. Now, though a good, sharp photograph
may be taken through a pinhole, the time required is so long that
photography of this sort has little practical value. What we want is a large
hole for the light to enter the camera by, and yet to secure a distinct image.
If we place a lens in the hole we can fulfil our wish. Fig. 107 shows a lens
in position, gathering up a number of rays from a point, a, and focussing
them on a point, b. If the lens has 1,000 times the area of the pinhole, it will
pass 1,000 times as many rays, and the image of a will be impressed on a
sensitized photographic plate 1,000 times more quickly.

Fig. 107.

THE IMAGE CAST BY A CONVEX LENS.

Fig. 108 shows diagrammatically how a convex lens forms an image.
From a and b, the extremities of the object, a simple ray is considered to
pass through the centre of the lens. This is not deflected at all. Two other
rays from the same points strike the lens above and below the centre
respectively. These are bent inwards and meet the central rays, or come to a
focus with them at A1 and B1. In reality a countless number of rays would
be transmitted from every point of the object and collected to form the
image.

Fig. 108.—Showing how an image is cast by a convex lens.

FOCUS.

We must now take special notice of that word heard so often in
photographic talk—"focus." What is meant by the focus or focal length of a
lens? Well, it merely signifies the distance between the optical centre of the
lens and the plane in which the image is formed.

Fig. 109.
We must here digress a moment to draw attention to the three simple
diagrams of Fig. 109. The object, o, in each case is assumed to be to the
right of the lens. In the topmost diagram the object is so far away from the
lens that all rays coming from a single point in it are practically parallel.
These converge to a focus at f. If the distance between f and the centre of
the lens is six inches, we say that the lens has a six-inch focal length. The
focal length of a lens is judged by the distance between lens and image
when the object is far away. To avoid confusion, this focal length is known
as the principal focus, and is denoted by the symbol f. In the middle
diagram the object is quite near the lens, which has to deal with rays
striking its nearer surface at an acuter angle than before (reckoning from the
centre). As the lens can only deflect their path to a fixed degree, they will
not, after passing the lens, come together until they have reached a point, f1,
further from the lens than f. The nearer we approach o to the lens, the
further away on the other side is the focal point, until a distance equal to
that of f from the lens is reached, when the rays emerge from the glass in a

parallel pencil. The rays now come to a focus no longer, and there can be no
image. If o be brought nearer than the focal distance, the rays would diverge
after passing through the lens.

RELATIVE POSITIONS OF OBJECT AND IMAGE.

Fig. 110.—Showing how the position of the image alters
relatively to the position of the object.
From what has been said above we deduce two main conclusions—(1.)
The nearer an object is brought to the lens, the further away from the lens
will the image be. (2.) If the object approaches within the principal focal
distance of the lens, no image will be cast by the lens. To make this plainer
we append a diagram (Fig. 110), which shows five positions of an object
and the relative positions of the image (in dotted lines). First, we note that
the line a b, or a b1, denotes the principal focal length of the lens, and a c,
or a c1, denotes twice the focal length. We will take the positions in order:
—

Position I. Object further away than 2f. Inverted image smaller than
object, at distance somewhat exceeding f.

Position II. Object at distance = 2f. Inverted image at distance = 2f, and
of size equal to that of object.

Position III Object nearer than 2f. Inverted image further away than 2f;
larger than the object.

Position IV. Object at distance = f. As rays are parallel after passing the
lens no image is cast.

Position V. Object at distance less than f. No real image—that is, one that
can be caught on a focussing screen—is now given by the lens, but a
magnified, erect, virtual image exists on the same side of the lens as the
object.

We shall refer to virtual images at greater length presently. It is hoped
that any reader who practises photography will now understand why it is
necessary to rack his camera out beyond the ordinary focal distance when
taking objects at close quarters. From Fig. 110 he may gather one
practically useful hint—namely, that to copy a diagram, etc., full size, both
it and the plate must be exactly 2f from the optical centre of the lens. And it
follows from this that the further he can rack his camera out beyond 2f the
greater will be the possible enlargement of the original.

CORRECTION OF LENSES FOR COLOUR.

We have referred to the separation of the spectrum colours of white light
by a prism. Now, a lens is one form of prism, and therefore sorts out the
colours. In Fig. 111 we assume that two parallel red rays and two parallel
violet rays from a distant object pass through a lens. A lens has most
bending effect on violet rays and least on red, and the other colours of the
spectrum are intermediately influenced. For the sake of simplicity we have
taken the two extremes only. You observe that the point r, in which the red
rays meet, is much further from the lens than is v, the meeting-point of the
violet rays. A photographer very seldom has to take a subject in which there
are not objects of several different colours, and it is obvious that if he used a
simple lens like that in Fig. 111 and got his red objects in good focus, the
blue and green portions of his picture would necessarily be more or less out
of focus.

Fig. 111.

Fig. 112.
This defect can fortunately be corrected by the method shown in Fig.
112. A compound lens is needed, made up of a crown glass convex element,
b, and a concave element, a, of flint glass. For the sake of illustration the
two parts are shown separated; in practice they would be cemented together,
forming one optical body, thicker in the centre than at the edges—a
meniscus lens in fact, since a is not so concave as b is convex. Now, it was
discovered by a Mr. Hall many years ago that if white light passed through
two similar prisms, one of flint glass the other of crown glass, the former
had the greater effect in separating the spectrum colours—that is, violet rays
were bent aside more suddenly compared with the red rays than happened
with the crown-glass prism. Look at Fig. 112. The red rays passing through
the flint glass are but little deflected, while the violet rays turn suddenly

outwards. This is just what is wanted, for it counteracts the unequal inward
refraction by b, and both sets of rays come to a focus in the same plane.
Such a lens is called achromatic, or colourless. If you hold a common
reading-glass some distance away from large print you will see that the
letters are edged with coloured bands, proving that the lens is not
achromatic. A properly corrected photographic lens would not show these
pretty edgings. Colour correction is necessary also for lenses used in
telescopes and microscopes.

SPHERICAL ABERRATION.

A lens which has been corrected for colour is still imperfect. If rays pass
through all parts of it, those which strike it near the edge will be refracted
more than those near the centre, and a blurred focus results. This is termed
spherical aberration. You will be able to understand the reason from Figs.
113 and 114. Two rays, a, are parallel to the axis and enter the lens near the
centre (Fig. 113). These meet in one plane. Two other rays, b, strike the lens
very obliquely near the edge, and on that account are both turned sharply
upwards, coming to a focus in a plane nearer the lens than a. If this
happened in a camera the results would be very bad. Either a or b would be
out of focus. The trouble is minimized by placing in front of the lens a plate
with a central circular opening in it (denoted by the thick, dark line in Fig.
114). The rays b of Fig. 113 are stopped by this plate, which is therefore
called a stop. But other rays from the same point pass through the hole.
These, however, strike the lens much more squarely above the centre, and
are not unduly refracted, so that they are brought to a focus in the same
plane as rays a.

Fig. 113.

Fig. 114.

DISTORTION OF IMAGE.

The lens we have been considering is a single meniscus, such as is used
in landscape photography, mounted with the convex side turned towards the
inside of the camera, and having the stop in front of it. If you possess a lens
of this sort, try the following experiment with it. Draw a large square on a
sheet of white paper and focus it on the screen. The sides instead of being
straight bow outwards: this is called barrel distortion. Now turn the lens

mount round so that the lens is
outwards and the stop inwards. The
sides of the square will appear to
bow towards the centre: this is pin-
cushion distortion. For a long time
opticians were unable to find a
remedy. Then Mr. George S.
Cundell suggested that two
meniscus lenses should be used in
combination, one on either side of
the stop, as in Fig 115. Each
produces distortion, but it is
counteracted by the opposite
distortion of the other, and a square
is represented as a square. Lenses of Fig. 115.—Section of a rectilinear
this kind are called rectilinear, or lens.
straight-line producing.

We have now reviewed the three chief defects of a lens—chromatic
aberration, spherical aberration, and distortion—and have seen how they
may be remedied. So we will now pass on to the most perfect of cameras,

THE HUMAN EYE.

The eye (Fig. 116) is nearly spherical in form, and is surrounded outside,
except in front, by a hard, horny coat called the sclerotica (s). In front is the
cornea (a), which bulges outwards, and acts as a transparent window to
admit light to the lens of the eye (c). Inside the sclerotica, and next to it,
comes the choroid coat; and inside that again is the retina, or curved
focussing screen of the eye, which may best be described as a network of
fibres ramifying from the optic nerve, which carries sight sensations to the
brain. The hollow of the ball is full of a jelly-like substance called the
vitreous humour; and the cavity between the lens and the cornea is full of
water.

We have already seen that, in focussing, the distance between lens and
image depends on the distance between object and lens. Now, the retina
cannot be pushed nearer to or pulled further away from its lens, like the
focussing screen of a camera. How, then, is the eye able to focus sharply
objects at distances varying from a foot to many miles?

Fig. 116.—Section of the human eye.
As a preliminary to the answer we must observe that the more convex a
lens is, the shorter is its focus. We will suppose that we have a box camera
with a lens of six-inch focus fixed rigidly in the position necessary for
obtaining a sharp image of distant objects. It so happens that we want to
take with it a portrait of a person only a few feet from the lens. If it were a
bellows camera, we should rack out the back or front. But we cannot do this
here. So we place in front of our lens a second convex lens which shortens
its principal focus; so that in effect the box has been racked out sufficiently.

Nature, however, employs a much more perfect method than this. The
eye lens is plastic, like a piece of india-rubber. Its edges are attached to
ligaments (l l), which pull outwards and tend to flatten the curve of its
surfaces. The normal focus is for distant objects. When we read a book the
eye adapts itself to the work. The ligaments relax and the lens decreases in
diameter while thickening at the centre, until its curvature is such as to
focus all rays from the book sharply on the retina. If we suddenly look
through the window at something outside, the ligaments pull on the lens
envelope and flatten the curves.

This wonderful lens is achromatic, and free from spherical aberration
and distortion of image. Nor must we forget that it is aided by an automatic
"stop," the iris, the central hole of which is named the pupil. We say that a
person has black, blue, or gray eyes according to the colour of the iris. Like
the lens, the iris adapts itself to all conditions, contracting when the light is
strong, and opening when the light is weak, so that as uniform an amount of
light as conditions allow may be admitted to the eye. Most modern camera
lenses are fitted with adjustable stops which can be made larger or smaller
by twisting a ring on the mount, and are named "iris" stops. The image of
anything seen is thrown on the retina upside down, and the brain reverses
the position again, so that we get a correct impression of things.

THE USE OF SPECTACLES.

Fig. 117a. Fig. 117b.

Fig. 118a. Fig. 118b.

The reader will now be able to understand without much trouble the
function of a pair of spectacles. A great many people of all ages suffer from
short-sight. For one reason or another the distance between lens and retina
becomes too great for a person to distinguish distant objects clearly. The
lens, as shown in Fig 117a, is too convex—has its minimum focus too short
—and the rays meet and cross before they reach the retina, causing general
confusion of outline. This defect is simply remedied by placing in front of
the eye (Fig. 117b) a concave lens, to disperse the rays somewhat before
they enter the eye, so that they come to a focus on the retina. If a person's
sight is thus corrected for distant objects, he can still see near objects quite
plainly, as the lens will accommodate its convexity for them. The scientific
term for short-sight is myopia. Long-sight, or hypermetropia, signifies that
the eyeball is too short or the lens too flat. Fig. 118a represents the normal
condition of a long-sighted eye. When looking at a distant object the eye
thickens slightly and brings the focus forward into the retina. But its
thickening power in such an eye is very limited, and consequently the rays
from a near object focus behind the retina. It is therefore necessary for a
long-sighted person to use convex spectacles for reading the newspaper. As
seen in Fig. 118b, the spectacle lens concentrates the rays before they enter
the eye, and so does part of the eye's work for it.

Returning for a moment to the diagram of the eye (Fig. 116), we notice a
black patch on the retina near the optic nerve. This is the "yellow spot."

Vision is most distinct when the image of the object looked at is formed on
this part of the retina. The "blind spot" is that point at which the optic nerve
enters the retina, being so called from the fact that it is quite insensitive to
light. The finding of the blind spot is an interesting little experiment. On a
card make a large and a small spot three inches apart, the one an eighth, the
other half an inch in diameter. Bring the card near the face so that an eye is
exactly opposite to each spot, and close the eye opposite to the smaller.
Now direct the other eye to this spot and you will find, if the card be moved
backwards and forwards, that at a certain distance the large spot, though
many times larger than its fellow, has completely vanished, because the rays
from it enter the open eye obliquely and fall on the "blind spot."

Chapter XIII.

THE MICROSCOPE, THE TELESCOPE, AND THE MAGIC-
LANTERN.

The simple microscope—Use of the simple microscope in the telescope—The
terrestrial telescope—The Galilean telescope—The prismatic telescope—The
reflecting telescope—The parabolic mirror—The compound microscope—The
magic-lantern—The bioscope—The plane mirror.

I N Fig. 119 is represented an eye looking at a vase, three inches high,
situated at a, a foot away. If we were to place another vase, b, six inches
high, at a distance of two feet; or c, nine inches high, at three feet; or d, a
foot high, at four feet, the image on the retina would in every case be of the
same size as that cast by a. We can therefore lay down the rule that the
apparent size of an object depends on the angle that it subtends at the eye.

Fig. 119.
To see a thing more plainly, we go nearer to it; and if it be very small, we
hold it close to the eye. There is, however, a limit to the nearness to which it
can be brought with advantage. The normal eye is unable to adapt its focus
to an object less than about ten inches away, termed the "least distance of
distinct vision."

THE SIMPLE MICROSCOPE.

Fig. 120.
A magnifying glass comes in useful when we want to examine an object
very closely. The glass is a lens of short focus, held at a distance somewhat
less than its principal focal length, f (see Fig. 120), from the object. The
rays from the head and tip of the pin which enter the eye are denoted by
continuous lines. As they are deflected by the glass the eye gets the
impression that a much longer pin is situated a considerable distance behind

the real object in the plane in which the refracted rays would meet if
produced backwards (shown by the dotted lines). The effect of the glass,
practically, is to remove it (the object) to beyond the least distance of
distinct vision, and at the same time to retain undiminished the angle it
subtends at the eye, or, what amounts to the same thing, the actual size of
the image formed on the retina.[22] It follows, therefore, that if a lens be of
such short focus that it allows us to see an object clearly at a distance of two
inches—that is, one-fifth of the least distance of distinct vision—we shall
get an image on the retina five times larger in diameter than would be
possible without the lens.

The two simple diagrams (Figs. 121 and 122) show why the image to be
magnified should be nearer to the lens than the principal focus, f. We have
already seen (Fig. 109) that rays coming from a point in the principal focal
plane emerge as a parallel pencil. These the eye can bring to a focus,
because it normally has a curvature for focussing parallel rays. But, owing
to the power of "accommodation," it can also focus diverging rays (Fig.
121), the eye lens thickening the necessary amount, and we therefore put
our magnifying glass a bit nearer than f to get full advantage of proximity.
If we had the object outside the principal focus, as in Fig. 122, the rays
from it would converge, and these could not be gathered to a sharp point by
the eye lens, as it cannot flatten more than is required for focussing parallel
rays.

Fig. 121.

Fig. 122.

USE OF THE SIMPLE MICROSCOPE IN THE TELESCOPE.

Fig. 123.

Let us now turn to Fig. 123. At a is a distant object, say, a hundred yards
away. b is a double convex lens, which has a focal length of twenty inches.
We may suppose that it is a lens in a camera. An inverted image of the

object is cast by the lens at c. If the eye were placed at c, it would
distinguish nothing. But if withdrawn to d, the least distance of distinct
vision,[23] behind c, the image is seen clearly. That the image really is at c is
proved by letting down the focussing screen, which at once catches it. Now,
as the focus of the lens is twice d, the image will be twice as large as the
object would appear if viewed directly without the lens. We may put this
into a very simple formula:—

focal length of lens
Magnification =
d

Fig. 124.

In Fig. 124 we have interposed between the eye and the object a small
magnifying glass of 2½-inch focus, so that the eye can now clearly see the
image when one-quarter d away from it. b already magnifies the image
twice; the eye-piece again magnifies it four times; so that the total
magnification is 2 × 4 = 8 times. This result is arrived at quickly by
dividing the focus of b (which corresponds to the object-glass of a
telescope) by the focus of the eye-piece, thus:—

20
=8
2½

The ordinary astronomical telescope has a very long focus object-glass
at one end of the tube, and a very short focus eye-piece at the other. To see
an object clearly one merely has to push in or pull out the eye-piece until its
focus exactly corresponds with that of the object-glass.

THE TERRESTRIAL TELESCOPE.

An astronomical telescope inverts images. This inversion is inconvenient
for other purposes. So the terrestrial telescope (such as is commonly used
by sailors) has an eye-piece compounded of four convex lenses which erect
as well as magnify the image. Fig. 125 shows the simplest form of
compound erecting eye-piece.

Fig. 125.

THE GALILEAN TELESCOPE.

Fig. 126.
A third form of telescope is that invented by the great Italian astronomer,
Galileo,[24] in 1609. Its principle is shown in Fig. 126. The rays transmitted
by the object-glass are caught, before coming to a focus, on a concave lens
which separates them so that they appear to meet in the paths of
convergence denoted by the dotted lines. The image is erect. Opera-glasses
are constructed on the Galilean principle.

THE PRISMATIC TELESCOPE.

In order to be able to use a long-focus object-glass without a long
focussing-tube, a system of glass reflecting prisms is sometimes employed,
as in Fig. 127. A ray passing through the object-glass is reflected from one

posterior surface of prism a on to the other posterior surface, and by it out
through the front on to a second prism arranged at right angles to it, which
passes the ray on to the compound eye-piece. The distance between object-
glass and eye-piece is thus practically trebled. The best-known prismatic
telescopes are the Zeiss field-glasses.

THE REFLECTING TELESCOPE.

We must not omit reference to
the reflecting telescope, so largely
used by astronomers. The front end
of the telescope is open, there being
no object-glass. Rays from the
object fall on a parabolic mirror
situated in the rear end of the tube.
This reflects them forwards to a
focus. In the Newtonian reflector a
plane mirror or prism is situated in
the axis of the tube, at the focus, to
reflect the rays through an eye-
piece projecting through the side of
the tube. Herschel's form of
reflector has the mirror set at an
angle to the axis, so that the rays are
Fig. 127. reflected direct into an eye-piece
pointing through the side of the
tube towards the mirror.

THE PARABOLIC MIRROR.

This mirror (Fig. 128) is of such a shape that all rays parallel to the axis
are reflected to a common point. In the marine searchlight a powerful arc
lamp is arranged with the arc at the focus of a parabolic reflector, which
sends all reflected light forward in a pencil of parallel rays. The most

powerful searchlight in existence gives a light equal to that of 350 million
candles.

Fig. 128.—A parabolic reflector.

THE COMPOUND MICROSCOPE.

We have already observed (Fig. 110) that the nearer an object
approaches a lens the further off behind it is the real image formed, until the

object has reached the focal distance, when no image at all is cast, as it is an
infinite distance behind the lens. We will assume that a certain lens has a
focus of six inches. We place a lighted candle four feet in front of it, and
find that a sharp diminished image is cast on a ground-glass screen held
seven inches behind it. If we now exchange the positions of the candle and
the screen, we shall get an enlarged image of the candle. This is a simple
demonstration of the law of conjugate foci—namely, that the distance
between the lens and an object on one side and that between the lens and
the corresponding image on the other bear a definite relation to each other;
and an object placed at either focus will cast an image at the other. Whether
the image is larger or smaller than the object depends on which focus it
occupies. In the case of the object-glass of a telescope the image was at
what we may call the short focus.

Fig. 129.—Diagram to explain the compound microscope.
Now, a compound microscope is practically a telescope with the object
at the long focus, very close to a short-focus lens. A greatly enlarged image
is thrown (see Fig. 129) at the conjugate focus, and this is caught and still
further magnified by the eye-piece. We may add that the object-glass, or
objective, of a microscope is usually compounded of several lenses, as is
also the eye-piece.

THE MAGIC-LANTERN.

The most essential features of a magic-lantern are:—(1) The source of
light; (2) the condenser for concentrating the light rays on to the slide; (3)
the lens for projecting a magnified image on to a screen.

Fig. 130 shows these diagrammatically. The illuminant is most
commonly an oil-lamp, or an acetylene gas jet, or a cylinder of lime heated
to intense luminosity by an oxy-hydrogen flame. The natural combustion of
hydrogen is attended by a great heat, and when the supply of oxygen is
artificially increased the temperature of the flame rises enormously. The
nozzle of an oxy-hydrogen jet has an interior pipe connected with the
cylinder holding one gas, and an exterior, and somewhat larger, pipe leading
from that containing the other, the two being arranged concentrically at the
nozzle. By means of valves the proportions of the gases can be regulated to
give the best results.

Fig. 130.—Sketch of the elements of a magic-lantern.

The condenser is set somewhat further from the illuminant than the
principal focal length of the lenses, so that the rays falling on them are bent
inwards, or to the slide.

The objective, or object lens, stands in front of the slide. Its position is
adjustable by means of a rack and a draw-tube. The nearer it is brought to
the slide the further away is the conjugate focus (see p. 239), and
consequently the image. The exhibitor first sets up his screen and lantern,
and then finds the conjugate foci of slide and image by racking the lens in
or out.

If a very short focus objective be used, subjects of microscopic
proportions can be projected on the screen enormously magnified. During
the siege of Paris in 1870–71 the Parisians established a balloon and pigeon
post to carry letters which had been copied in a minute size by photography.
These copies could be enclosed in a quill and attached to a pigeon's wing.
On receipt, the copies were placed in a special lantern and thrown as large
writing on the screen. Micro-photography has since then made great strides,
and is now widely used for scientific purposes, one of the most important
being the study of the crystalline formations of metals under different
conditions.

THE BIOSCOPE.

"Living pictures" are the most recent improvement in magic-lantern
entertainments. The negatives from which the lantern films are printed are
made by passing a ribbon of sensitized celluloid through a special form of
camera, which feeds the ribbon past the lens in a series of jerks, an
exposure being made automatically by a revolving shutter during each rest.
The positive film is placed in a lantern, and the intermittent movement is
repeated; but now the source of illumination is behind the film, and light
passes outwards through the shutter to the screen. In the Urban bioscope the
film travels at the rate of fifteen miles an hour, upwards of one hundred
exposures being made every second.

The impression of continuous movement arises from the fact that the eye
cannot get rid of a visual impression in less than one-tenth of a second. So
that if a series of impressions follow one another more rapidly than the eye
can rid itself of them the impressions will overlap, and give one of motion,
if the position of some of the objects, or parts of the objects, varies slightly
in each succeeding picture.[25]

THE PLANE MIRROR.

Fig. 131.
This chapter may conclude with a glance at the common looking-glass.
Why do we see a reflection in it? The answer is given graphically by Fig.
131. Two rays, a b, a c, from a point a strike the mirror m at the points b and
c. Lines b n, c o, drawn from these points perpendicular to the mirror are
called their normals. The angles a b n, a c o are the angles of incidence of
rays a b, a c. The paths which the rays take after reflection must make
angles with b n and c o respectively equal to a b n, a c o. These are the
angles of reflection. If the eye is so situated that the rays enter it as in our
illustration, an image of the point a is seen at the point a1, in which the lines
d b, e c meet when produced backwards.

Fig. 132.

When the vertical mirror is replaced by a horizontal reflecting surface,
such as a pond (Fig. 132), the same thing happens. The point at which the
ray from the reflection of the spire's tip to the eye appears to pass through
the surface of the water must be so situated that if a line were drawn
perpendicular to it from the surface the angles made by lines drawn from
the real spire tip and from the observer's eye to the base of the
perpendicular would be equal.
[22] Glazebrook, "Light," p. 157.

[23] Glazebrook, "Light," p. 157.

[24] Galileo was severely censured and imprisoned for daring to maintain that
the earth moved round the sun, and revolved on its axis.

[25] For a full account of Animated Pictures the reader might advantageously
consult "The Romance of Modern Invention," pp. 166 foll.

Chapter XIV.

SOUND AND MUSICAL INSTRUMENTS.

Nature of sound—The ear—Musical instruments—The vibration of strings—The
sounding-board and the frame of a piano—The strings—The striking mechanism

—The quality of a note.

S OUND differs from light, heat, and electricity in that it can be
propagated through matter only. Sound-waves are matter-waves, not
ether-waves. This can be proved by placing an electric bell under the bell-
glass of an air-pump and exhausting all the air. Ether still remains inside the
glass, but if the bell be set in motion no sound is audible. Admit air, and the
clang of the gong is heard quite plainly.

Sound resembles light and heat, however, thus far, that it can be
concentrated by means of suitable lenses and curved surfaces. An echo is a
proof of its reflection from a surface.

Before dealing with the various appliances used for producing sound-
waves of a definite character, let us examine that wonderful natural
apparatus

THE EAR,

through which we receive those sensations which we call sound.

Fig. 133.—Diagrammatic sketch of the parts of the ear.
Fig. 133 is a purely diagrammatic section of the ear, showing the various
parts distorted and out of proportion. Beginning at the left, we have the
outer ear, the lobe, to gather in the sound-waves on to the membrane of the

tympanum, or drum, to which is attached the first of a series of ossicles, or
small bones. The last of these presses against an opening in the inner ear, a
cavity surrounded by the bones of the head. Inside the inner ear is a watery
fluid, p, called perilymph ("surrounding water"), immersed in which is a
membranic envelope, m, containing endolymph ("inside water"), also full of
fluid. Into this fluid project e e e, the terminations of the auditory nerve,
leading to the brain.

When sound-waves strike the tympanum, they cause it to move inwards
and outwards in a series of rapid movements. The ossicles operated by the
tympanum press on the little opening o, covered by a membrane, and every
time they push it in they slightly squeeze the perilymph, which in turn
compresses the endolymph, which affects the nerve-ends, and telegraphs a
sensation of sound to the brain.

In Fig. 134 we have a more developed sketch, giving in fuller detail,
though still not in their actual proportions, the components of the ear. The
ossicles m, i, and s are respectively the malleus (hammer), incus (anvil), and
stapes (stirrup). Each is attached by ligaments to the walls of the middle ear.
The tympanum moves the malleus, the malleus the incus, and the incus the
stapes, the last pressing into the opening o of Fig. 133, which is
scientifically known as the fenestra ovalis, or oval window. As liquids are
practically incompressible, nature has made allowance for the squeezing in
of the oval window membrane, by providing a second opening, the round
window, also covered with a membrane. When the stapes pushes the oval
membrane in, the round membrane bulges out, its elasticity sufficing to put
a certain pressure on the perilymph (indicated by the dotted portion of the
inner ear).

Fig. 134.—Diagrammatic section of the ear, showing the
various parts.
The inner ear consists of two main parts, the cochlea—so called from its
resemblance in shape to a snail's shell—and the semicircular canals. Each
portion has its perilymph and endolymph, and contains a number of the
nerve-ends, which are, however, most numerous in the cochlea. We do not
know for certain what the functions of the canals and the cochlea are; but it
is probable that the former enables us to distinguish between the intensity or
loudness of sounds and the direction from which they come, while the latter
enables us to determine the pitch of a note. In the cochlea are about 2,800
tiny nerve-ends, called the rods of Corti. The normal ear has such a range as
to give about 33 rods to the semitone. The great scientist Helmholtz has
advanced the theory that these little rods are like tiny tuning-forks, each
responding to a note of a certain pitch; so that when a string of a piano is
sounded and the air vibrations are transmitted to the inner ear, they affect
only one of these rods and the part of the brain which it serves, and we have
the impression of one particular note. It has been proved by experiment that
a very sensitive ear can distinguish between sounds varying in pitch by only
1⁄64th of a semitone, or but half the range of any one Corti fibre. This

difficulty Helmholtz gets over by suggesting that in such an ear two
adjacent fibres are affected, but one more than the other.

A person who has a "good ear" for music is presumably one whose Corti
rods are very perfect. Unlucky people like the gentleman who could only
recognize one tune, and that because people took off their hats when it
commenced, are physically deficient. Their Corti rods cannot be properly
developed.

What applies to one single note applies also to the elements of a musical
chord. A dozen notes may sound simultaneously, but the ear is able to
assimilate each and blend it with its fellows; yet it requires a very sensitive
and well-trained ear to pick out any one part of a harmony and concentrate
the brain's attention on that part.

The ear has a much larger range than the eye. "While the former ranges
over eleven octaves, but little more than a single octave is possible to the
latter. The quickest vibrations which strike the eye, as light, have only about
twice the rapidity of the slowest; whereas the quickest vibrations which
strike the ear, as a musical sound, have more than two thousand times the
rapidity of the slowest."[26] To come to actual figures, the ordinary ear is
sensitive to vibrations ranging from 16 to 38,000 per second. The bottom
and top notes of a piano make respectively about 40 and 4,000 vibrations a
second. Of course, some ears, like some eyes, cannot comprehend the
whole scale. The squeak of bats and the chirrup of crickets are inaudible to
some people; and dogs are able to hear sounds far too shrill to affect the
human auditory apparatus.

Not the least interesting part of this wonderful organ is the tympanic
membrane, which is provided with muscles for altering its tension
automatically. If we are "straining our ears" to catch a shrill sound, we
tighten the membrane; while if we are "getting ready" for a deep, loud
report like that of a gun, we allow the drum to slacken.

The Eustachian tube (Fig. 134) communicates with the mouth. Its
function is probably to keep the air-pressure equal on both sides of the
drum. When one catches cold the tube is apt to become blocked by mucus,
causing unequal pressure and consequent partial deafness.

Before leaving this subject, it will be well to remind our more youthful
readers that the ear is delicately as well as wonderfully made, and must be
treated with respect. Sudden shouting into the ear, or a playful blow, may
have most serious effects, by bursting the tympanum or injuring the
arrangement of the tiny bones putting it in communication with the inner
ear.

MUSICAL INSTRUMENTS.

These are contrivances for producing sonorous shocks following each
other rapidly at regular intervals. Musical sounds are distinguished from
mere noises by their regularity. If we shake a number of nails in a tin box,
we get only a series of superimposed and chaotic sensations. On the other
hand, if we strike a tuning-fork, the air is agitated a certain number of times
a second, with a pleasant result which we call a note.

We will begin our excursion into the region of musical instruments with
an examination of that very familiar piece of furniture,

THE PIANOFORTE,

which means literally the "soft-strong." By many children the piano is
regarded as a great nuisance, the swallower-up of time which could be
much more agreeably occupied, and is accordingly shown much less respect
than is given to a phonograph or a musical-box. Yet the modern piano is a
very clever piece of work, admirably adapted for the production of sweet
melody—if properly handled. The two forms of piano now generally used
are the upright, with vertical sound-board and wires, and the grand, with
horizontal sound-board.[27]

THE VIBRATION OF STRINGS.

As the pianoforte is a stringed instrument, some attention should be
given to the subject of the vibration of strings. A string in a state of tension
emits a note when plucked and allowed to vibrate freely. The pitch of the
note depends on several conditions:—(1) The diameter of the string; (2) the
tension of the string; (3) the length of the string; (4) the substance of the
string. Taking them in order:—(1.) The number of vibrations per second is
inversely proportional to the diameter of the string: thus, a string one-
quarter of an inch in diameter would vibrate only half as often in a given
time as a string one-eighth of an inch in diameter. (2.) The length remaining
the same, the number of vibrations is directly proportional to the square
root of the tension: thus, a string strained by a 16-lb. weight would vibrate
four times as fast as it would if strained by a 1-lb. weight. (3.) The number
of vibrations is inversely proportional to the length of the string: thus, a
one-foot string would vibrate twice as fast as a two-foot string, strained to
the same tension, and of equal diameter and weight. (4.) Other things being
equal, the rate of vibration is inversely proportional to the square root of the
density of the substance: so that a steel wire would vibrate more rapidly
than a platinum wire of equal diameter, length, and tension. These facts are
important to remember as the underlying principles of stringed instruments.

Now, if you hang a wire from a cord, and hang a heavy weight from the
wire, the wire will be in a state of high tension, and yield a distinct note if
struck. But the volume of sound will be very small, much too small for a
practical instrument. The surface of the string itself is so limited that it sets
up but feeble motions in the surrounding air. Now hang the wire from a
large board and strike it again. The volume of sound has greatly increased,
because the string has transmitted its vibrations to the large surface of the
board.

To get the full sound-value of the vibrations of a string, we evidently
ought to so mount the string that it may influence a large sounding surface.
In a violin this is effected by straining the strings over a "bridge" resting on
a hollow box made of perfectly elastic wood. Draw the bow across a string.

The loud sound heard proceeds not from the string only, but also from the
whole surface of the box.

THE SOUNDING-BOARD AND FRAME OF A PIANO.

A piano has its strings strained across a frame of wood or steel, from a
row of hooks in the top of the frame to a row of tapering square-ended pins
in the bottom, the wires passing over sharp edges near both ends. The tuner
is able, on turning a pin, to tension its strings till it gives any desired note.
Readers may be interested to learn that the average tension of a string is 275
lbs., so that the total strain on the frame of a grand piano is anything
between 20 and 30 tons.

To the back of the frame is attached the sounding-board, made of spruce
fir (the familiar Christmas tree). This is obtained from Central and Eastern
Europe, where it is carefully selected and prepared, as it is essential that the
timber should be sawn in such a way that the grain of the wood runs in the
proper direction.

THE STRINGS.

These are made of extremely strong steel wire of the best quality. If you
examine the wires of your piano, you will see that they vary in thickness,
the thinnest being at the treble end of the frame. It is found impracticable to
use wires of the same gauge and the same tension throughout. The makers
therefore use highly-tensioned thick wires for the bass, and finer, shorter
wires for the treble, taking advantage of the three factors—weight, tension,
and length—which we have noticed above. The wires for the deepest notes
are wrapped round with fine copper wire to add to their weight without
increasing their diameter at the tuning-pins. There are about 600 yards
(roughly one-third of a mile) of wire in a grand piano.

THE STRIKING MECHANISM.

We now pass to the apparatus for putting the strings in a state of
vibration. The grand piano mechanism shown in Fig. 135 may be taken as

typical of the latest improvements. The essentials of an effective
mechanism are:—(1) That the blow delivered shall be sharp and certain; (2)
that the string shall be immediately "damped," or have its vibration checked
if required, so as not to interfere with the succeeding notes of other strings;
(3) that the hammer shall be able to repeat the blows in quick succession.
The hammer has a head of mahogany covered with felt, the thickness of
which tapers gradually and regularly from an inch and a quarter at the bass
end to three-sixteenths of an inch at the extreme treble notes. The entire
eighty-five hammers for the piano are covered all together in one piece, and
then they are cut apart from each other. The consistency of the covering is
very important. If too hard, it yields a harsh note, and must be reduced to
the right degree by pricking with a needle. In the diagram the felt is
indicated by the dotted part.

Fig. 135.—The striking mechanism of a "grand" piano.
The action carriage which operates the hammer is somewhat
complicated. When the key is depressed, the left end rises, and pushes up
the whole carriage, which is pivoted at one end. The hammer shank is
raised by the jack b pressing upon a knob, n, called the notch, attached to
the under side of the shank. When the jack has risen to a certain point, its
arm, b1, catches against the button c and jerks it from under the notch at the
very moment when the hammer strikes, so that it may not be blocked

against the string. As it rebounds, the hammer is caught on the repetition
lever r, which lifts it to allow of perfect repetition.

The check catches the tail of the hammer head during its descent when
the key is raised, and prevents it coming back violently on the carriage and
rest. The tail is curved so as to wedge against the check without jamming in
any way. The moment the carriage begins to rise, the rear end of the key
lifts a lever connected with the damper by a vertical wire, and raises the
damper of the string. If the key is held down, the vibrations continue for a
long time after the blow; but if released at once, the damper stifles them as
the hammer regains its seat. A bar, l, passing along under all the damper
lifters, is raised by depressing the loud pedal. The soft pedal slides the
whole keyboard along such a distance that the hammers strike two only out
of the three strings allotted to all except the bass notes, which have only one
string apiece, or two, according to their depth or length. In some pianos the
soft pedal presses a special damper against the strings; and a third kind of
device moves the hammers nearer the strings so that they deliver a lighter
blow. These two methods of damping are confined to upright pianos.

A high-class piano is the result of very careful workmanship. The
mechanism of each note must be accurately regulated by its tiny screws to a
minute fraction of an inch. It must be ensured that every hammer strikes its
blow at exactly the right place on the string, since on this depends the
musical value of the note. The adjustment of the dampers requires equal
care, and the whole work calls for a sensitive ear combined with skilled
mechanical knowledge, so that the instrument may have a light touch,
strength, and certainty of action throughout the whole keyboard.

THE QUALITY OF A NOTE.

If two strings, alike in all respects and equally tensioned, are plucked,
both will give the same note, but both will not necessarily have the same
quality of tone. The quality, or timbre, as musicians call it, is influenced by
the presence of overtones, or harmonics, in combination with the

fundamental, or deepest, tone of the string. The fact is, that while a
vibrating string vibrates as a whole, it also vibrates in parts. There are, as it
were, small waves superimposed on the big fundamental waves. Points of
least motion, called nodes, form on the string, dividing it into two, three,
four, five, etc., parts, which may be further divided by subsidiary nodes.
The string, considered as halved by one node, gives the first overtone, or
octave of the fundamental. It may also vibrate as three parts, and give the
second overtone, or twelfth of the fundamental;[28] and as four parts, and
give the third overtone, the double octave.

Now, if a string be struck at a point corresponding to a node, the
overtones which require that point for a node will be killed, on account of
the excessive motion imparted to the string at that spot. Thus to hit it at the
middle kills the octave, the double octave, etc.; while to hit it at a point one-
third of the length from one end stifles the twelfth and all its sub-multiples.

A fundamental note robbed of all its harmonics is hard to obtain, which
is not a matter for regret, as it is a most uninteresting sound. To get a rich
tone we must keep as many useful harmonics as possible, and therefore a
piano hammer is so placed as to strike the string at a point which does not
interfere with the best harmonics, but kills those which are objectionable.
Pianoforte makers have discovered by experiment that the most pleasing
tone is excited when the point against which the hammer strikes is one-
seventh to one-ninth of the length of the wire from one end.

The nature of the material which does the actual striking is also of
importance. The harder the substance, and the sharper the blow, the more
prominent do the harmonics become; so that the worker has to regulate
carefully both the duration of the blow and the hardness of the hammer
covering.
[26] Tyndall, "On Sound," p. 75.
[27] A Broadwood "grand" is made up of 10,700 separate pieces, and in its
manufacture forty separate trades are concerned.

[28] Twelve notes higher up the scale.

Chapter XV.

WIND INSTRUMENTS.

Longitudinal vibration—Columns of air—Resonance of columns of air—Length and
tone—The open pipe—The overtones of an open pipe—Where overtones are
used—The arrangement of the pipes and pedals—Separate sound-boards—
Varieties of stops—Tuning pipes and reeds—The bellows—Electric and
pneumatic actions—The largest organ in the world—Human reeds.

LONGITUDINAL VIBRATION.

I N stringed instruments we are concerned only with the transverse
vibrations of a string—that is, its movements in a direction at right
angles to the axis of the string. A string can also vibrate longitudinally—that
is, in the direction of its axis—as may be proved by drawing a piece of
resined leather along a violin string. In this case the harmonics "step up" at
the same rate as when the movements were transverse.

Let us substitute for a wire a stout bar of metal fixed at one end only.
The longitudinal vibrations of this rod contain overtones of a different ratio.
The first harmonic is not an octave, but a twelfth. While a tensioned string
is divided by nodes into two, three, four, five, six, etc., parts, a rod fixed at
one end only is capable of producing only those harmonics which
correspond to division into three, five, seven, nine, etc., parts. Therefore a
free-end rod and a wire of the same fundamental note would not have the
same timbre, or quality, owing to the difference in the harmonics.

COLUMNS OF AIR.

In wind instruments we employ, instead of rods or wires, columns of air
as the vibrating medium. The note of the column depends on its length. In
the "penny whistle," flute, clarionet, and piccolo the length of the column is

altered by closing or opening apertures in the substance encircling the
column.

RESONANCE OF COLUMNS OF AIR.

Why does a tube closed at one end, such as the shank of a key, emit a
note when we blow across the open end? The act of blowing drives a thin
sheet of air against the edge of the tube and causes it to vibrate. The
vibrations are confused, some "pulses" occurring more frequently than
others. If we blew against the edge of a knife or a piece of wood, we should
hear nothing but a hiss. But when, as in the case which we are considering,
there is a partly-enclosed column of air close to the pulses, this selects those
pulses which correspond to its natural period of vibration, and augments
them to a sustained and very audible musical sound.

In Fig. 136, 1 is a pipe, closed at the bottom and open at the top. A
tuning-fork of the same note as the pipe is struck and held over it so that the
prongs vibrate upwards and downwards. At the commencement of an
outward movement of the prongs the air in front of them is compressed.
This impulse, imparted to the air in the pipe, runs down the column, strikes
the bottom, and returns. Just as it reaches the top the prong is beginning to
move inwards, causing a rarefaction of the air behind it. This effect also
travels down and back up the column of air in the pipe, reaching the prong
just as it arrives at the furthest point of the inward motion. The process is
repeated, and the column of air in the pipe, striking on the surrounding
atmosphere at regular intervals, greatly increases the volume of sound. We
must observe that if the tuning-fork were of too high or too low a note for
the column of air to move in perfect sympathy with it, this increase of
sound would not result. Now, when we blow across the end, we present, as
it were, a number of vibrating tuning-forks to the pipe, which picks out
those air-pulses with which it sympathizes.

LENGTH AND TONE.

The rate of vibration
is found to be inversely
proportional to the
length of the pipe.
Thus, the vibrations of
a two-foot pipe are
twice as rapid as those
of a four-foot pipe, and
the note emitted by the
former is an octave
higher than that of the
latter. A one-foot pipe
gives a note an octave
higher still. We are
here speaking of the
fundamental tones of
the pipes. With them,
as in the case of
strings, are associated
the overtones, or
harmonics, which can
be brought into
prominence by
increasing the pressure
of the blast at the top of Fig 136.—Showing how the harmonics of a
the pipe. Blow very "stopped" pipe are formed.
hard on your key, and
the note suddenly changes to one much shriller. It is the twelfth of the
fundamental, of which it has completely got the upper hand.

We must now put on our thinking-caps and try to understand how this
comes about. First, let us note that the vibration of a body (in this case a
column of air) means a motion from a point of rest to a point of rest, or

from node to node. In the air-column in Fig. 136, 1, there is only one point
of rest for an impulse—namely, at the bottom of the pipe. So that to pass
from node to node the impulse must pass up the pipe and down again. The
distance from node to node in a vibrating body is called a ventral segment.
Remember this term. Therefore the pipe represents a semi-ventral segment
when the fundamental note is sounding.

When the first overtone is sounded the column divides itself into two
vibrating parts. Where will the node between them be? We might naturally
say, "Half-way up." But this cannot be so; for if the node were so situated,
an impulse going down the pipe would only have to travel to the bottom to
find another node, while an impulse going up would have to travel to the
top and back again—that is, go twice as far. So the node forms itself one-
third of the distance down the pipe. From b to a (Fig. 136, 2) and back is
now equal to from b to c. When the second overtone is blown (Fig. 136, 3)
a third node forms. The pipe is now divided into five semi-ventral segments.
And with each succeeding overtone another node and ventral segment are
added.

The law of vibration of a column of air is that the number of vibrations
is directly proportional to the number of semi-ventral segments into which
the column of air inside the pipe is divided.[29] If the fundamental tone
gives 100 vibrations per second, the first overtone in a closed pipe must
give 300, and the second 500 vibrations.

THE OPEN PIPE.

A pipe open at both ends is capable of emitting a note. But we shall find,
if we experiment, that the note of a stopped pipe is an octave lower than
that of an open pipe of equal length. This is explained by Fig. 137, 1. The
air-column in the pipe (of the same length as that in Fig. 136) divides itself,
when an end is blown across, into two equal portions at the node b, the
natural point to obtain equilibrium. A pulse will pass from a or a1 to b and

back again in half the time required to pass from a to b and back in Fig.
136, 1; therefore the note is an octave higher.

THE OVERTONES OF AN
OPEN PIPE.

The first overtone
results when nodes
form as in Fig. 137, 2,
at points one-quarter of
the length of the pipe
from the ends, giving
one complete ventral
segment and two semi-
ventral segments. The
vibrations now are
twice as rapid as
before. The second
overtone requires three
nodes, as in Fig. 137, 3.
The rate has now
trebled. So that, while
the overtones of a
closed pipe rise in the
ratio 1, 3, 5, 7, etc.,
those of an open pipe
rise in the proportion 1,
2, 3, 4, etc.
Fig. 137.—Showing how harmonics of an open
pipe are formed, b, b1, and c are "nodes." The WHERE OVERTONES ARE
arrows indicate the distance travelled by a USED.
sound impulse from a node to a node.
In the flute, piccolo,
and clarionet, as well
as in the horn class of instrument, the overtones are as important as the

fundamental notes. By artificially altering the length of the column of air,
the fundamental notes are also altered, while the harmonics of each
fundamental are produced at will by varying the blowing pressure; so that a
continuous chromatic, or semitonal, scale is possible throughout the
compass of the instrument.

THE ORGAN.

From the theory of acoustics[30] we pass to the practical application, and
concentrate our attention upon the grandest of all wind instruments, the pipe
organ. This mechanism has a separate pipe for every note, properly
proportioned. A section of an ordinary wooden pipe is given in Fig. 138.
Wind rushes up through the foot of the pipe into a little chamber, closed by
a block of wood or a plate except for a narrow slit, which directs it against
the sharp lip a, and causes a fluttering, the proper pulse of which is
converted by the air-column above into a musical sound.

In even the smallest organs more than one pipe is actuated by one key on
the keyboard, for not only do pipes of different shapes give different
qualities of tone, but it is found desirable to have ranks of pipes with their
bottom note of different pitches. The length of an open pipe is measured
from the edge of the lip to the top of the pipe; of a stopped pipe, from the
lip to the top and back again. When we speak of a 16 or 8 foot rank, or stop,
we mean one of which the lowest note in the rank is that produced by a 16
or 8 foot open pipe, or their stopped equivalents (8 or 4 foot). In a big organ
we find 32, 16, 8, 4, and 2 foot stops, and some of these repeated a number
of times in pipes of different shape and construction.

THE ARRANGEMENT OF THE PIPES.

We will now study briefly the mechanism of a very simple single-
keyboard organ, with five ranks of pipes, or stops.

It is necessary to arrange matters so that the pressing down of one key
may make all five of the pipes belonging to it speak, or only four, three,

two, or one, as
we may desire.
The pipes are
mounted in
rows on a
sound-board,
which is built
up in several
layers. At the
top is the
upper board;
below it come
the sliders,
Fig. 139.—The table of a sound- one for each
board.
stop; and
underneath
that the table. In Fig. 139 we see part of the table from
below. Across the under side are fastened parallel bars
with spaces (shown black) left between them. Two
other bars are fastened across the ends, so that each
groove is enclosed by wood at the top and on all sides.
The under side of the table has sheets of leather glued
or otherwise attached to it in such a manner that no air
can leak from one groove to the next. Upper board,
sliders, and table are pierced with rows of holes, to
permit the passage of wind from the grooves to the Fig. 138.—
pipes. The grooves under the big pipes are wider than Section of an
ordinary wooden
those under the small pipes, as they have to pass more
"flue" pipe.
air. The bars between the grooves also vary in width
according to the weight of the pipes which they have
to carry. The sliders can be moved in and out a short distance in the
direction of the axis of the rows of pipes. There is one slider under each
row. When a slider is in, the holes in it do not correspond with those in the

table and upper board, so that no wind can get from the grooves to the rank
over that particular slider. Fig. 140 shows the manner in which the sliders
are operated by the little knobs (also called stops) projecting from the
casing of the organ within convenient reach of the performer's hands. One
stop is in, the other drawn out.

In Fig. 141 we see the table, etc.,
in cross section, with a slider out,
putting the pipes of its rank in
communication with the grooves.
The same diagram shows us in
section the little triangular pallets
which admit air from the wind-chest
to the grooves; and Fig. 142 gives
us an end section of table, sliders,
and wind-chest, together with the
rods, etc., connecting the key to its
pallet. When the key is depressed,
the sticker (a slight wooden rod) is Fig. 140.
pushed up. This rocks a backfall, or
pivoted lever, to which is attached
the pulldown, a wire penetrating the bottom of the wind-chest to the pallet.
As soon as the pallet opens, wind rushes into the groove above through the
aperture in the leather bottom, and thence to any one of the pipes of which
the slider has been drawn out. (The sliders in Fig. 142 are solid black.) It is
evident that if the sound-board is sufficiently deep from back to front, any
number of rows of pipes may be placed on it.

PEDALS.

The organ pedals are connected to the pallets by an action similar to that
of the keys. The pedal stops are generally of deep tone, 32-foot and 16-foot,
as they have to sustain the bass part of the musical harmonies. By means of
couplers one or more of the keyboard stops may be linked to the pedals.

SEPARATE SOUND-BOARDS.

The keyboard of a very large
organ has as many as five manuals,
or rows of keys. Each manual
operates what is practically a
separate organ mounted on its own
sound-board.

Fig. 141.

Fig. 142.

Fig. 143.—General section of a two-manual organ.
The manuals are arranged in steps, each slightly overhanging that below.
Taken in order from the top, they are:—(1.) Echo organ, of stops of small
scale and very soft tone, enclosed in a "swell-box." (2.) Solo organ, of stops
imitating orchestral instruments. The wonderful "vox humana" stop also
belongs to this manual. (3.) Swell organ, contained in a swell-box, the front
and sides of which have shutters which can be opened and closed by the
pressure of the foot on a lever, so as to regulate the amount of sound
proceeding from the pipes inside. (4.) Great organ, including pipes of
powerful tone. (5.) Choir organ, of soft, mellow stops, often enclosed in a
swell-box. We may add to these the pedal organ, which can be coupled to
any but the echo manual.

VARIETIES OF STOPS.

We have already remarked that the quality of a stop depends on the
shape and construction of the pipe. Some pipes are of wood, others of
metal. Some are rectangular, others circular. Some have parallel sides,
others taper or expand towards the top. Some are open, others stopped.

The two main classes into which organ pipes may be divided are:—(1.)
Flue pipes, in which the wind is directed against a lip, as in Fig. 138. (2.)
Reed pipes—that is, pipes used in combination with a simple device for
admitting air into the bottom of the pipe in a series of gusts. Fig. 144 shows
a striking reed, such as is found in the ordinary motor horn. The elastic
metal tongue when at rest stands a very short distance away from the orifice
in the reed. When wind is blown through the reed the tongue is sucked
against the reed, blocks the current, and springs away again. A free reed has
a tongue which vibrates in a slot without actually touching the sides.
Harmonium and concertina reeds are of this type. In the organ the reed
admits air to a pipe of the correct length to sympathize with the rate of the
puffs of air which the reed passes. Reed pipes expand towards the top.

TUNING PIPES AND REEDS.

Fig. 144.—A reed pipe.
Pipes are tuned by adjusting their length. The plug at the top of a stopped
pipe is pulled out or pushed in a trifle to flatten or sharpen the note
respectively. An open pipe, if large, has a tongue cut in the side at the top,
which can be pressed inwards or outwards for the purpose of correcting the
tone. Small metal pipes are flattened by contracting the tops inwards with a

metal cone like a candle-extinguisher placed over the top and tapped; and
sharpened by having the top splayed by a cone pushed in point downwards.
Reeds of the striking variety (see Fig. 144) have a tuning-wire pressing on
the tongue near the fixed end. The end of this wire projects through the
casing. By moving it, the length of the vibrating part of the tongue is
adjusted to correctness.

BELLOWS.

Different stops require different wind-pressures, ranging from 1⁄10 lb. to
1 lb. to the square inch, the reeds taking the heaviest pressures. There must
therefore be as many sets of bellows and wind-chests as there are different
pressures wanted. A very large organ consumes immense quantities of air
when all the stops are out, and the pumping has to be done by a powerful
gas, water, or electric engine. Every bellows has a reservoir (see Fig. 143)
above it. The top of this is weighted to give the pressure required. A valve
in the top opens automatically as soon as the reservoir has expanded to a
certain fixed limit, so that there is no possibility of bursting the leather
sides.

Fig. 145.—The keyboard and part of the pneumatic
mechanism of the Hereford Cathedral organ. c,
composition pedals for pushing out groups of stops; p (at
bottom), pedals; p p (at top), pipes carrying compressed
air; m, manuals (4); s s, stops.

ELECTRIC AND PNEUMATIC ACTIONS.

We have mentioned in connection with railway signalling that the
signalman is sometimes relieved of the hard manual labour of moving
signals and points by the employment of electric and pneumatic auxiliaries.
The same is true of organs and organists. The touch of the keys has been
greatly lightened by making the keys open air-valves or complete electric
circuits which actuate the mechanism for pulling down the pallets. The
stops, pedals, and couplers also employ "power." Not only are the
performer's muscles spared a lot of heavy work when compressed air and
electricity aid him, but he is able to have the console, or keyboard, far away
from the pipes. "From the console, the player, sitting with the singers, or in
any desirable part of the choir or chancel, would be able to command the
working of the whole of the largest organ situated afar at the western end of
the nave; would draw each stop in complete reliance on the sliders and the
sound-board fulfilling their office; ... and—marvel of it all—the player,
using the swell pedal in his ordinary manner, would obtain crescendo and
diminuendo with a more perfect effect than by the old way."[31]

In cathedrals it is no uncommon thing for the different sound-boards to
be placed in positions far apart, so that to the uninitiated there may appear
to be several independent organs scattered about. Yet all are absolutely
under the control of a man who is sitting away from them all, but connected
with them by a number of tubes or wires.

The largest organ in the world is that in the Town Hall, Sydney. It has a
hundred and twenty-six speaking stops, five manuals, fourteen couplers,
and forty-six combination studs. The pipes, about 8,000 in number, range

from the enormous 64-foot contra-trombone to some only a fraction of an
inch in length. The organ occupies a space 85 feet long and 26 feet deep.

HUMAN REEDS.

The most wonderful of all musical reeds is found in the human throat, in
the anatomical part called the larynx, situated at the top of the trachea, or
windpipe.

Slip a piece of rubber tubing over the end of a pipe, allowing an inch or
so to project. Take the free part of the tube by two opposite points between
the first fingers and thumbs and pull it until the edges are stretched tight.
Now blow through it. The wind, forcing its way between the two rubber
edges, causes them and the air inside the tube to vibrate, and a musical note
results. The more you strain the rubber the higher is the note.

The larynx works on this principle. The windpipe takes the place of the
glass pipe; the two vocal cords represent the rubber edges; and the
arytenoid muscles stand instead of the hands. When contracted, these
muscles bring the edges of the cords nearer to one another, stretch the cords,
and shorten the cords. A person gifted with a "very good ear" can, it has
been calculated, adjust the length of the vocal cords to 1⁄17000th of an inch!

Simultaneously with the adjustment of the cords is effected the
adjustment of the length of the windpipe, so that the column of air in it may
be of the right length to vibrate in unison. Here again is seen a wonderful
provision of nature.

The resonance of the mouth cavity is also of great importance. By
altering the shape of the mouth the various harmonics of any fundamental
note produced by the larynx are rendered prominent, and so we get the
different vocal sounds. Helmholtz has shown that the fundamental tone of
any note is represented by the sound oo. If the mouth is adjusted to bring
out the octave of the fundamental, o results. a is produced by accentuating

the second harmonic, the twelfth; ee by developing the second and fourth
harmonics; while for ah the fifth and seventh must be prominent.

When we whistle we transform the lips into a reed and the mouth into a
pipe. The tension of the lips and the shape of the mouth cavity decide the
note. The lips are also used as a reed for blowing the flute, piccolo, and all
the brass band instruments of the cornet order. In blowing a coach-horn the
various harmonics of the fundamental note are brought out by altering the
lip tension and the wind pressure. A cornet is practically a coach-horn rolled
up into a convenient shape and furnished with three keys, the depression of
which puts extra lengths of tubing in connection with the main tube—in
fact, makes it longer. One key lowers the fundamental note of the horn half
a tone; the second, a full tone; the third, a tone and a half. If the first and
third are pressed down together, the note sinks two tones; if the second and
third, two and a half tones; and simultaneous depression of all three gives a
drop of three tones. The performer thus has seven possible fundamental
notes, and several harmonics of each of these at his command; so that by a
proper manipulation of the keys he can run up the chromatic scale.

We should add that the cornet tube is an "open" pipe. So is that of the
flute. The clarionet is a "stopped" pipe.
[29] It is obvious that in Fig. 136, 2, a pulse will pass from a to b and back in
one-third the time required for it to pass from a to b and back in Fig. 136, 1.

[30] The science of hearing; from the Greek verb, ἀκούειν, "to hear."

[31] "Organs and Tuning," p. 245.

Chapter XVI.

TALKING-MACHINES.

The phonograph—The recorder—The reproducer—The gramophone—The making
of records—Cylinder records—Gramophone records.

I N the Patent Office Museum at South Kensington is a curious little piece
of machinery—a metal cylinder mounted on a long axle, which has at
one end a screw thread chased along it. The screw end rotates in a socket
with a thread of equal pitch cut in it. To the other end is attached a handle.
On an upright near the cylinder is mounted a sort of drum. The membrane
of the drum carries a needle, which, when the membrane is agitated by the
air-waves set up by human speech, digs into a sheet of tinfoil wrapped
round the cylinder, pressing it into a helical groove turned on the cylinder
from end to end. This construction is the first phonograph ever made.
Thomas Edison, the "wizard of the West," devised it in 1876; and from this
rude parent have descended the beautiful machines which record and
reproduce human speech and musical sounds with startling accuracy.

Fig. 146.—The "governor" of a phonograph.

We do not propose to trace here the development of the talking-machine;
nor will it be necessary to describe in detail its mechanism, which is
probably well known to most readers, or could be mastered in a very short
time on personal examination. We will content ourselves with saying that
the wax cylinder of the phonograph, or the ebonite disc of the gramophone,
is generally rotated by clockwork concealed in the body of the machine.
The speed of rotation has to be very carefully governed, in order that the

record may revolve under the reproducing point at a uniform speed. The
principle of the governor commonly used appears in Fig. 146. The last
pinion of the clockwork train is mounted on a shaft carrying two triangular
plates, a and c, to which are attached three short lengths of flat steel spring
with a heavy ball attached to the centre of each. a is fixed; c moves up the
shaft as the balls fly out, and pulls with it the disc d, which rubs against the
pad p (on the end of a spring) and sets up sufficient friction to slow the
clockwork. The limit rate is regulated by screw s.

THE PHONOGRAPH.

Though the recording and reproducing apparatus of a phonograph gives
very wonderful results, its construction is quite simple. At the same time, it
must be borne in mind that an immense amount of experimenting has been
devoted to finding out the most suitable materials and forms for the parts.

Fig. 147.—Section of an Edison Bell phonograph recorder.
The recorder (Fig. 147) is a little circular box about one and a half
inches in diameter.[32] From the top a tube leads to the horn. The bottom is a
circular plate, c c, hinged at one side. This plate supports a glass disc, d,
about 1⁄150th of an inch thick, to which is attached the cutting stylus—a tiny
sapphire rod with a cup-shaped end having very sharp edges. Sound-waves
enter the box through the horn tube; but instead of being allowed to fill the
whole box, they are concentrated by the shifting nozzle n on to the centre of
the glass disc through the hole in c c. You will notice that n has a ball end,
and c c a socket to fit n exactly, so that, though c c and n move up and
down very rapidly, they still make perfect contact. The disc is vibrated by
the sound-impulses, and drives the cutting point down into the surface of

the wax cylinder, turning below it in a clockwork direction. The only dead
weight pressing on s is that of n, c c, and the glass diaphragm.

As the cylinder revolves, the recorder
is shifted continuously along by a leading
screw having one hundred or more
threads to the inch cut on it, so that it
traces a continuous helical groove from
one end of the wax cylinder to the other.
This groove is really a series of very
minute indentations, not exceeding
1⁄1000th of an inch in depth.[33] Seen under

a microscope, the surface of the record is
a succession of hills and valleys, some
Fig. 148.—Perspective view much larger than others (Fig. 151, a). A
of a phonograph recorder. loud sound causes the stylus to give a
vigorous dig, while low sounds scarcely
move it at all. The wonderful thing about this sound-recording is, that not
only are the fundamental tones of musical notes impressed, but also the
harmonics, which enable us to decide at once whether the record is one of a
cornet, violin, or banjo performance. Furthermore, if several instruments are
playing simultaneously near the recorder's horn, the stylus catches all the
different shades of tone of every note of a chord. There are, so to speak,
minor hills and valleys cut in the slopes of the main hills and valleys.

Fig. 149.—Section of the reproducer of an Edison Bell
phonograph.
The reproducer (Fig. 149) is somewhat more complicated than the
recorder. As before, we have a circular box communicating with the horn of
the instrument. A thin glass disc forms a bottom to the box. It is held in
position between rubber rings, r r, by a screw collar, c. To the centre is
attached a little eye, from which hangs a link, l. Pivoted at p from one edge
of the box is a floating weight, having a circular opening immediately under
the eye. The link passes through this to the left end of a tiny lever, which
rocks on a pivot projecting from the weight. To the right end of the lever is
affixed a sapphire bar, or stylus, with a ball end of a diameter equal to that
of the cutting point of the recorder. The floating weight presses the stylus

against the record, and
also keeps the link
between the rocking
lever of the glass
diaphragm in a state of
tension. Every blow
given to the stylus is
therefore transmitted
by the link to the
diaphragm, which
vibrates and sends an
air-impulse into the
horn. As the impulses
are given at the same
rate as those which
agitated the diaphragm
of the recorder, the
sounds which they
represent are accurately
reproduced, even to the
harmonics of a musical Fig. 150.—Perspective view of a phonograph
note. reproducer.

THE GRAMOPHONE.

This effects the same purpose as the phonograph, but in a somewhat
different manner. The phonograph recorder digs vertically downwards into
the surface of the record, whereas the stylus of the gramophone wags from
side to side and describes a snaky course (Fig. 151b). It makes no difference
in talking-machines whether the reproducing stylus be moved sideways or
vertically by the record, provided that motion is imparted by it to the
diaphragm.

Fig. Fig.
151a. 151b.

Fig. 151c.—Section of a gramophone reproducer.

In Fig. 151c the construction of the gramophone reproducer is shown in
section. a is the cover which screws on to the bottom b, and confines the
diaphragm d between itself and a rubber ring. The portion b is elongated
into a tubular shape for connection with the horn, an arm of which slides
over the tube and presses against the rubber ring c to make an air-tight joint.
The needle-carrier n is attached at its upper end to the centre of the
diaphragm. At a point indicated by the white dot a pin passes through it and
the cover. The lower end is tubular to accommodate the steel points, which

have to be replaced after passing once over a record. A screw, s, working in
a socket projecting from the carrier, holds the point fast. The record moves
horizontally under the point in a plane perpendicular to the page. The
groove being zigzag, the needle vibrates right and left, and rotating the
carrier a minute fraction of an inch on the pivot, shakes the glass diaphragm
and sends waves of air into the horn.

The gramophone is a reproducing instrument only. The records are made
on a special machine, fitted with a device for causing the recorder point to
describe a spiral course from the circumference to the centre of the record
disc. Some gramophone records have as many as 250 turns to the inch. The
total length of the tracing on a ten-inch "concert" record is about 1,000 feet.

THE MAKING OF RECORDS.

For commercial purposes it would not pay to make every record
separately in a recording machine. The expense of employing good singers
and instrumentalists renders such a method impracticable. All the records
we buy are made from moulds, the preparation of which we will now
briefly describe.

CYLINDER, OR PHONOGRAPH RECORDS.

First of all, a wax record is made in the ordinary way on a recording
machine. After being tested and approved, it is hung vertically and centrally
from a rotating table pivoted on a vertical metal spike passing up through
the record. On one side of the table is a piece of iron. On each side of the
record, and a small distance away, rises a brass rod enclosed in a glass tube.
The top of the rods are hooked, so that pieces of gold leaf may be
suspended from them. A bell-glass is now placed over the record, table, and
rods, and the air is sucked out by a pump. As soon as a good vacuum has
been obtained, the current from the secondary circuit of an induction coil is
sent into the rods supporting the gold leaves, which are volatilized by the
current jumping from one to the other. A magnet, whirled outside the bell-
glass, draws round the iron armature on the pivoted table, and consequently

revolves the record, on the surface of which a very thin coating of gold is
deposited. The record is next placed in an electroplating bath until a copper
shell one-sixteenth of an inch thick has formed all over the outside. This is
trued up on a lathe and encased in a brass tube. The "master," or original
wax record, is removed by cooling it till it contracts sufficiently to fall out
of the copper mould, on the inside surface of which are reproduced, in
relief, the indentations of the wax "master."

Copies are made from the mould by immersing it in a tank of melted
wax. The cold metal chills the wax that touches it, so that the mould soon
has a thick waxen lining. The mould and copy are removed from the tank
and mounted on a lathe, which shapes and smooths the inside of the record.
The record is loosened from the mould by cooling. After inspection for
flaws, it is, if found satisfactory, packed in cotton-wool and added to the
saleable stock.

Gramophone master records are made on a circular disc of zinc, coated
over with a very thin film of acid-proof fat. When the disc is revolved in the
recording machine, the sharp stylus cuts through the fat and exposes the
zinc beneath. On immersion in a bath of chromic acid the bared surfaces are
bitten into, while the unexposed parts remain unaffected. When the etching
is considered complete, the plate is carefully cleaned and tested. A negative
copper copy is made from it by electrotyping. This constitutes the mould.
From it as many as 1,000 copies may be made on ebonite plates by
combined pressure and heating.

[32] The Edison Bell phonograph is here referred to.

[33] Some of the sibilant or hissing sounds of the voice are computed to be represented by
depressions less than a millionth of an inch in depth. Yet these are reproduced very clearly!

Chapter XVII.

WHY THE WIND BLOWS.

Why the wind blows—Land and sea breezes—Light air and moisture—The barometer—The
column barometer—The wheel barometer—A very simple barometer—The aneroid barometer
—Barometers and weather—The diving-bell—The diving-dress—Air-pumps—Pneumatic
tyres—The air-gun—The self-closing door-stop—The action of wind on oblique surfaces—
The balloon—The flying-machine.

W HEN a child's rubber ball gets slack through a slight leakage of air, and loses
some of its bounce, it is a common practice to hold it for a few minutes in front
of the fire till it becomes temporarily taut again. Why does the heat have this effect on
the ball? No more air has been forced into the ball. After perusing the chapter on the
steam-engine the reader will be able to supply the answer. "Because the molecules of
air dash about more vigorously among one another when the air is heated, and by
striking the inside of the ball with greater force put it in a state of greater tension."

If we heat an open jar there is no pressure developed, since the air simply expands
and flows out of the neck. But the air that remains in the jar, being less in quantity than
when it was not yet heated, weighs less, though occupying the same space as before. If
we took a very thin bladder and filled it with hot air it would therefore float in colder
air, proving that heated air, as we should expect, tends to rise. The fire-balloon employs
this principle, the air inside the bag being kept artificially warm by a fire burning in
some vessel attached below the open neck of the bag.

Now, the sun shines with different degrees of heating power at different parts of the
world. Where its effect is greatest the air there is hottest. We will suppose, for the sake
of argument, that, at a certain moment, the air envelope all round the globe is of equal
temperature. Suddenly the sun shines out and heats the air at a point, a, till it is many
degrees warmer than the surrounding air. The heated air expands, rises, and spreads out
above the cold air. But, as a given depth of warm air has less weight than an equal depth
of cold air, the cold air at once begins to rush towards b and squeeze the rest of the
warm air out. We may therefore picture the atmosphere as made up of a number of

colder currents passing along the surface of the earth to replace warm currents rising
and spreading over the upper surface of the cold air. A similar circulation takes place in
a vessel of heated water (see p. 17).

LAND AND SEA BREEZES.

A breeze which blows from the sea on to the land during the day often reverses its
direction during the evening. Why is this? The earth grows hot or cold more rapidly
than the sea. When the sun shines hotly, the land warms quickly and heats the air over
it, which becomes light, and is displaced by the cooler air over the sea. When the sun
sets, the earth and the air over it lose their warmth quickly, while the sea remains at
practically the same temperature as before. So the balance is changed, the heavier air
now lying over the land. It therefore flows seawards, and drives out the warmer air
there.

LIGHT AIR AND MOISTURE.

Light, warm air absorbs moisture. As it cools, the moisture in it condenses. Breathe
on a plate, and you notice that a watery film forms on it at once. The cold surface
condenses the water suspended in the warm breath. If you wish to dry a damp room you
heat it. Moisture then passes from the walls and objects in the room to the atmosphere.

THE BAROMETER.

This property of air is responsible for the changes in weather. Light, moisture-laden
air meets cold, dry air, and the sudden cooling forces it to release its moisture, which
falls as rain, or floats about as clouds. If only we are able to detect the presence of
warm air-strata above us, we ought to be in a position to foretell the weather.

We can judge of the specific gravity of the air in our neighbourhood by means of the
barometer, which means "weight-measurer." The normal air-pressure at sea-level on our
bodies or any other objects is about 15 lbs. to the square inch—that is to say, if you
could imprison and weigh a column of air one inch square in section and of the height
of the world's atmospheric envelope, the scale would register 15 lbs. Many years ago
(1643) Torricelli, a pupil of Galileo, first calculated the pressure by a very simple
experiment. He took a long glass tube sealed at one end, filled it with mercury, and,
closing the open end with the thumb, inverted the tube and plunged the open end below
the surface of a tank of mercury. On removing his thumb he found that the mercury
sank in the tube till the surface of the mercury in the tube was about 30 inches in a
vertical direction above the surface of the mercury in the tank. Now, as the upper end
was sealed, there must be a vacuum above the mercury. What supported the column?

The atmosphere. So it was evident that the downward pressure of the mercury exactly
counterbalanced the upward pressure of the air. As a mercury column 30 inches high
and 1 inch square weighs 15 lbs., the air-pressure on a square inch obviously is the
same.

Fig. 152.—A
Fortin
barometer.

FORTIN'S COLUMN BAROMETER

is a simple Torricellian tube, t, with the lower end submerged in a little glass tank of
mercury (Fig. 152). The bottom of this tank is made of washleather. To obtain a
"reading" the screw s, pressing on the washleather, is adjusted until the mercury in the
tank rises to the tip of the little ivory point p. The reading is the figure of the scale on
the face of the case opposite which the surface of the column stands.

Fig. 153.

THE WHEEL BAROMETER

also employs the mercury column (Fig. 153). The lower end of the tube is turned up and
expanded to form a tank, c. The pointer p, which travels round a graduated dial, is
mounted on a spindle carrying a pulley, over which passes a string with a weight at
each end. The heavier of the weights rests on the top of the mercury. When the
atmospheric pressure falls, the mercury in c rises, lifting this weight, and the pointer
moves. This form of barometer is not so delicate or reliable as Fortin's, or as the siphon
barometer, which has a tube of the same shape as the wheel instrument, but of the same
diameter from end to end except for a contraction at the bend. The reading of a siphon
is the distance between the two surfaces of the mercury.

A VERY SIMPLE BAROMETER

is made by knocking off the neck of a small bottle, filling the body with water, and
hanging it up by a string in the position shown (Fig. 154). When the atmospheric
pressure falls, the water at the orifice bulges outwards; when it rises, the water retreats
till its surface is slightly concave.

Fig. 154.

THE ANEROID BAROMETER.

On account of their size and weight, and the comparative difficulty of transporting
them without derangement of the mercury column, column barometers are not so
generally used as the aneroid variety. Aneroid means "without moisture," and in this
particular connection signifies that no liquid is used in the construction of the
barometer.

Fig. 155 shows an aneroid in detail. The most noticeable feature is the vacuum
chamber, v c, a circular box which has a top and bottom of corrugated but thin and
elastic metal. Sections of the box are shown in Figs. 156, 157. It is attached at the
bottom to the base board of the instrument by a screw (Fig. 156). From the top rises a
pin, p, with a transverse hole through it to accommodate the pin k e, which has a
triangular section, and stands on one edge.

Fig. 155.—An aneroid barometer.

Returning to Fig. 155, we see that p projects through s, a powerful spring of sheet-
steel. To this is attached a long arm, c, the free end of which moves a link rotating,
through the pin e, a spindle mounted in a frame, d. The spindle moves arm f. This pulls
on a very minute chain wound round the pointer spindle b, in opposition to a hairspring,
h s. b is mounted on arm h, which is quite independent of the rest of the aneroid.

Fig. 156. Fig. 157.
The vacuum chamber of an aneroid barometer extended and compressed.

The vacuum chamber is exhausted during manufacture and sealed. It would
naturally assume the shape of Fig. 157, but the spring s, acting against the atmospheric

pressure, pulls it out. As the pressure varies, so does the spring rise or sink; and the
slightest movement is transmitted through the multiplying arms c, e, f, to the pointer.

A good aneroid is so delicate that it will register the difference in pressure caused by
raising it from the floor to the table, where it has a couple of feet less of air-column
resting upon it. An aneroid is therefore a valuable help to mountaineers for determining
their altitude above sea-level.

BAROMETERS AND WEATHER.

We may now return to the consideration of forecasting the weather by movements of
the barometer. The first thing to keep in mind is, that the instrument is essentially a
weight recorder. How is weather connected with atmospheric weight?

In England the warm south-west wind generally brings wet weather, the north and
east winds fine weather; the reason for this being that the first reaches us after passing
over the Atlantic and picking up a quantity of moisture, while the second and third have
come overland and deposited their moisture before reaching us.

A sinking of the barometer heralds the approach of heated air—that is, moist air—
which on meeting colder air sheds its moisture. So when the mercury falls we expect
rain. On the other hand, when the "glass" rises, we know that colder air is coming, and
as colder air comes from a dry quarter we anticipate fine weather. It does not follow
that the same conditions are found in all parts of the world. In regions which have the
ocean to the east or the north, the winds blowing thence would be the rainy winds,
while south-westerly winds might bring hot and dry weather.

THE DIVING-BELL.

Water is nearly 773 times as heavy as air. If we submerge a barometer a very little
way below the surface of a water tank, we shall at once observe a rise of the mercury
column. At a depth of 34 feet the pressure on any submerged object is 15 lbs. to the
square inch, in addition to the atmospheric pressure of 15 lbs. per square inch—that is,
there would be a 30-lb. absolute pressure. As a rule, when speaking of hydraulic
pressures, we start with the normal atmospheric pressure as zero, and we will here
observe the practice.

Fig. 158.—A diving bell.
The diving-bell is used to enable people to work under water without having
recourse to the diving-dress. A sketch of an ordinary diving-bell is given in Fig. 158. It
may be described as a square iron box without a bottom. At the top are links by which it
is attached to a lowering chain, and windows, protected by grids; also a nozzle for the
air-tube.

Fig. 159.

A simple model bell (Fig. 159) is easily made out of a glass tumbler which has had a
tap fitted in a hole drilled through the bottom. We turn off the tap and plunge the glass
into a vessel of water. The water rises a certain way up the interior, until the air within
has been compressed to a pressure equal to that of the water at the level of the surface
inside. The further the tumbler is lowered, the higher does the water rise inside it.

Evidently men could not work in a diving-bell which is invaded thus by water. It is
imperative to keep the water at bay. This we can do by attaching a tube to the tap (Fig.
160) and blowing into the tumbler till the air-pressure exceeds that of the water, which
is shown by bubbles rising to the surface. The diving-bell therefore has attached to it a
hose through which air is forced by pumps from the atmosphere above, at a pressure
sufficient to keep the water out of the bell. This pumping of air also maintains a fresh
supply of oxygen for the workers.

Fig. 160.

Inside the bell is tackle for grappling any object that has to be moved, such as a
heavy stone block. The diving-bell is used mostly for laying submarine masonry. "The
bell, slung either from a crane on the masonry already built above sea-level, or from a
specially fitted barge, comes into action. The block is lowered by its own crane on to
the bottom. The bell descends upon it, and the crew seize it with tackle suspended
inside the bell. Instructions are sent up as to the direction in which the bell should be
moved with its burden, and as soon as the exact spot has been reached the signal for
lowering is given, and the stone settles on to the cement laid ready for it."[34]

For many purposes it is necessary that the worker should have more freedom of
action than is possible when he is cooped up inside an iron box. Hence the invention of
the

DIVING-DRESS,

which consists of two main parts, the helmet and the dress proper. The helmet (Fig.
161) is made of copper. A breastplate, b, shaped to fit the shoulders, has at the neck a
segmental screw bayonet-joint. The headpiece is fitted with a corresponding screw,
which can be attached or removed by one-eighth of a turn. The neck edge of the dress,
which is made in one piece, legs, arms, body and all, is attached to the breastplate by
means of the plate p1, screwed down tightly on it by the wing-nuts n n, the bolts of
which pass through the breastplate. Air enters the helmet through a valve situated at the
back, and is led through tubes along the inside to the front. This valve closes
automatically if any accident cuts off the air supply, and encloses sufficient air in the
dress to allow the diver to regain the surface. The outlet valve o v can be adjusted by
the diver to maintain any pressure. At the sides of the headpiece are two hooks, h, over
which pass the cords connecting the heavy lead weights of 40 lbs. each hanging on the
diver's breast and back. These weights are also attached to the knobs k k. A pair of
boots, having 17 lbs. of lead each in the soles, complete the dress. Three glazed
windows are placed in the headpiece, that in the front, r w, being removable, so that the
diver may gain free access to the air when he is above water without being obliged to
take off the helmet.

Fig. 161.—A diver's helmet.

By means of telephone wires built into the life-line (which passes under the diver's
arms and is used for lowering and hoisting) easy communication is established between
the diver and his attendants above. The transmitter of the telephone is placed inside the
helmet between the front and a side window, the receiver and the button of an electric
bell in the crown. This last he can press by raising his head. The life-line sometimes
also includes the wires for an electric lamp (Fig. 162) used by the diver at depths to
which daylight cannot penetrate.

The pressure on a diver's body increases in the ratio of 4⅓ lbs. per square inch for
every 10 feet that he descends. The ordinary working limit is about 150 feet, though
"old hands" are able to stand greater pressures. The record is held by one James Hooper,
who, when removing the cargo of the Cape Horn sunk off the South American coast,
made seven descents of 201 feet, one of which lasted for forty-two minutes.

Fig. 162.—Diver's
electric lamp.

A sketch is given (Fig. 163) of divers working below water with pneumatic tools,
fed from above with high-pressure air. Owing to his buoyancy a diver has little
depressing or pushing power, and he cannot bore a hole in a post with an auger unless
he is able to rest his back against some firm object, or is roped to the post. Pneumatic
chipping tools merely require holding to their work, their weight offering sufficient
resistance to the very rapid blows which they make.

Fig. 163.—Divers at work below water
with pneumatic tools.

AIR-PUMPS.

Fig. 164. Fig. 165.

Mention having been made of the air-pump, we append diagrams (Figs. 164, 165) of
the simplest form of air-pump, the cycle tyre inflator. The piston is composed of two
circular plates of smaller diameter than the barrel, holding between them a cup leather.
During the upstroke the cup collapses inwards and allows air to pass by it. On the
downstroke (Fig. 165) the edges of the cup expand against the barrel, preventing the
passage of air round the piston. A double-action air-pump requires a long, well-fitting
piston with a cup on each side of it, and the addition of extra valves to the barrel, as the
cups under these circumstances cannot act as valves.

PNEUMATIC TYRES.

Fig. 166. Fig. 167.

The action of the pneumatic tyre in reducing vibration and increasing the speed of a
vehicle is explained by Figs. 166, 167. When the tyre encounters an obstacle, such as a
large stone, it laps over it (Fig. 166), and while supporting the weight on the wheel,
reduces the deflection of the direction of movement. When an iron-tyred wheel meets a
similar obstacle it has to rise right over it, often jumping a considerable distance into
the air. The resultant motions of the wheel are indicated in each case by an arrow. Every
change of direction means a loss of forward velocity, the loss increasing with the
violence and extent of the change. The pneumatic tyre also scores because, on account
of its elasticity, it gives a "kick off" against the obstacle, which compensates for the
resistance during compression.

Fig. 168.—Section of the mechanism of an air-gun.

THE AIR-GUN.

This may be described as a valveless air-pump. Fig. 168 is a section of a "Gem" air-
gun, with the mechanism set ready for firing. In the stock of the gun is the cylinder, in
which an accurately fitting and hollow piston moves. A powerful helical spring, turned
out of a solid bar of steel, is compressed between the inside end of the piston and the
upper end of the butt. To set the gun, the catch is pressed down so that its hooked end
disengages from the stock, and the barrel is bent downwards on pivot p. This slides the
lower end of the compressing lever towards the butt, and a projection on the guide b,
working in a groove, takes the piston with it. When the spring has been fully
compressed, the triangular tip of the rocking cam r engages with a groove in the
piston's head, and prevents recoil when the barrel is returned to its original position. On
pulling the trigger, the piston is released and flies up the cylinder with great force, and
the air in the cylinder is compressed and driven through the bore of the barrel, blocked
by the leaden slug, to which the whole energy of the expanding spring is transmitted
through the elastic medium of the air.

There are several other good types of air-gun, all of which employ the principles
described above.

THE SELF-CLOSING DOOR-STOP

is another interesting pneumatic device. It consists of a cylinder with an air-tight piston,
and a piston rod working through a cover at one end. The other end of the cylinder is
pivoted to the door frame. When the door is opened the piston compresses a spring in
the cylinder, and air is admitted past a cup leather on the piston to the upper part of the
cylinder. This air is confined by the cup leather when the door is released, and escapes
slowly through a leak, allowing the spring to regain its shape slowly, and by the agency
of the piston rod to close the door.

THE ACTION OF WIND ON OBLIQUE SURFACES.

Why does a kite rise? Why does a boat sail across the wind? We can supply an
answer almost instinctively in both cases, "Because the wind pushes the kite or sail
aside." It will, however, be worth while to look for a more scientific answer. The kite
cannot travel in the direction of the wind because it is confined by a string. But the face
is so attached to the string that it inclines at an angle to the direction of the wind. Now,
when a force meets an inclined surface which it cannot carry along with it, but which is
free to travel in another direction, the force may be regarded as resolving itself into two
forces, coming from each side of the original line. These are called the component
forces.

Fig. 169.

To explain this we give a simple sketch of a kite in the act of flying (Fig. 169). The
wind is blowing in the direction of the solid arrow a. The oblique surface of the kite
resolves its force into the two components indicated by the dotted arrows b and c. Of
these c only has lifting power to overcome the force of gravity. The kite assumes a
position in which force c and gravity counterbalance one another.

Fig. 170.

A boat sailing across the wind is acted on in a similar manner (Fig. 170). The wind
strikes the sail obliquely, and would thrust it to leeward were it not for the opposition of
the water. The force a is resolved into forces b and c, of which c propels the boat on the
line of its axis. The boat can be made to sail even "up" the wind, her head being brought
round until a point is reached at which the force b on the boat, masts, etc., overcomes
the force c. The capability of a boat for sailing up wind depends on her "lines" and the
amount of surface she offers to the wind.

THE BALLOON

is a pear-shaped bag—usually made of silk—filled with some gas lighter than air. The
tendency of a heavier medium to displace a lighter drives the gas upwards, and with it
the bag and the wicker-work car attached to a network encasing the bag. The tapering
neck at the lower end is open, to permit the free escape of gas as the atmospheric
pressure outside diminishes with increasing elevation. At the top of the bag is a wooden
valve opening inwards, which can be drawn down by a rope passing up to it through the
neck whenever the aeronaut wishes to let gas escape for a descent. He is able to cause a
very rapid escape by pulling another cord depending from a "ripping piece" near the top
of the bag. In case of emergency this is torn away bodily, leaving a large hole. The
ballast (usually sand) carried enables him to maintain a state of equilibrium between the
upward pull of the gas and the downward pull of gravity. To sink he lets out gas, to rise
he throws out ballast; and this process can be repeated until the ballast is exhausted.
The greatest height ever attained by aeronauts is the 7¼ miles, or 37,000 feet, of
Messrs. Glaisher and Coxwell on September 5, 1862. The ascent nearly cost them their
lives, for at an elevation of about 30,000 feet they were partly paralyzed by the
rarefaction of the air, and had not Mr. Coxwell been able to pull the valve rope with his
teeth and cause a descent, both would have died from want of air.

Fig. 171.

The flying-machine, which scientific engineers have so long been trying to produce,
will probably be quite independent of balloons, and will depend for its ascensive
powers on the action of air on oblique surfaces. Sir Hiram Maxim's experimental air-
ship embodied the principles shown by Fig. 171. On a deck was mounted an engine, e,
extremely powerful for its weight. This drove large propellers, s s. Large aeroplanes, of
canvas stretched over light frameworks, were set up overhead, the forward end
somewhat higher than the rear. The machine was run on rails so arranged as to prevent
it rising. Unfortunately an accident happened at the first trial and destroyed the
machine.

In actual flight it would be necessary to have a vertical rudder for altering the
horizontal direction, and a horizontal "tail" for steering up or down. The principle of an
aeroplane is that of the kite, with this difference, that, instead of moving air striking a
captive body, a moving body is propelled against more or less stationary air. The
resolution of forces is shown by the arrows as before.

Up to the present time no practical flying-machine has appeared. But experimenters
are hard at work examining the conditions which must be fulfilled to enable man to
claim the "dominion of the air."
[34] The "Romance of Modern Mechanism," p. 243

Chapter XVIII.

HYDRAULIC MACHINERY.

The siphon—The bucket pump—The force-pump—The most marvellous pump—The blood
channels—The course of the blood—The hydraulic press—Household water-supply fittings—
The ball-cock—The water-meter—Water-supply systems—The household filter—Gas traps—
Water engines—The cream separator—The "hydro."

I N the last chapter we saw that the pressure of the atmosphere is 15 lbs. to the square
inch. Suppose that to a very long tube having a sectional area of one square inch we
fit an air-tight piston (Fig. 172), and place the lower end of the tube in a vessel of water.
On raising the piston a vacuum would be created in the tube, did not the pressure of the
atmosphere force water up into the tube behind the piston. The water would continue to
rise until it reached a point 34 feet perpendicularly above the level of the water in the
vessel. The column would then weigh 15 lbs., and exactly counterbalance the
atmospheric pressure; so that a further raising of the piston would not raise the water

any farther. At sea-level, therefore, the lifting power of a pump by suction is limited to
34 feet. On the top of a lofty mountain, where the air-pressure is less, the height of the
column would be diminished—in fact, be proportional to the pressure.

Fig. 172. Fig. 173.

THE SIPHON

is an interesting application of the principle of suction. By its own weight water may be
made to lift water through a height not exceeding 34 feet. This is explained by Fig. 173.
The siphon pipe, a b c d, is in the first instance filled by suction. The weight of the
water between a and b counter-balances that between b and c. But the column c d
hangs, as it were, to the heels of b c, and draws it down. Or, to put it otherwise, the
column b d, being heavier than the column b a, draws it over the topmost point of the
siphon. Any parting between the columns, provided that b a does not exceed 34 feet, is

impossible, as the pressure of the atmosphere on the mouth of b a is sufficient to
prevent the formation of a vacuum.

THE BUCKET PUMP.

We may now pass to the commonest form of pump used in houses, stables, gardens,
etc. (Fig. 174). The piston has a large hole through it, over the top of which a valve is
hinged. At the bottom of the barrel is a second valve, also opening upwards, seated on
the top of the supply pipe. In sketch (a) the first upstroke is in progress. A vacuum
forms under the piston, or plunger, and water rises up the barrel to fill it. The next
diagram (b) shows the first downstroke. The plunger valve now opens and allows water
to rise above the piston, while the lower closes under the pressure of the water above
and the pull of that below. During the second upstroke (c) the water above the piston is
raised until it overflows through the spout, while a fresh supply is being sucked in
below.

Fig. 174.

THE FORCE-PUMP.

Fig. 175. Force-pump; suction stroke. Fig. 176. Force-pump; delivery stroke.

For driving water to levels above that of the pump a somewhat different arrangement
is required. One type of force-pump is shown in Figs. 175, 176. The piston now is solid,
and the upper valve is situated in the delivery pipe. During an upstroke this closes, and
the other opens; the reverse happening during a downstroke. An air-chamber is
generally fitted to the delivery pipe when water is to be lifted to great heights or under
high pressure. At each delivery stroke the air in the chamber is compressed, absorbing
some of the shock given to the water in the pipe by the water coming from the pump;
and its expansion during the next suction stroke forces the water gradually up the pipe.
The air-chamber is a very prominent feature of the fire-engine.

A double-action force-pump is seen in Fig. 177, making an upward stroke. Both
sides of the piston are here utilized, and the piston rod works through a water-tight
stuffing-box. The action of the pump will be easily understood from the diagram.

Fig. 177.

THE MOST MARVELLOUS PUMP

known is the heart. We give in Fig. 178 a diagrammatic sketch of the system of blood
circulation in the human body, showing the heart, the arteries, and the veins, big and
little. The body is supposed to be facing the reader, so that the left lung, etc., is to his
right.

Fig. 178.—A diagrammatic representation of the
circulatory system of the blood.
The heart, which forces the blood through the body, is a large muscle (of about the
size of the clenched fist) with four cavities. These are respectively known as the right
and left auricles, and the right and left ventricles. They are arranged in two pairs, the
auricle uppermost, separated by a fleshy partition. Between each auricle and its
ventricle is a valve, which consists of strong membranous flaps, with loose edges
turned downwards. The left-side valve is the mitral valve, that between the right auricle

and ventricle the tricuspid valve. The edges of the valves fall together when the heart
contracts, and prevent the passage of blood. Each ventricle has a second valve through
which it ejects the blood. (That of the right ventricle has been shown double for the
sake of convenience.)

The action of the heart is this:—The auricles and ventricles expand; blood rushes
into the auricles from the channels supplying them, and distends them and the
ventricles; the auricles contract and fill the ventricles below quite full (there are no
valves above the auricles, but the force of contraction is not sufficient to return the
blood to the veins); the ventricles contract; the mitral and tricuspid valves close; the
valves leading to the arteries open; blood is forced out of the ventricles.

THE BLOOD CHANNELS

are of two kinds—(1) The arteries, which lead the blood into the circulatory system; (2)
the veins, which lead the blood back to the heart. The arteries divide up into branches,
and these again divide into smaller and smaller arteries. The smallest, termed
capillaries (Latin, capillus, a hair), are minute tubes having an average diameter of
1⁄3000th of an inch. These permeate every part of the body. The capillary arteries lead

into the smallest veins, which unite to form larger and larger veins, until what we may
call the main streams are reached. Through these the blood flows to the heart.

There are three main points of difference between arteries and veins. In the first
place, the larger arteries have thick elastic walls, and maintain their shape even when
empty. This elasticity performs the function of the air-chamber of the force-pump.
When the ventricles contract, driving blood into the arteries, the walls of the latter
expand, and their contraction pushes the blood steadily forward without shock. The
capillaries have very thin walls, so that fluids pass through them to and from the body,
feeding it and taking out waste matter. The veins are all thin-walled, and collapse when
empty. Secondly, most veins are furnished with valves, which prevent blood flowing the
wrong way. These are similar in principle to those of the heart. Arteries have no valves.
Thirdly, arteries are generally deeply set, while many of the veins run near the surface
of the body. Those on the front of the arm are specially visible. Place your thumb on
them and run it along towards the wrist, and you will notice that the veins distend
owing to the closing of the valves just mentioned.

Arterial blood is red, and comes out from a cut in gulps, on account of the
contraction of the elastic walls. If you cut a vein, blue blood issues in a steady stream.
The change of colour is caused by the loss of oxygen during the passage of the blood
through the capillaries, and the absorption of carbon dioxide from the tissues.

The lungs are two of the great purifiers of the blood. As it circulates through them, it
gives up the carbon dioxide which it has absorbed, and receives pure oxygen in
exchange. If the air of a room is "foul," the blood does not get the proper amount of
oxygen. For this reason it is advisable for us to keep the windows of our rooms open as
much as possible both day and night. Fatigue is caused by the accumulation of carbon
dioxide and other impurities in the blood. When we run, the heart pumps blood through
the lungs faster than they can purify it, and eventually our muscles become poisoned to
such an extent that we have to stop from sheer exhaustion.

THE COURSE OF THE BLOOD.

It takes rather less than a minute for a drop of blood to circulate from the heart
through the whole system and back to the heart.

We may briefly summarize the course of the circulation of the blood thus:—It is
expelled from the left ventricle into the aorta and the main arteries, whence it passes
into the smaller arteries, and thence into the capillaries of the brain, stomach, kidneys,
etc. It here imparts oxygen to the body, and takes in impurities. It then enters the veins,
and through them flows back to the right auricle; is driven into the right ventricle; is
expelled into the pulmonary (lung) arteries; enters the lungs, and is purified. It returns
to the left auricle through the pulmonary veins; enters the left auricle, passes to left
ventricle, and so on.

A healthy heart beats from 120 times per minute in a one-year-old infant to 60 per
minute in a very aged person. The normal rate for a middle-aged adult is from 80 to 70
beats.

Heart disease signifies the failure of the heart valves to close properly. Blood passes
back when the heart contracts, and the circulation is much enfeebled. By listening
through a stethoscope the doctor is able to tell whether the valves are in good order. A
hissing sound during the beat indicates a leakage past the valves; a thump, or "clack,"
that they shut completely.

THE HYDRAULIC PRESS.

It is a characteristic of fluids and gases that if pressure be brought to bear on any part
of a mass of either class of bodies it is transmitted equally and undiminished in all
directions, and acts with the same force on all equal surfaces, at right angles to those
surfaces. The great natural philosopher Pascal first formulated this remarkable fact, of
which a simple illustration is given in Fig. 179. Two cylinders, a and b, having a bore of
one and two inches respectively, are connected by a pipe. Water is poured in, and

pistons fitting the cylinders accurately and of equal weight are inserted. On piston b is
placed a load of 10 lbs. To prevent a rising above the level of b, it must be loaded
proportionately. The area of piston a is four times that of b, so that if we lay on it a 40-
lb. weight, neither piston will move. The walls of the cylinders and connecting pipe are
also pressed outwards in the ratio of 10 lbs. for every part of their interior surface which
has an area equal to that of piston b.

Fig. 179.
The hydraulic press is an application of this law. Cylinder b is represented by a force
pump of small bore, capable of delivering water at very high pressures (up to 10 tons
per square inch). In the place of a we have a stout cylinder with a solid plunger, p (Fig.
180), carrying the table on which the object to be pressed is placed. Bramah, the
inventor of the hydraulic press, experienced great difficulty in preventing the escape of
water between the top of the cylinder and the plunger. If a "gland" packing of the type
found in steam-cylinders were used, it failed to hold back the water unless it were
screwed down so tightly as to jam the plunger. He tried all kinds of expedients without
success; and his invention, excellent though it was in principle, seemed doomed to
failure, when his foreman, Henry Maudslay,[35] solved the problem in a simple but most
masterly manner. He had a recess turned in the neck of the cylinder at the point
formerly occupied by the stuffing-box, and into this a leather collar of U-section
(marked solid black in Fig. 180) was placed with its open side downwards. When water

reached it, it forced the edges apart, one
against the plunger, the other against the
walls of the recess, with a degree of
tightness proportionate to the pressure.
On water being released from the
cylinder the collar collapsed, allowing the
plunger to sink without friction.

The principle of the hydraulic press is
employed in lifts; in machines for
bending, drilling, and riveting steel
plates, or forcing wheels on or off their
axles; for advancing the "boring shield"
of a tunnel; and for other purposes too
numerous to mention.

HOUSEHOLD WATER-SUPPLY FITTINGS.
Fig. 180.—The cylinder and ram of a
Among these, the most used is the tap, hydraulic press.
or cock. When a house is served by the
town or district water supply, the fitting
of proper taps on all pipes connected with the supply is stipulated for by the water-
works authorities. The old-fashioned "plug" tap is unsuitable for controlling high-
pressure water on account of the suddenness with which it checks the flow. Lest the
reader should have doubts as to the nature of a plug tap, we may add that it has a
tapering cone of metal working in a tapering socket. On the cone being turned till a hole
through it is brought into line with the channel of the tap, water passes. A quarter turn
closes the tap.

Fig. 181.—A screw-down water cock.
Its place has been taken by the screw-down cock. A very common and effective
pattern is shown in Fig. 181. The valve v, with a facing of rubber, leather, or some other
sufficiently elastic substance, is attached to a pin, c, which projects upwards into the
spindle a of the tap. This spindle has a screw thread on it engaging with a collar, b.
When the spindle is turned it rises or falls, allowing the valve to leave its seating, v s, or
forcing it down on to it. A packing p in the neck of b prevents the passage of water
round the spindle. To open or close the tap completely is a matter of several turns,
which cannot be made fast enough to produce a "water-hammer" in the pipes by
suddenly arresting the flow. The reader will easily understand that if water flowing at
the rate of several miles an hour is abruptly checked, the shock to the pipes carrying it
must be very severe.

THE BALL-COCK

is used to feed a cistern automatically with water, and prevent the water rising too far in
the cistern (Fig. 182). Water enters the cistern through a valve, which is opened and
closed by a plug faced with rubber. The lower extremity of the plug is flattened, and has
a rectangular hole cut in it. Through this passes a lever, l, attached at one end to a
hollow copper sphere, and pivoted at the other on the valve casing. This casing is not
quite circular in section, for two slots are cast in the circumference to allow water to
pass round the plug freely when the valve is open. The buoyancy of the copper sphere is

sufficient to force the plug's face up towards its seating as the valve rises, and to cut off
the supply entirely when a certain level has been attained. If water is drawn off, the
sphere sinks, the valve opens, and the loss is made good.

Fig. 182.—An automatic ball-valve.

THE WATER-METER.

Fig. 183.
Some consumers pay a sum quarterly for the privilege of a water supply, and the
water company allows them to use as much as they require. Others, however, prefer to
pay a fixed amount for every thousand gallons used. In such cases, a water-meter is
required to record the consumption. We append a sectional diagram of Kennedy's patent
water-meter (Fig. 183), very widely used. At the bottom is the measuring cylinder,
fitted with a piston, (6), which is made to move perfectly water-tight and free from
friction by means of a cylindrical ring of india-rubber, rolling between the body of the
piston and the internal surface of the cylinder. The piston rod (25), after passing through

a stuffing-box in the cylinder cover, is attached to a rack, (15), which gears with a cog,
(13), fixed on a shaft. As the piston moves up and down, this cog is turned first in one
direction, then in the other. To this shaft is connected the index mechanism (to the
right). The cock-key (24) is so constructed that it can put either end of the measuring
cylinder in communication with the supply or delivery pipes, if given a quarter turn (see
Fig. 184). The weighted lever (14) moves loosely on the pinion shaft through part of a
circle. From the pinion project two arms, one on each side of the lever. When the lever
has been lifted by one of these past the vertical position, it falls by its own weight on to
a buffer-box rest, (18). In doing so, it strikes a projection on the duplex lever (19),
which is joined to the cock-key, and gives the latter a quarter turn.

In order to follow the working of the meter, we must keep an eye on Figs. 183 and
184 simultaneously. Water is entering from a, the supply pipe. It flows through the cock
downwards through channel d into the lower half of the cylinder. The piston rises,
driving out the water above it through c to the delivery pipe b. Just as the piston
completes its stroke the weight, raised by the rack and pinion, topples over, and strikes
the key-arm, which it sends down till stopped by the buffer-box. The tap is then at right
angles to the position shown in Fig. 184, and water is directed from a down c into the
top of the cylinder, forcing the piston down, while the water admitted below during the
last stroke is forced up the passage d, and out by the outlet b. Before the piston has
arrived at the bottom of the cylinder, the lifter will have lifted the weighted lever from
the buffer-box, and raised it to a vertical position; from there it will have fallen on the
right-hand key-arm, and have brought the cock-key to its former position, ready to
begin another upward stroke.

Fig. 184.
The index mechanism makes allowance for the fact that the bevel-wheel on the
pinion shaft has its direction reversed at the beginning of every stroke of the piston.
This bevel engages with two others mounted loosely on the little shaft, on which is
turned a screw thread to revolve the index counter wheels. Each of these latter bevels
actuates the shaft through a ratchet; but while one turns the shaft when rotating in a
clockwise direction only, the other engages it when making an anti-clockwise
revolution. The result is that the shaft is always turned in the same direction.

WATER-SUPPLY SYSTEMS.

The water for a town or a district supply is got either from wells or from a river. In
the former case it may be assumed to be free from impurities. In the latter, there is need
for removing all the objectionable and dangerous matter which river water always
contains in a greater or less degree. This purification is accomplished by first leading
the water into large settling tanks, where the suspended matter sinks to the bottom. The
water is then drawn off into filtration beds, made in the following manner. The bottom
is covered with a thick layer of concrete. On this are laid parallel rows of bricks, the
rows a small distance apart. Then come a layer of bricks or tiles placed close together; a

layer of coarse gravel; a layer of finer gravel; and a thick layer of sand at the top. The
sand arrests any solid matter in the water as it percolates to the gravel and drains below.
Even the microbes,[36] of microscopic size, are arrested as soon as the film of mud has
formed on the top of the sand. Until this film is formed the filter is not in its most
efficient condition. Every now and then the bed is drained, the surface mud and sand
carefully drained off, and fresh sand put in their place. A good filter bed should not pass
more than from two to three gallons per hour for every square foot of surface, and it
must therefore have a large area.

It is sometimes necessary to send the water through a succession of beds, arranged in
terraces, before it is sufficiently pure for drinking purposes.

THE HOUSEHOLD FILTER.

When there is any doubt as to the wholesomeness of the water supply, a small filter
is often used. The microbe-stopper is usually either charcoal, sand, asbestos, or baked
clay of some kind. In Fig. 185 we give a section of a Maignen filter. r is the reservoir
for the filtered water; a the filter case proper; d a conical perforated frame; b a jacket of
asbestos cloth secured top and bottom by asbestos cords to d; c powdered carbon,
between which and the asbestos is a layer of special chemical filtering medium. A
perforated cap, e, covers in the carbon and prevents it being disturbed when water is
poured in. The carbon arrests the coarser forms of matter; the asbestos the finer. The
asbestos jacket is easily removed and cleansed by heating over a fire.

Fig. 185.
The most useful form of household filter is one which can be attached to a tap
connected with the main. Such a filter is usually made of porcelain or biscuit china. The
Berkefeld filter has an outer case of iron, and an interior hollow "candle" of porcelain
from which a tube passes through the lid of the filter to a storage tank for the filtered
water. The water from the main enters the outer case, and percolates through the
porcelain walls to the internal cavity and thence flows away through the delivery pipe.

Whatever be the type of filter used it must be cleansed at proper intervals. A foul
filter is very dangerous to those who drink the water from it. It has been proved by tests
that, so far from purifying the water, an inefficient and contaminated filter passes out

water much more highly charged with microbes than it was before it entered. We must
not therefore think that, because water has been filtered, it is necessarily safe. The
reverse is only too often the case.

GAS TRAPS.

Dangerous microbes can be breathed as well as drunk into the human system. Every
communication between house and drains should be most carefully "trapped." The
principle of a gas trap between, say, a kitchen sink and the drain to carry off the water is
given in Fig. 186. Enough water always remains in the bend to rise above the level of
the elbow, effectually keeping back any gas that there may be in the pipe beyond the
bend.

Fig. 186.—A trap for foul air.

WATER-ENGINES.

Before the invention of the steam-engine human industries were largely dependent
on the motive power of the wind and running water. But when the infant nursed by Watt
and Stephenson had grown into a giant, both of these natural agents were deposed from
the important position they once held. Windmills in a state of decay crown many of our
hilltops, and the water-wheel which formerly brought wealth to the miller now rots in
its mountings at the end of the dam. Except for pumping and moving boats and ships,
wind-power finds its occupation gone. It is too uncertain in quantity and quality to find
a place in modern economics. Water-power, on the other hand, has received a fresh

lease of life through the invention of machinery so scientifically designed as to use
much more of the water's energy than was possible with the old-fashioned wheel.

Fig. 187.—A Pelton wheel which develops 5,000 horse-
power. Observe the shape of the double buckets.
The turbine, of which we have already spoken in our third chapter, is now the
favourite hydraulic engine. Some water-turbines work on much the same principle as
the Parsons steam-turbine; others resemble the De Laval. Among the latter the Pelton
wheel takes the first place. By the courtesy of the manufacturers we are able to give
some interesting details and illustrations of this device.

Fig. 188.—Pelton wheel mounted, with nozzle in position.
The wheel, which may be of any diameter from six inches to ten feet, has buckets set
at regular intervals round the circumference, sticking outwards. Each bucket, as will be
gathered from our illustration of an enormous 5,000 h.p. wheel (Fig. 187), is composed
of two cups. A nozzle is so arranged as to direct water on the buckets just as they reach
the lowest point of a revolution (see Fig. 188). The water strikes the bucket on the
partition between the two cups, which turns it right and left round the inside of the cups.
The change of direction transfers the energy of the water to the wheel.

Fig. 189.—Speed regulator for Pelton wheel.

The speed of the wheel may be automatically regulated by a deflecting nozzle (Fig.
189), which has a ball and socket joint to permit of its being raised or lowered by a
centrifugal governor, thus throwing the stream on or off the buckets. The power of the
wheel is consequently increased or diminished to meet the change of load, and a
constant speed is maintained. When it is necessary to waste as little water as possible, a
concentric tapered needle may be fitted inside the nozzle. When the nozzle is in its
highest position the needle tip is withdrawn; as the nozzle sinks the needle protrudes,
gradually decreasing the discharge area of the nozzle.

Pelton wheels are designed to run at all speeds and to use water of any pressure. At
Manitou, Colorado, is an installation of three wheels operated by water which leaves
the nozzle at the enormous pressure of 935 lbs. per square inch. It is interesting to note
that jets of very high-pressure water offer astonishing resistance to any attempt to
deflect their course. A three-inch jet of 500-lb. water cannot be cut through by a blow
from a crowbar.

In order to get sufficient pressure for working hydraulic machinery in mines,
factories, etc., water is often led for many miles in flumes, or artificial channels, along
the sides of valleys from the source of supply to the point at which it is to be used. By
the time that point is reached the difference between the gradients of the flume and of
the valley bottom has produced a difference in height of some hundreds of feet.

Fig. 190.—The Laxey water-wheel, Isle of
Man. In the top right-hand corner is a Pelton
wheel of proportionate size required to do the
same amount of work with the same
consumption of water at the same pressure.
The full-page illustration on p. 380 affords a striking testimony to the wonderful
progress made in engineering practice during the last fifty years. The huge water-wheel
which forms the bulk of the picture is that at Laxey, in the Isle of Man. It is 72½ feet in
diameter, and is supposed to develop 150 horse-power, which is transmitted several
hundreds of feet by means of wooden rods supported at regular intervals. The power
thus transmitted operates a system of pumps in a lead mine, raising 250 gallons of water
per minute, to an elevation of 1,200 feet. The driving water is brought some distance to
the wheel in an underground conduit, and is carried up the masonry tower by pressure,
flowing over the top into the buckets on the circumference of the wheel.

The little cut in the upper corner represents a Pelton wheel drawn on the same scale,
which, given an equal supply of water at the same pressure, would develop the same
power as the Laxey monster. By the side of the giant the other appears a mere toy.

THE CREAM SEPARATOR.

In 1864 Denmark went to war with Germany, and emerged from the short struggle
shorn of the provinces of Lauenburg, Holstein, and Schleswig. The loss of the two last,
the fairest and most fertile districts of the kingdom, was indeed grievous. The Danish
king now ruled only over a land consisting largely of moor, marsh, and dunes,
apparently worthless for any purpose. But the Danes, with admirable courage, entered
upon a second struggle, this time with nature. They made roads and railways, dug
irrigation ditches, and planted forest trees; and so gradually turned large tracts of what
had been useless country into valuable possessions. Agriculture being much depressed,
owing to the low price of corn, they next gave their attention to the improvement of
dairy farming. Labour-saving machinery of all kinds was introduced, none more
important than the device for separating the fatty from the watery constituents of milk.
It would not be too much to say that the separator is largely responsible for the present
prosperity of Denmark.

Fig. 191.—Section of a Cream Separator.
How does it work? asks the reader. Centrifugal force[37] is the governing principle.
To explain its application we append a sectional illustration (Fig. 191) of Messrs.
Burmeister and Wain's hand-power separator, which may be taken as generally
representative of this class of machines. Inside a circular casing is a cylindrical bowl, d,

mounted on a shaft which can be revolved 5,000 times a minute by means of the cog-
wheels and the screw thread chased on it near the bottom extremity. Milk flows from
the reservoir r (supported on a stout arm) through tap a into a little distributer on the
top of the separator, and from it drops into the central tube c of the bowl. Falling to the
bottom, it is flung outwards by centrifugal force, finds an escape upwards through the
holes a a, and climbs up the perforated grid e, the surface of which is a series of
pyramidical excrescences, and finally reaches the inner surface of the drum proper. The
velocity of rotation is so tremendous that the heavier portions of the milk—that is, the
watery—crowd towards the point furthest from the centre, and keep the lighter fatty
elements away from contact with the sides of the drum. In the diagram the water is
represented by small circles, the cream by small crosses.

As more milk enters the drum it forces upwards what is already there. The cap of the
drum has an inner jacket, f, which at the bottom all but touches the side of the drum.
The distance between them is the merest slit; but the cream is deflected up outside f
into space e, and escapes through a hole one-sixteenth of an inch in diameter
perforating the plate g. The cream is flung into space k and trickles out of spout b, while
the water flies into space h and trickles away through spout a.

THE "HYDRO.,"

used in laundries for wringing clothes by centrifugal force, has a solid outer casing and
an inner perforated cylindrical cage, revolved at high speed by a vertical shaft. The wet
clothes are placed in the cage, and the machine is started. The water escapes through the
perforations and runs down the side of the casing to a drain. After a few minutes the
clothes are dry enough for ironing. So great is the centrifugal force that they are
consolidated against the sides of the cage, and care is needed in their removal.
[35] Inventor of the lathe slide-rest.

[36] Living germs; some varieties the cause of disease.

[37] That is, centre-fleeing force. Water dropped on a spinning top rushes towards the
circumference and is shot off at right angles to a line drawn from the point of parting to the
centre of the top.

Chapter XIX.

HEATING AND LIGHTING.

The hot-water supply—The tank system—The cylinder system—How a lamp works—Gas and
gasworks—Automatic stoking—A gas governor—The gas meter—Incandescent gas lighting.

HOT-WATER SUPPLY.

A WELL-EQUIPPED house is nowadays expected to contain efficient apparatus for
supplying plenty of hot water at all hours of the day. There is little romance about
the kitchen boiler and the pipes which the plumber and his satellites have sometimes to
inspect and put right, but the methods of securing a proper circulation of hot water
through the house are sufficiently important and interesting to be noticed in these pages.

In houses of moderate size the kitchen range does the heating. The two systems of
storing and distributing the heated water most commonly used are—(1) The tank
system; (2) the cylinder system.

THE TANK SYSTEM

is shown diagrammatically in Fig. 192. The boiler is situated at the back of the range,
and when a "damper" is drawn the fire and hot gases pass under it to a flue leading to
the chimney. The almost boiling water rises to the top of the boiler and thence finds its
way up the flow pipe into the hot-water tank a, displacing the somewhat colder water
there, which descends through the return pipe to the bottom of the boiler.

Water is drawn off from the flow pipe. This pipe projects some distance through the
bottom of a, so that the hottest portion of the contents may be drawn off first. A tank
situated in the roof, and fed from the main by a ball-cock valve, communicates with a
through the siphon pipe s. The bend in this pipe prevents the ascent of hot water, which
cannot sink through water colder than itself. From the top of a an expansion pipe is led
up and turned over the cold-water tank to discharge any steam which may be generated
in the boiler.

A hot-water radiator for warming the house may be connected to the flow and return
pipes as shown. Since it opens a "short circuit" for the circulation, the water in the tank
above will not be so well heated while it is in action. If cocks are fitted to the radiator
pipes, the amount of heat thus deflected can be governed.

Fig. 192.—The "tank" system of hot-water supply.
A disadvantage of the tank system is that the tank, if placed high enough to supply
all flows, is sometimes so far from the boiler that the water loses much of its heat in the
course of circulation. Also, if for any reason the cold water fails, tank a may be entirely
emptied, circulation cease, and the water in the boiler and pipes boil away rapidly.

THE CYLINDER SYSTEM

(Fig. 193) is open to neither of these objections. Instead of a rectangular tank up aloft,
we now have a large copper cylinder situated in the kitchen near the range. The flow
and return pipes are continuous, and the cold supply enters the bottom of the cylinder
through a pipe with a siphon bend in it. As before, water is drawn off from the flow
pipe, and a radiator may be put in the circuit. Since there is no draw-off point below the
top of the cylinder, even if the cold supply fails the cylinder will remain full, and the
failure will be discovered long before there is any danger of the water in it boiling
away.

Fig. 193.—The "cylinder" system of hot-water supply.
Boiler explosions are due to obstructions in the pipes. If the expansion pipe and the
cold-water supply pipe freeze, there is danger of a slight accumulation of steam; and if
one of the circulation pipes is also blocked, steam must generate until "something has to
go,"[38] which is naturally the boiler. Assuming that the pipes are quite full to the points
of obstruction, the fracture would result from the expansion of the water. Steam cannot
generate unless there be a space above the water. But the expanding water has stored up

the heat which would have raised steam, and the moment expansion begins after
fracture this energy is suddenly let loose. Steam forms instantaneously, augmenting the
effects of the explosion. From this it will be gathered that all pipes should be properly
protected against frost; especially near the roof.

Another cause of disaster is the furring up of the pipes with the lime deposited by
hard water when heated. When hard water is used, the pipes will sooner or later be
blocked near the boiler; and as the deposit is too hard to be scraped away, periodical
renewals are unavoidable.

HOW A LAMP WORKS.

From heating we turn to lighting, and first to the ordinary paraffin lamp. The two
chief things to notice about this are the wick and the chimney. The wick, being made of
closely-woven cotton, draws up the oil by what is known as capillary attraction. If you
dip the ends of two glass tubes, one half an inch, the other one-eighth of an inch in
diameter, into a vessel of water, you will notice that the water rises higher in the smaller
tube. Or get two clean glass plates and lay them face to face, touching at one end, but
kept slightly apart at the other by some small object. If they are partly submerged
perpendicularly, the water will rise between the plates—furthest on the side at which
the two plates touch, and less and less as the other edge is approached. The tendency of
liquids to rise through porous bodies is a phenomenon for which we cannot account.

Mineral oil contains a large proportion of carbon and hydrogen; it is therefore
termed hydro-carbon. When oil reaches the top of a lighted wick, the liquid is heated
until it turns into gas. The carbon and hydrogen unite with the oxygen of the air. Some
particles of the carbon apparently do not combine at once, and as they pass through the
fiery zone of the flame are heated to such a temperature as to become highly luminous.
It is to produce these light-rays that we use a lamp, and to burn our oil efficiently we
must supply the flame with plenty of oxygen, with more than it could naturally obtain.
So we surround it with a transparent chimney of special glass. The air inside the
chimney is heated, and rises; fresh air rushes in at the bottom, and is also heated and
replaced. As the air passes through, the flame seizes on the oxygen. If the wick is turned
up until the flame becomes smoky and flares, the point has been passed at which the
induced chimney draught can supply sufficient oxygen to combine with the carbon of
the vapour, and the "free" carbon escapes as smoke.

The blower-plate used to draw up a fire (Fig. 194) performs exactly the same
function as the lamp chimney, but on a larger scale. The plate prevents air passing

straight up the chimney over the coals, and compels it to find a way through the fire
itself to replace the heated air rising up the chimney.

Fig. 194.—Showing how a blower-
plate draws up the fire.

GAS AND GASWORKS.

A lamp is an apparatus for converting hydro-carbon mineral oil into gas and burning
it efficiently. The gas-jet burns gases produced by driving off hydro-carbon vapours
from coal in apparatus specially designed for the purpose. Gas-making is now, in spite
of the competition of electric lighting, so important an industry that we shall do well to
glance at the processes which it includes. Coal gas may be produced on a very small
scale as follows:—Fill a tin canister (the joints of which have been made by folding the
metal, not by soldering) with coal, clap on the lid, and place it, lid downwards, in a
bright fire, after punching a hole in the bottom. Vapour soon begins to issue from the
hole. This is probably at first only steam, due to the coal being more or less damp. But
if a lighted match be presently applied the vapour takes fire, showing that coal gas

proper is coming off. The flame lasts for a long time. When it dies the canister may be
removed and the contents examined. Most of the carbon remains in the form of coke. It
is bulk for bulk much lighter than coal, for the hydrogen, oxygen, and other gases, and
some of the carbon have been driven off by the heat. The coke itself burns if placed in a
fire, but without any smoke, such as issues from coal.

Fig. 195.—Sketch of the apparatus used in the manufacture of coal
gas.
Our home-made gas yields a smoky and unsatisfactory flame, owing to the presence
of certain impurities—ammonia, tar, sulphuretted hydrogen, and carbon bisulphide. A
gas factory must be equipped with means of getting rid of these objectionable
constituents. Turning to Fig. 195, which displays very diagrammatically the main
features of a gas plant, we observe at the extreme right the retorts, which correspond to
our canister. These are usually long fire-brick tubes of d-section, the flat side at the
bottom. Under each is a furnace, the flames of which play on the bottom, sides, and
inner end of the retort. The outer end projecting beyond the brickwork seating has an
iron air-tight door for filling the retort through, immediately behind which rises an iron
exit pipe, a, for the gases. Tar, which vaporizes at high temperatures, but liquefies at
ordinary atmospheric heat, must first be got rid of. This is effected by passing the gas
through the hydraulic main, a tubular vessel half full of water running the whole length
of the retorts. The end of pipe a dips below the surface of the water, which condenses
most of the tar and steam. The partly-purified gas now passes through pipe b to the
condensers, a series of inverted U-pipes standing on an iron chest with vertical cross
divisions between the mouths of each U. These divisions dip into water, so that the gas

has to pass up one leg of a U, down the other, up the first leg of the second pipe, and so
on, till all traces of the tar and other liquid constituents have condensed on the inside of
the pipe, from which they drop into the tank below.

The next stage is the passage of the scrubber, filled with coke over which water
perpetually flows. The ammonia gas is here absorbed. There still remain the
sulphuretted hydrogen and the carbon bisulphide, both of which are extremely offensive
to the nostrils. Slaked lime, laid on trays in an air-tight compartment called the lime
purifier, absorbs most of the sulphurous elements of these; and the coal gas is then fit
for use. On leaving the purifiers it flows into the gasometer, or gasholder, the huge
cake-like form of which is a very familiar object in the environs of towns. The
gasometer is a cylindrical box with a domed top, but no bottom, built of riveted steel
plates. It stands in a circular tank of water, so that it may rise and fall without any
escape of gas. The levity of the gas, in conjunction with weights attached to the ends of
chains working over pulleys on the framework surrounding the holder, suffices to raise
the holder.

Fig. 196.—The largest gasholder in the world:
South Metropolitan Gas Co., Greenwich Gas
Works. Capacity, 12,158,600 cubic feet.

Some gasometers have an enormous capacity. The record is at present held by that
built for the South Metropolitan Gas Co., London, by Messrs. Clayton & Son of Leeds.
This monster (of which we append an illustration, Fig. 196) is 300 feet in diameter and

180 feet high. When fully extended it holds 12,158,600 cubic feet of gas. Owing to its
immense size, it is built on the telescopic principle in six "lifts," of 30 feet deep each.
The sides of each lift, or ring, except the topmost, have a section shaped somewhat like
the letter N. Two of the members form a deep, narrow cup to hold water, in which the
"dip" member of the ring above it rises and falls.

Fig. 197.—Drawing retorts. (Photo by F. Marsh.)

AUTOMATIC STOKING.

The labour of feeding the retorts with coal and removing the coke is exceedingly
severe. In the illustration on p. 400 (made from a very fine photograph taken by Mr. F.
Marsh of Clifton) we see a man engaged in "drawing" the retorts through the iron doors
at their outer ends. Automatic machinery is now used in large gasworks for both
operations. One of the most ingenious stokers is the De Brouwer, shown at work in Fig.
198. The machine is suspended from an overhead trolley running on rails along the face
of the retorts. Coal falls into a funnel at the top of the telescopic pipe p from hoppers in
the story above, which have openings, h h, controlled by shutters. The coal as it falls is
caught by a rubber belt working round part of the circumference of the large wheel w
and a number of pulleys, and is shot into the mouth of the retort. The operator is seen
pulling the handle which opens the shutter of the hopper above the feed-tube, and
switching on the 4 h.p. electric motor which drives the belt and moves the machine
about. One of these feeders will charge a retort 20 feet long in twenty-two seconds.

Fig. 198.—De Brouwer automatic retort
charger.

A GAS GOVERNOR.

Some readers may have noticed that late at night a gas-jet, which a few hours before
burned with a somewhat feeble flame when the tap was turned fully on, now becomes
more and more vigorous, and finally may flare up with a hissing sound. This is because
many of the burners fed by the main supplying the house have been turned off, and
consequently there is a greater amount of gas available for the jets still burning, which
therefore feel an increased pressure. As a matter of fact, the pressure of gas in the main
is constantly varying, owing partly to the irregularity of the delivery from the
gasometer, and partly to the fact that the number of burners in action is not the same for
many minutes together. It must also be remembered that houses near the gasometer end
of the main will receive their gas at a higher pressure than those at the other end. The
gas stored in the holders may be wanted for use in the street lamps a few yards away, or
for other lamps several miles distant. It is therefore evident that if there be just enough
pressure to give a good supply to the nearest lamp, there will be too little a short
distance beyond it, and none at all at the extreme point; so that it is necessary to put on
enough pressure to overcome the friction on all these miles of pipe, and give just
enough gas at the extreme end. It follows that at all intermediate points the pressure is
excessive. Gas of the average quality is burned to the greatest advantage, as regards its

light-giving properties, when its pressure is equal to that of a column of water half an
inch high, or about 1⁄50 lb. to the square inch. With less it gives a smoky, flickering
light, and with more the combustion is also imperfect.

Every house supply should
therefore be fitted with a gas
governor, to keep the pressure
constant. A governor frequently
used, the Stott, is shown in section
in Fig. 199. Gas enters from the
main on the right, and passes into a
circular elbow, d, which has top
and bottom apertures closed by the
valves v v. Attached to the valve
shaft is a large inverted cup of
metal, the tip of which is immersed
in mercury. The pressure at which
the governor is to act is determined
by the weights w, with which the
Fig. 199. valve spindle is loaded at the top.
As soon as this pressure is
exceeded, the gas in c c lifts the metal cup, and v v are pressed against their seats, so
cutting off the supply. Gas cannot escape from c c, as it has not sufficient pressure to
force its way through the mercury under the lip of the cup. Immediately the pressure in
c c falls, owing to some of the gas being used up, the valves open and admit more gas.
When the fluctuations of pressure are slight, the valves never close completely, but
merely throttle the supply until the pressure beyond them falls to its proper level—that
is, they pass just as much gas as the burners in use can consume at the pressure
arranged for.

Governors of much larger size, but working on much the same principle, are fitted to
the mains at the point where they leave the gasometers. They are not, however,
sensitive to local fluctuations in the pipes, hence the necessity for separate governors in
the house between the meter and the burners.

THE GAS-METER

commonly used in houses acts on the principle shown in Fig. 200. The air-tight casing
is divided by horizontal and vertical divisions into three gas-chambers, b, c, and d. Gas

enters at a, and passes to the valve chamber b. The slide-valves of this allow it to pass
into c and d, and also into the two circular leather bellows e, f, which are attached to the
central division g, but are quite independent of one another.

We will suppose that in the illustration the
valves are admitting gas to chamber c and
bellows f. The pressure in c presses the circular
head of e towards the division g, expelling the
contents of the bellows through an outlet pipe
(not shown) to the burners in operation within
the house. Simultaneously the inflation of f
forces the gas in chamber d also through the
outlet. The head-plates of the bellows are
attached to rods and levers (not shown)
working the slide-valves in b. As soon as e is
fully in, and f fully expanded, the valves begin
to open and put the inlet pipe in communication
with d and e, and allow the contents of f and c
to escape to the outlet. The movements of the
valve mechanism operate a train of counting
wheels, visible through a glass window in the
side of the case. As the bellows have a definite
capacity, every stroke that they give means that Fig. 200.—Sketch of the bellows
a certain volume of gas has been ejected either and chambers of a "dry" gas
from them or from the chambers in which they meter.
move: this is registered by the counter. The
apparatus practically has two double-action cylinders (of which the bellows ends are the
pistons) working on the same principle as the steam-cylinder (Fig. 21). The valves have
three ports—the central, or exhaust, leading to the outlet, the outer ones from the inlet.
The bellows are fed through channels in the division g.

INCANDESCENT GAS LIGHTING.

The introduction of the electric arc lamp and the incandescent glow-lamp seemed at
one time to spell the doom of gas as an illuminating agent. But the appearance in 1886
of the Welsbach incandescent mantle for gas-burners opened a prosperous era in the
history of gas lighting.

The luminosity of a gas flame depends on the number of carbon particles liberated
within it, and the temperature to which these particles can be heated as they pass
through the intensely hot outside zone of the flame. By enriching the gas in carbon
more light is yielded, up to a certain point, with a flame of a given temperature. To
increase the heat of the flame various devices were tried before the introduction of the
incandescent mantle, but they were found to be too short-lived to have any commercial
value. Inventors therefore sought for methods by which the emission of light could be
obtained from coal gas independently of the incandescence of the carbon particles in
the flame itself; and step by step it was discovered that gas could be better employed
merely as a heating agent, to raise to incandescence substances having a higher
emissivity of light than carbon.

Dr. Auer von Welsbach found that the substances most suitable for incandescent
mantles were the oxides of certain rare metals, thorium, and cerium. The mantle is
made by dipping a cylinder of cotton net into a solution of nitrate of thorium and
cerium, containing 99 per cent. of the former and 1 per cent. of the latter metal. When
the fibres are sufficiently soaked, the mantle is withdrawn, squeezed, and placed on a
mould to dry. It is next held over a Bunsen gas flame and the cotton is burned away,
while the nitrates are converted into oxides. The mantle is now ready for use, but very
brittle. So it has to undergo a further dipping, in a solution of gun-cotton and alcohol, to
render it tough enough for packing. When it is required for use, it is suspended over the
burner by an asbestos thread woven across the top, a light is applied to the bottom, and
the collodion burned off, leaving nothing but the heat-resisting oxides.

The burner used with a mantle is constructed on the Bunsen principle. The gas is
mixed, as it emerges from the jet, with sufficient air to render its combustion perfect.
All the carbon is burned, and the flame, though almost invisible, is intensely hot. The
mantle oxides convert the heat energy of the flame into light energy. This is proved not
only by the intense whiteness of the mantle, but by the fact that the heat issuing from
the chimney of the burner is not nearly so great when the mantle is in position as when
it is absent.

The incandescent mantle is more extensively used every year. In Germany 90 per
cent. of gas lighting is on the incandescent system, and in England about 40 per cent.
We may notice, as an interesting example of the fluctuating fortunes of invention, that
the once doomed gas-burner has, thanks to Welsbach's mantle, in many instances
replaced the incandescent electric lamps that were to doom it.
[38] If, of course, there is no safety-valve in proper working order included in the
installation.

Chapter XX.

VARIOUS MECHANISMS.

Clocks and Watches:—A short history of timepieces—The construction of timepieces—The
driving power—The escapement—Compensating pendulums—The spring balance—The
cylinder escapement—The lever escapement—Compensated balance-wheels—Keyless
winding mechanism for watches—The hour hand train. Locks:—The Chubb lock—The Yale
lock. The Cycle:—The gearing of a cycle—The free wheel—The change-speed gear.
Agricultural Machines:—The threshing-machine—Mowing-machines. Some Natural
Phenomena:—Why sun-heat varies in intensity—The tides—Why high tide varies daily.

CLOCKS AND WATCHES.
A SHORT HISTORY OF TIMEPIECES.

T HE oldest device for measuring time is the sun-dial. That of Ahaz mentioned in the
Second Book of Kings is the earliest dial of which we have record. The obelisks of
the Egyptians and the curious stone pillars of the Druidic age also probably served as
shadow-casters.

The clepsydra, or water-clock, also of great antiquity, was the first contrivance for
gauging the passage of the hours independently of the motion of the earth. In its
simplest form it was a measure into which water fell drop by drop, hour levels being
marked on the inside. Subsequently a very simple mechanism was added to drive a
pointer—a float carrying a vertical rack, engaging with a cog on the pointer spindle; or
a string from the float passed over a pulley attached to the pointer and rotated it as the
float rose, after the manner of the wheel barometer (Fig. 153). In 807 a.d. Charlemagne
received from the King of Persia a water-clock which struck the hours. It is thus
described in Gifford's "History of France":—"The dial was composed of twelve small
doors, which represented the division of the hours. Each door opened at the hour it was
intended to represent, and out of it came a small number of little balls, which fell one
by one, at equal distances of time, on a brass drum. It might be told by the eye what
hour it was by the number of doors that were open, and by the ear by the number of
balls that fell. When it was twelve o'clock twelve horsemen in miniature issued forth at
the same time and shut all the doors."

Sand-glasses were introduced about 330 a.d. Except for special purposes, such as
timing sermons and boiling eggs, they have not been of any practical value.

The clepsydra naturally suggested to the mechanical mind the idea of driving a
mechanism for registering time by the force of gravity acting on some body other than
water. The invention of the weight-driven clock is attributed, like a good many other
things, to Archimedes, the famous Sicilian mathematician of the third century b.c.; but
no record exists of any actual clock composed of wheels operated by a weight prior to
1120 a.d. So we may take that year as opening the era of the clock as we know it.

About 1500 Peter Hele of Nuremberg invented the mainspring as a substitute for the
weight, and the watch appeared soon afterwards (1525 a.d.). The pendulum was first
adopted for controlling the motion of the wheels by Christian Huygens, a distinguished
Dutch mechanician, in 1659.

To Thomas Tompion, "the father of English watchmaking," is ascribed the honour of
first fitting a hairspring to the escapement of a watch, in or about the year 1660. He
also introduced the cylinder escapement now so commonly used in cheap watches.
Though many improvements have been made since his time, Tompion manufactured
clocks and watches which were excellent timekeepers, and as a reward for the benefits
conferred on his fellows during his lifetime, he was, after death, granted the exceptional
honour of a resting-place in Westminster Abbey.

THE CONSTRUCTION OF TIMEPIECES.

A clock or watch contains three main elements:—(1) The source of power, which
may be a weight or a spring; (2) the train of wheels operated by the driving force; (3)
the agent for controlling the movements of the train—this in large clocks is usually a
pendulum, in small clocks and watches a hairspring balance. To these may be added, in
the case of clocks, the apparatus for striking the hour.

THE DRIVING POWER.

Weights are used only in large clocks, such as one finds in halls, towers, and
observatories. The great advantage of employing weights is that a constant driving
power is exerted. Springs occupy much less room than weights, and are indispensable
for portable timepieces. The employment of them caused trouble to early experimenters
on account of the decrease in power which necessarily accompanies the uncoiling of a
wound-up spring. Jacob Zech of Prague overcame the difficulty in 1525 by the
invention of the fusee, a kind of conical pulley interposed between the barrel, or circular
drum containing the mainspring, and the train of wheels which the spring has to drive.
The principle of the "drum and fusee" action will be understood from Fig. 201. The
mainspring is a long steel ribbon fixed at one end to an arbor (the watchmaker's name

for a spindle or axle), round which it is tightly wound. The arbor and spring are inserted
in the barrel. The arbor is prevented from turning by a ratchet, b, and click, and
therefore the spring in its effort to uncoil causes the barrel to rotate.

A string of catgut (or a very fine chain) is
connected at one end to the circumference of
the drum, and wound round it, the other end
being fixed to the larger end of the fusee, which
is attached to the driving-wheel of the watch or
clock by the intervention of a ratchet and click
(not shown). To wind the spring the fusee is
turned backward by means of a key applied to
the square end a of the fusee arbor, and this
draws the string from off the drum on to the
fusee. The force of the spring causes the fusee
to rotate by pulling the string off it, coil by coil,
and so drives the train of wheels. But while the
mainspring, when fully wound, turns the fusee
by uncoiling the string from the smallest part of
the fusee, it gets the advantage of the larger
radius as its energy becomes lessened.

The fusee is still used for marine
Fig. 201. chronometers, for some clocks that have a
mainspring and pendulum, and occasionally for
watches. In the latter it has been rendered unnecessary by the introduction of the going-
barrel by Swiss watchmakers, who formed teeth on the edge of the mainspring barrel to
drive the train of wheels. This kind of drum is called "going" because it drives the
watch during the operation of winding, which is performed by rotating the drum arbor
to which the inner end of the spring is attached. A ratchet prevents the arbor from being
turned backwards by the spring. The adoption of the going-barrel has been made
satisfactory by the improvements in the various escapement actions.

THE ESCAPEMENT.

The spring or weight transmits its power through a train of cogs to the escapement,
or device for regulating the rate at which the wheels are to revolve. In clocks a
pendulum is generally used as the controlling agent. Galileo, when a student at Pisa,
noticed that certain hanging lamps in the cathedral there swung on their cords at an

equal rate; and on investigation he discovered the
principle that the shorter a pendulum is the more
quickly will it swing to and fro. As has already been
observed, Huygens first applied the principle to the
governing of clocks. In Fig. 202 we have a simple
representation of the "dead-beat" escapement
commonly used in clocks. The escape-wheel is
mounted on the shaft of the last cog of the driving
train, the pallet on a spindle from which depends a
split arm embracing the rod and the pendulum. We
must be careful to note that the pendulum controls
motion only; it does not cause movement.

The escape-wheel revolves in a clockwise
direction. The two pallets a and b are so designed that Fig. 202.
only one can rest on the teeth at one time. In the
sketch the sloping end of b has just been forced upwards by the pressure of a tooth. This
swings the pallet and the pendulum. The momentum of the latter causes a to descend,
and at the instant when b clears its tooth a catches and holds another. The left-hand side
of a, called the locking-face, is part of a circle, so that the escape-wheel is held
motionless as long as it touches a: hence the term, "dead beat"—that is, brought to a
dead stop. As the pendulum swings back, to the left, under the influence of gravity, a is
raised and frees the tooth. The wheel jerks round, and another tooth is caught by the
locking-face of b. Again the pendulum swings to the right, and the sloping end of b is
pushed up once more, giving the pendulum fresh impetus. This process repeats itself as
long as the driving power lasts—for weeks, months, or years, as the case may be, and
the mechanism continues to be in good working order.

COMPENSATING PENDULUMS.

Metal expands when heated; therefore a steel pendulum which is of the exact length
to govern a clock correctly at a temperature of 60° would become too long at 80°, and
slow the clock, and too short at 40°, and cause it to gain. In common clocks the
pendulum rod is often made of wood, which maintains an almost constant length at all
ordinary temperatures. But for very accurate clocks something more efficient is
required. Graham, the partner of Thomas Tompion, took advantage of the fact that
different kinds of metal have different ratios of expansion to produce a self-
compensating pendulum on the principle illustrated by Fig. 203. He used steel for the
rod, and formed the bob, or weighted end, of a glass jar containing mercury held in a

stirrup; the mercury being of such a height that, as the pendulum rod lengthened with a
rise of temperature, the mercury expanded upwards sufficiently to keep the distance
between the point of suspension and the centre of gravity of the bob always the same.
With a fall of temperature the rod shortened, while the mercury sank in the jar. This
device has not been improved upon, and is still used in observatories and other places
where timekeepers of extreme precision are required. The milled nut s in Fig. 203 is
fitted at the end of the pendulum rod to permit the exact adjustment of the pendulum's
length.

For watches, chronometers, and small clocks

THE SPRING BALANCE

takes the place of the pendulum. We still have an escape-wheel with teeth of a suitable
shape to give impulses to the controlling agent. There are two forms of spring
escapement, but as both employ a hairspring and balance-wheel we will glance at these
before going further.

Fig. 203.

The hairspring is made of very fine steel ribbon, tempered to extreme elasticity, and
shaped to a spiral. The inner end is attached to the arbor of the balance-wheel, the outer
end to a stud projecting from the plate of the watch. When the balance-wheel, impelled
by the escapement, rotates, it winds up the spring. The energy thus stored helps the
wheel to revolve the other way during the locking of a tooth of the escape-wheel. The
time occupied by the winding and the unwinding depends upon the length of the spring.
The strength of the impulse makes no difference. A strong impulse causes the spring to
coil itself up more than a weak impulse would; but inasmuch as more energy is stored
the process of unwinding is hastened. To put the matter very simply—a strong impulse
moves the balance-wheel further, but rotates it quickly; a weak impulse moves it a
shorter distance, but rotates it slowly. In fact, the principle of the pendulum is also that
of the hairspring; and the duration of a vibration depends on the length of the rod in the
one case, and of the spring in the other.

Motion is transmitted to the balance by one of two methods. Either (1) directly, by a
cylinder escapement; or (2) indirectly, through a lever.

THE CYLINDER ESCAPEMENT

is seen in Fig. 204. The escape-wheel has sharp teeth
set on stalks. (One tooth is removed to show the
stalk.) The balance-wheel is mounted on a small steel
cylinder, with part of the circumference cut away at
the level of the teeth, so that if seen from above it
would appear like a in our illustration. A tooth is just
beginning to shove its point under the nearer edge of
the opening. As it is forced forwards, b is revolved in
a clockwise direction, winding up the hairspring.
Fig. 204.—"Cylinder" watch When the tooth has passed the nearer edge it flies
escapement. forward, striking the inside of the further wall of the
cylinder, which holds it while the spring uncoils. The
tooth now pushes its way past the other edge, accelerating the unwinding, and, as it
escapes, the next tooth jumps forward and is arrested by the outside of the cylinder. The
balance now reverses its motion, is helped by the tooth, is wound up, locks the tooth,
and so on.

THE LEVER ESCAPEMENT

is somewhat more complicated. The escape-wheel teeth are locked and unlocked by the
pallets p p1 projecting from a lever which moves on a pivot (Fig. 205). The end of the
lever is forked, and has a square notch in it. On the arbor of the balance-wheel is a
roller, or plate, r, which carries a small pin, i. Two pins, b b, projecting from the plate of
the watch prevent the lever moving too far. We must further notice the little pin c on the
lever, and a notch in the edge of the roller.

Fig. 205.—"Lever" watch escapement.

In the illustration a tooth has just passed under the "impulse face" b of p1. The lever
has been moved upwards at the right end; and its forked end has given an impulse to r,
and through it to the balance-wheel. The spring winds up. The pin c prevents the lever
dropping, because it no longer has the notch opposite to it, but presses on the
circumference of r. As the spring unwinds it strikes the lever at the moment when the
notch and c are opposite. The lever is knocked downwards, and the tooth, which had
been arrested by the locking-face a of pallet p, now presses on the impulse face b,
forcing the left end of the lever up. The impulse pin i receives a blow, assisting the
unwinding of the spring, and c again locks the lever. The same thing is repeated in
alternate directions over and over again.

COMPENSATING BALANCE-WHEELS.

The watchmaker has had to overcome the same difficulty as the clockmaker with
regard to the expansion of the metal in the controlling agent. When a metal wheel is
heated its spokes lengthen, and the rim recedes from the centre. Now, let us suppose
that we have two rods of equal weight, one three feet long, the other six feet long. To an
end of each we fasten a 2-lb. weight. We shall find it much easier to wave the shorter
rod backwards and forwards quickly than the other. Why? Because the weight of the
longer rod has more leverage over the hand than has that of the shorter rod. Similarly,
if, while the mass of the rim of a wheel remains constant, the length of the spokes

varies, the effort needed to rotate the wheel to and fro at a constant rate must vary also.
Graham got over the difficulty with a rod by means of the compensating pendulum.
Thomas Earnshaw mastered it in wheels by means of the compensating balance, using
the same principle—namely, the unequal expansion of different metals. Any one who
owns a compensated watch will see, on stopping the tiny fly-wheel, that it has two
spokes (Fig. 206), each carrying an almost complete semicircle of rim attached to it. A
close examination shows that the rim is compounded of an outer strip of brass welded
to an inner lining of steel. The brass element expands more with heat and contracts
more with cold than steel; so that when the spokes become elongated by a rise of
temperature, the pieces bend inwards at their free ends (Fig. 207); if the temperature
falls, the spokes are shortened, and the rim pieces bend outwards (Fig. 208).[39] This
ingenious contrivance keeps the leverage of the rim constant within very fine limits.
The screws s s are inserted in the rim to balance it correctly, and very fine adjustment is
made by means of the four tiny weights w w. In ships' chronometers,[40] the rim pieces
are sub-compensated towards their free ends to counteract slight errors in the primary
compensation. So delicate is the compensation that a daily loss or gain of only half a
second is often the limit of error.

Fig. 206. Fig. 207. Fig. 208.
A "compensating" watch balance, at normal, super-normal, and sub-normal
temperatures.

KEYLESS WINDING MECHANISM FOR WATCHES.

The inconvenience attaching to a key-wound watch caused the Swiss manufacturers
to put on the market, in 1851, watches which dispensed with a separate key. Those of
our readers who carry keyless watches will be interested to learn how the winding and
setting of the hands is effected by the little serrated knob enclosed inside the pendant
ring.

There are two forms of "going-barrel" keyless mechanism—(1) The rocking bar; (2)
the shifting sleeve. The rocking bar device is shown in Figs. 209, 210. The milled head
m turns a cog, g, which is always in gear with a cog, f. This cog gears with two others, a
and b, mounted at each end of the rocker r, which moves on pivot s. A spring, s p,
attached to the watch plate presses against a small stud on the rocking bar, and keeps a
normally in gear with c, mounted on the arbor of the mainspring.

Fig. 209.—The winding mechanism of a keyless watch.

To wind the watch, m is turned so as to give f an anti-clockwise motion. The teeth of
f now press a downwards and keep it in gear with c while the winding is done. A spring
click (marked solid black) prevents the spring uncoiling (Fig. 209). If f is turned in a
clockwise direction it lifts a and prevents it biting the teeth of c, and no strain is thrown
on c.

To set the hands, the little push-piece p is pressed inwards by the thumb (Fig. 210) so
as to depress the right-hand end of r and bring b into gear with d, which in turn moves
e, mounted on the end of the minute-hand shaft. The hands can now be moved in either
direction by turning m. On releasing the push-piece the winding-wheels engage again.

The shifting sleeve mechanism has a bevel pinion in the place of g (Fig. 209) gearing
with the mainspring cog. The shaft of the knob m is round where it passes through the
bevel and can turn freely inside it, but is square below. On the square part is mounted a
little sliding clutch with teeth on the top corresponding with the other teeth on the under
side of the bevel-wheel, and teeth similar to those of g (Fig. 209) at the end. The clutch
has a groove cut in the circumference, and in this lies the end of a spring lever which
can be depressed by the push-piece. The mechanism much resembles on a small scale
the motor car changing gear (Fig. 49). Normally, the clutch is pushed up the square part
of the knob shaft by the spring so as to engage with the bevel and the winding-wheels.
On depressing the clutch by means of the push-piece it gears with the minute-hand
pinion, and lets go of the bevel.

Fig. 210.—The hand-setting mechanism in action.
In one form of this mechanism the push-piece is dispensed with, and the minute-
wheel pinion is engaged by pulling the knob upwards.

THE HOUR-HAND TRAIN.

Fig. 211.—The hour-hand train of a clock.
The teeth of the mainspring drum gear with a cog on the minute-hand shaft, which
also carries one of the cogs of the escapement train. The shaft is permitted by the
escapement to revolve once an hour. Fig. 211 shows diagrammatically how this is
managed. The hour-hand shaft a (solid black) can be moved round inside the cog b,
driven by the mainspring drum. It carries a cog, c. This gears with a cog, d, having three
times as many teeth. The cog e, united to d, drives cog f, having four times as many
teeth as e. To f is attached the collar g of the hour-hand. f and g revolve outside the
minute-hand shaft. On turning a, c turns d and e, e turns f and the hour-hand, which
revolves ⅓ of ¼ = 1⁄12 as fast as a.[41]

LOCKS.
On these unfortunately necessary mechanisms a great deal of ingenuity has been
expended. With the advance of luxury and the increased worship of wealth, it becomes
more and more necessary to guard one's belongings against the less scrupulous
members of society.

Fig. 212.

The simplest form of lock, such as is found in desks and very cheap articles, works
on the principle shown in Fig. 212. The bolt is split at the rear, and the upper part bent
upwards to form a spring. The under edge has two notches cut in it, separated by a
curved excrescence. The key merely presses the bolt upwards against the spring, until
the notch, engaging with the frame, moves it backwards or forwards until the spring
drives the tail down into the other notch. This primitive device affords, of course, very
little security. An advance is seen in the

TUMBLER LOCK.

Fig. 213.
The bolt now can move only in a horizontal direction. It has an opening cut in it with
two notches (Figs. 213, 214). Behind the bolt lies the tumbler t (indicated by the dotted
line), pivoted at the angle on a pin. From the face of the tumbler a stud, s, projects
through the hole in the bolt. This stud is forced into one or other of the notches by the
spring, s1, which presses on the tail of the tumbler.

Fig. 214.
In Fig. 213 the key is about to actuate the locking mechanism. The next diagram
(Fig. 214) shows how the key, as it enters the notch on the lower side of the bolt to
move it along, also raises the tumbler stud clear of the projection between the two

notches. By the time that the bolt has been fully "shot," the key leaves the under notch
and allows the tumbler stud to fall into the rear locking-notch.

A lock of this type also can be picked very easily, as the picker has merely to lift the
tumbler and move the bolt along. Barron's lock, patented in 1778, had two tumblers and
two studs; and the opening in the bolt had notches at the top as well as at the bottom
(Fig. 215). This made it necessary for both tumblers to be raised simultaneously to
exactly the right height. If either was not lifted sufficiently, a stud could not clear its
bottom notch; if either rose too far, it engaged an upper notch. The chances therefore
were greatly against a wrong key turning the lock.

Fig. 215.—The bolt of a Barron lock.

THE CHUBB LOCK

is an amplification of this principle. It usually has several tumblers of the shape shown
in Fig. 216. The lock stud in these locks projects from the bolt itself, and the openings,
or "gates," through which the stud must pass as the lock moves, are cut in the tumblers.
It will be noticed that the forward notch of the tumbler has square serrations in the
edges. These engage with similar serrations in the bolt stud and make it impossible to
raise the tumbler if the bolt begins to move too soon when a wrong key is inserted.

Fig. 216.—Tumbler of Chubb lock.

Fig. 217 is a Chubb key with eight steps. That nearest the head (8) operates a
circular revolving curtain, which prevents the introduction of picking tools when a key
is inserted and partly turned, as the key slot in the curtain is no longer opposite that in
the lock. Step 1 moves the bolt.

Fig. 217.—A Chubb key.

In order to shoot the bolt the height of the key steps must be so proportioned to the
depth of their tumblers that all the gates in the tumblers are simultaneously raised to the
right level for the stud to pass through them, as in Fig. 218. Here you will observe that
the tumbler d on the extreme right (lifted by step 2 of the key) has a stud, d s, projecting
from it over the other tumblers. This is called the detector tumbler. If a false key or
picking tool is inserted it is certain to raise one of the tumblers too far. The detector is
then over-lifted by the stud d s, and a spring catch falls into a notch at the rear. It is now
impossible to pick the lock, as the detector can be released only by the right key
shooting the bolt a little further in the locking direction, when a projection on the rear
of the bolt lifts the catch and allows the tumbler to fall. The detector also shows that the
lock has been tampered with, since even the right key cannot move the bolt until the
overlocking has been performed.

Fig. 218.—A Chubb key raising all
the tumblers to the correct height.

Each tumbler step of a large Chubb key can be given one of thirty different heights;
the bolt step one of twenty. By merely transposing the order of the steps in a six-step
key it is possible to get 720 different combinations. By diminishing or increasing the
heights the possible combinations may be raised to the enormous total of 7,776,000!

Fig. 219.—Section of a Yale lock.

THE YALE LOCK,

which comes from America, works on a quite different system. Its most noticeable
feature is that it permits the use of a very small key, though the number of combinations
possible is still enormous (several millions). In our illustrations (Figs. 219, 220, 221)
we show the mechanism controlling the turning of the key. The keyhole is a narrow
twisted slot in the face of a cylinder, g (Fig. 219), which revolves inside a larger fixed

cylinder, f. As the key is pushed in, the notches in its upper edge raise up the pins a1,
1 1 1 1
b , c , d , e , until their tops exactly reach the surface of g, which can now be revolved
by the key in Fig. 220, and work the bolt through the medium of the arm h. (The bolt
itself is not shown.) If a wrong key is inserted, either some of the lower pins will
project upwards into the fixed cylinder f (see Fig. 221), or some of the pins in f will
sink into g. It is then impossible to turn the key.

Fig. 220.—Yale key turning.

There are other well-known locks, such as those invented by Bramah and Hobbs.
But as these do not lend themselves readily to illustration no detailed account can be
given. We might, however, notice the time lock, which is set to a certain hour, and can
be opened by the right key or a number of keys in combination only when that hour is
reached. Another very interesting device is the automatic combination lock. This may
have twenty or more keys, any one of which can lock it; but the same one must be used
to unlock it, as the key automatically sets the mechanism in favour of itself. With such a
lock it would be possible to have a different key for every day in the month; and if any
one key got into wrong hands it would be useless unless it happened to be the one
which last locked the lock.

Fig. 221.—The wrong key inserted. The pins do not allow
the lock to be turned.

THE CYCLE.
There are a few features of this useful and in some ways wonderful contrivance which
should be noticed. First,

THE GEARING OF A CYCLE.

To a good many people the expression "geared to 70 inches," or 65, or 80, as the
case may be, conveys nothing except the fact that the higher the gear the faster one
ought to be able to travel. Let us therefore examine the meaning of such a phrase before
going farther.

The safety cycle is always "geared up"—that is, one turn of the pedals will turn the
rear wheel more than once. To get the exact ratio of turning speed we count the teeth on
the big chain-wheel, and the teeth on the small chain-wheel attached to the hub of the
rear wheel, and divide the former by the latter. To take an example:—The teeth are 75
and 30 in number respectively; the ratio of speed therefore = 75⁄30 = 5⁄2 = 2½. One turn
of the pedal turns the rear wheel 2½ times. The gear of the cycle is calculated by
multiplying this result by the diameter of the rear wheel in inches. Thus a 28-inch wheel
would in this case give a gear of 2½ × 28 = 70 inches.

One turn of the pedals on a machine of this gear would propel the rider as far as if he
were on a high "ordinary" with the pedals attached directly to a wheel 70 inches in

diameter. The gearing is raised or lowered by altering the number ratio of the teeth on
the two chain-wheels. If for the 30-tooth wheel we substituted one of 25 teeth the
gearing would be—
75⁄25 × 28 inches = 84 inches.

A handy formula to remember is, gearing = T/t × D, where T = teeth on large chain-
wheel; t = teeth on small chain-wheel; and D = diameter of driving-wheel in inches.

Two of the most important improvements recently added to the cycle are—(1) The
free wheel; (2) the change-speed gear.

THE FREE WHEEL

is a device for enabling the driving-wheel to overrun the pedals when the rider
ceases pedalling; it renders the driving-wheel "free" of the driving gear. It is a ratchet
specially suited for this kind of work. From among the many patterns now marketed we
select the Micrometer free-wheel hub (Fig. 222), which is extremely simple. The
ratchet-wheel r is attached to the hub of the driving-wheel. The small chain-wheel (or
"chain-ring," as it is often called) turns outside this, on a number of balls running in a
groove chased in the neck of the ratchet. Between these two parts are the pawls, of half-
moon shape. The driving-wheel is assumed to be on the further side of the ratchet. To
propel the cycle the chain-ring is turned in a clockwise direction. Three out of the six
pawls at once engage with notches in the ratchet, and are held tightly in place by the
pressure of the chain-ring on their rear ends. The other three are in a midway position.

When the rider ceases to pedal, the chain-
ring becomes stationary, but the ratchet
continues to revolve. The pawls offer no
resistance to the ratchet teeth, which push them
up into the semicircular recesses in the chain-
ring. Each one rises as it passes over a tooth. It
is obvious that driving power cannot be
transmitted again to the road wheel until the
chain-wheel is turned fast enough to overtake
the ratchet.

THE CHANGE-SPEED GEAR.

Fig. 222. A gain in speed means a loss in power, and
vice versâ. By gearing-up a cycle we are able to

make the driving-wheel revolve faster than the pedals, but at the expense of control
over the driving-wheel. A high-geared cycle is fast on the level, but a bad hill-climber.
The low-geared machine shows to disadvantage on the flat, but is a good hill-climber.
Similarly, the express engine must have large driving-wheels, the goods engine small
driving-wheels, to perform their special functions properly.

In order to travel fast over level country, and yet be able to mount hills without
undue exertion, we must be able to do what the motorist does—change gear. Two-speed
and three-speed gears are now very commonly fitted to cycles. They all work on the
same principle, that of the epicyclic train of cog-wheels, the mechanisms being so
devised that the hub turns more slowly than, at the same speed as, or faster than the
small chain-wheel,[42] according to the wish of the rider.

We do not propose to do more here than explain the principle of the epicyclic train,
which means "a wheel on (or running round) a wheel." Lay a footrule on the table and
roll a cylinder along it by the aid of a second rule, parallel to the first, but resting on the
cylinder. It will be found that, while the cylinder advances six inches, the upper rule
advances twice that distance. In the absence of friction the work done by the agent
moving the upper rule is equal to that done in overcoming the force which opposes the
forward motion of the cylinder; and as the distance through which the cylinder
advances is only half that through which the upper rule advances, it follows that the
force which must act on the upper rule is only half as great as that overcome in moving
the cylinder. The carter makes use of this principle when he puts his hand to the top of a
wheel to help his cart over an obstacle.

Fig. 223. Fig. 224. Fig. 225.

Now see how this principle is applied to the change-speed gear. The lower rule is
replaced by a cog-wheel, c (Fig. 223); the cylinder by a cog, b, running round it; and
the upper rule by a ring, a, with internal teeth. We may suppose that a is the chain-ring,

b a cog mounted on a pin projecting from the hub, and c a cog attached to the fixed
axle. It is evident that b will not move so fast round c as a does. The amount by which a
will get ahead of b can be calculated easily. We begin with the wheels in the position
shown in Fig. 223. A point, i, on a is exactly over the topmost point of c. For the sake of
convenience we will first assume that instead of b running round c, b is revolved on its
axis for one complete revolution in a clockwise direction, and that a and c move as in
Fig. 224. If b has 10 teeth, c 30, and a 40, a will have been moved 10⁄40 = ¼ of a
revolution in a clockwise direction, and c 10⁄30 = ⅓ of a revolution in an anti-clockwise
direction.

Now, coming back to what actually does happen, we shall be able to understand how
far a rotates round c relatively to the motion of b, when c is fixed and B rolls (Fig. 225).
b advances ⅓ of distance round c; a advances ⅓ + ¼ = 7⁄12 of distance round b. The

fractions, if reduced to a common denominator, are as 4:7, and this is equivalent to 40
(number of teeth on a): 40 + 30 (teeth on a + teeth on c.)

To leave the reader with a very clear idea we will summarize the matter thus:—If T
= number of teeth on a, t = number of teeth on c, then movement of a: movement of b::
T + t: T.

Here is a two-speed hub. Let us count the teeth. The chain-ring (= a) has 64 internal
teeth, and the central cog (= c) on the axle has 16 teeth. There are four cogs (= b)
equally spaced, running on pins projecting from the hub-shell between a and c. How
much faster than b does a run round c? Apply the formula:—Motion of a: motion of b::
64 + 16: 64. That is, while a revolves once, b and the hub and the driving-wheel will
revolve only 64⁄80 = ⅘ of a turn. To use scientific language, b revolves 20 per cent.
slower than a.

This is the gearing we use for hill-climbing. On the level we want the driving-wheel
to turn as fast as, or faster than, the chain-ring. To make it turn at the same rate, both a
and c must revolve together. In one well-known gear this is effected by sliding c along
the spindle of the wheel till it disengages itself from the spindle, and one end locks with
the plate which carries a. Since b is now being pulled round at the bottom as well as the
top, it cannot rotate on its own axis any longer, and the whole train revolves solidly—
that is, while a turns through a circle b does the same.

To get an increase of gearing, matters must be so arranged that the drive is
transmitted from the chain-wheel to b, and from a to the hub. While b describes a circle,
a and the driving-wheel turn through a circle and a part of a circle—that is, the driving-

wheel revolves faster than the hub. Given the same number of teeth as before, the
proportional rates will be a = 80, b = 64, so that the gear rises 25 per cent.

By means of proper mechanism the power is transmitted in a three-speed gear either
(1) from chain-wheel to a, a to b, b to wheel = low gear; or (2) from chain-wheel to a
and c simultaneously = solid, normal, or middle gear; or (3) from chain-wheel to b, b to
a, a to wheel = high gear. In two-speed gears either 1 or 3 is omitted.

AGRICULTURAL MACHINES.
THE THRESHING-MACHINE.

Bread would not be so cheap as it is were the flail still the only means of separating the
grain from the straw. What the cream separator has done for the dairy industry (p. 384),
the threshing-machine has done for agriculture. A page or two ought therefore to be
spared for this useful invention.

Fig. 226.—Section of a threshing machine.

In Fig. 226 a very complete fore-and-aft section of the machine is given. After the
bands of the sheaves have been cut, the latter are fed into the mouth of the drum a by
the feeder, who stands in the feeding-box on the top of the machine. The drum revolves
at a very high velocity, and is fitted with fluted beaters which act against a steel
concave, or breastwork, b, the grain being threshed out of the straw in passing between
the two. The breastwork is provided with open wires, through which most of the
threshed grain, cavings (short straws), and chaff passes on to a sloping board. The straw

is flung forward on to the shakers c, which gradually move the straw towards the open
end and throw it off. Any grain, etc., that has escaped the drum falls through the shakers
on to d, and works backwards to the caving riddles, or moving sieves, e. The main
blower, by means of a revolving fan, n, sends air along the channel x upwards through
these riddles, blowing the short straws away to the left. The grain, husks, and dust fall
through e on to g, over the end of which they fall on to the chaff riddle, h. A second
column of air from the blower drives the chaff away. The heavy grain, seeds, dust, etc.,
fall on to i, j, and k in turn, and are shaken until only the grain remains to pass along l
to the elevator bottom, m. An endless band with cups attached to it scoops up the grain,
carries it aloft, and shoots it into hopper p. It then goes through the shakers q, r, is
dusted by the back end blower, s, and slides down t into the open end of the rotary
screen-drum u, which is mounted on the slope, so that as it turns the grain travels
gradually along it. The first half of the screen has wires set closely together. All the
small grain that falls through this, called "thirds," passes into a hopper, and is collected
in a sack attached to the hopper mouth. The "seconds" fall through the second half of
the drum, more widely spaced, into their sack; and the "firsts" fall out of the end and
through a third spout.

MOWING-MACHINES.

Fig. 227.

The ordinary lawn—mower employs a revolving reel, built up of spirally-arranged
knives, the edges of which pass very close to a sharp plate projecting from the frame of
the mower. Each blade, as it turns, works along the plate, giving a shearing cut to any
grass that may be caught between the two cutting edges. The action is that of a pair of

scissors (Fig. 227), one blade representing the fixed, the other the moving knife. If you
place a cylinder of wood in the scissors it will be driven forward by the closing of the
blades, and be marked by them as it passes along the edges. The same thing happens
with grass, which is so soft that it is cut right through.

HAY-CUTTER.

The hay-cutter is another adaptation of the same principle. A cutter-bar is pulled
rapidly backwards and forwards in a frame which runs a few inches above the ground
by a crank driven by the wheels through gearing. To the front edge of the bar are
attached by one side a number of triangular knives. The frame carries an equal number
of spikes pointing forward horizontally. Through slots in these the cutter-bar works, and
its knives give a drawing cut to grass caught between them and the sides of the spikes.

SOME NATURAL PHENOMENA.
WHY SUN-HEAT VARIES IN INTENSITY.

The more squarely parallel heat-rays strike a surface the greater will be the number that
can affect that surface. This is evident from Figs. 228, 229, where a b is an equal
distance in both cases. The nearer the sun is to the horizon, the more obliquely do its
rays strike the earth. Hence midday is necessarily warmer than the evening, and the
tropics, where the sun stands overhead, are hotter than the temperate zones, where, even
in summer at midday, the rays fall more or less on the slant.

Fig. 228. Fig. 229.

The atmospheric envelope which encompasses the earth tends to increase the effect
of obliquity, since a slanting ray has to travel further through it and is robbed of more
heat than a vertical ray.

THE TIDES.

All bodies have an attraction for one another. The earth attracts the moon, and the
moon attracts the earth. Now, though the effect of this attraction is not visible as regards
the solid part of the globe, it is strongly manifested by the water which covers a large
portion of the earth's surface. The moon attracts the water most powerfully at two
points, that nearest to it and that furthest away from it; as shown on an exaggerated
scale in Fig. 230. Since the earth and the water revolve as one mass daily on their axis,
every point on the circumference would be daily nearest to and furthest from the moon
at regular intervals, and wherever there is ocean there would be two tides in that period,
were the moon stationary as regards the earth. (It should be clearly understood that the
tides are not great currents, but mere thickenings of the watery envelope. The inrush of
the tide is due to the temporary rise of level.)

Fig. 230. Fig. 231.

WHY HIGH TIDE VARIES DAILY.

The moon travels round the earth once in twenty-eight days. In Fig. 231 the point a
is nearest the moon at, say, twelve noon. At the end of twenty-four hours it will have
arrived at the same position by the compass, but yet not be nearest to the moon, which
has in that period moved on 1⁄28th of a revolution round the earth.[43] Consequently high

tide will not occur till a has reached position b and overtaken the moon, as it were,
which takes about an hour on the average. This explains why high tide occurs at
intervals of more than twelve hours.

Fig. 232.—Relative positions of sun, Fig. 233.—Relative positions of sun,
moon, and earth at "spring" tides. moon, and earth at "neap" tides.

NEAP TIDES AND SPRING TIDES.

The sun, as well as the moon, attracts the ocean, but with less power, owing to its
being so much further away. At certain periods of the month, sun, earth, and moon are
all in line. Sun and moon then pull together, and we get the highest, or spring tides (Fig.
232). When sun and moon pull at right angles to one another—namely, at the first and
third quarters—the excrescence caused by the moon is flattened (Fig. 233), and we get
the lowest, or neap tides.
[39] In both Figs. 207 and 208 the degree of expansion is very greatly exaggerated.

[40] As the sun passes the meridian (twelve o'clock, noon) the chronometer's reading is
taken, and the longitude, or distance east or west of Greenwich, is reckoned by the difference
in time between local noon and that of the chronometer.

[41] For much of the information given here about clocks and watches the author is
indebted to "The History of Watches," by Mr. J.F. Kendal.

[42] We shall here notice only those gears which are included in the hub of the driving-
wheel.

[43] The original position of the moon is indicated by the dotted circle.

INDEX.
Note.—Figures in italics signify that an illustration of
the thing referred to appears on the page.

Aberration, spherical, of lens, 243.

Acoustics, 294.

Achromatic lens, 243.

Action carriage of piano, 283.

Advancing the spark, 102.

Air-gun, 342.

Air-pump for cycle tyres, 340;
for Westinghouse brake, 199.

Alternating currents, 164;
dynamo, 164.

Amperage, 125.

Angle of advance, 57, 58;
incidence, 268;
reflection, 268.

Aorta, 360.

Arc lamp, 182.

Archimedes, 412.

Armature, 162.

Arteries, 358.

Arterial blood, 359.

Atmospheric pressure, 350.

Auditory nerve, 272.

Automatic brakes, 188;
signalling, 228;
stoker, 399.

Backfall, 298.

Balance-wheel, 419.

Ball cock, 366, 367.

Balloon, fire, 323;
gas, 347.

Barometer, aneroid, 328, 329;
and weather, 331;
Fortin's, 326;
meaning of, 325;
simple, 328;
wheel, 327.

Beau de Rochas, 89.

Bell, diving, 332;
electric, 119, 120.

Bellows of organ, 303.

Bioscope, 266.

Blades, turbine, 81, 83.

Block system, 201, 212.

Blood, arterial, 359;

circulation of, 356, 357, 360;
venous, 359.

Blower-plate, 393, 394.

Boat, sails of, 346.

Boiler, Babcock and Wilcox, 21, 22;
explosions, 34, 391;
fire-tube, 21;
fittings, 31;
Lancashire, 25, 26;
locomotive, 20, 23;
multitubular, 21;
principle of, 15;
stored energy in, 32;
vertical, 25;
water supply to, 39;
water-tube, 21.

Brakes, hydraulic, 188;
motor car, 110;
railway, 187;
vacuum, 189, 190, 191;
Westinghouse, 194, 195, 197.

Bramah, 363, 437.

Breezes, land and sea, 324.

Brushes of dynamo, 161, 172.

Bunsen burner, 409.

Burning-glass, 232.

Camera, the, 233;
pinhole, 234, 235.

Canals, semicircular, 273.

Capillary attraction, 392;
veins, 358.

Carbon dioxide, 27, 359;
monoxide, 27.

Carburetter, 98, 99.

Cardan shaft, 93.

Carmania, the, 83.

Centrifugal force, 382.

Change-speed gear, 105, 442.

Chassis of motor car, 92.

Circulation of water in a boiler, 17, 18, 19;
of water in a motor car, 95, 97.

Clarionet, 308.

Clock, first weight-driven, 412;
water, 410.

Clutch of motor car, 105.

Coal, as fuel, 15;
gas, 394;
gas making, 394;
gas plant, 396;
gas, purification of, 397.

Cochlea, 273.

Coherer, 140.

Coil, Ruhmkorff, 121.

Coke, 395.

Combinations in Chubb lock, 436;
Yale lock, 436.

Combustion, 26, 393;
perfect, 28.

Compensating gear, 107, 108.

Compound engines, 59;
arrangement of, 61;
invention of, 59.

Compound locomotives, 62.

Compound microscope, 261.

Condenser, marine, 71, 72;
of Ruhmkorff coil, 123.

Conduit, 176.

Convex lens, image cast by, 236.

Conjugate foci, 262.

Cornet, 308.

Corti, rods of, 274.

Coxwell, 348.

Cream separator, 381, 383.

Current, reversal of electric, 130, 131;
transformation of, 124.

Cushioning of steam, 55.

Cycle, gearing of, 439.