249 files changed, 29730 insertions, 0 deletions

diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..6833f05
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,3 @@
+* text=auto
+*.txt text
+*.md text
diff --git a/28553-8.txt b/28553-8.txt
new file mode 100644
index 0000000..dd99905
--- /dev/null
+++ b/28553-8.txt
@@ -0,0 +1,9549 @@
+The Project Gutenberg EBook of How it Works, by Archibald Williams
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: How it Works
+       Dealing in simple language with steam, electricity, light,
+       heat, sound, hydraulics, optics, etc., and with their
+       applications to
+
+Author: Archibald Williams
+
+Release Date: April 10, 2009 [EBook #28553]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK HOW IT WORKS ***
+
+
+
+
+Produced by Steven Gibbs, Greg Bergquist and the Online
+Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+Transcriber's Note
+
+The punctuation and spelling from the original text have been faithfully
+preserved. Only obvious typographical errors have been corrected.
+Subscripts are represented as X_1. Superscripts are represented by X^1.
+
+
+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.
+
+[Illustration: 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
+
+
+
+
+PREFACE.
+
+
+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 on the magnetic needle--Method
+of reversing the current--Sounding instruments--Telegraphic
+relays--Recording telegraphs--High-speed telegraphy      127
+
+
+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 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      322
+
+
+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 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      410
+
+
+
+
+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?
+
+If 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.
+
+[Illustration: FIG. 1.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 4.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 1/2 to 3/4 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.
+
+[Illustration: 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.
+
+[Illustration: 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_ (CO_2). 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 CO_2 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.
+
+[Illustration: FIG. 10.--Sketch showing how the "forced draught" is
+produced in a stokehold and how it affects the furnaces.]
+
+[Illustration: 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 3-1/2 miles. In the case of a Lancashire
+boiler at equal pressure the distance would be 2-1/2 miles; of a
+locomotive boiler, at 125 lbs., 1-1/2 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.
+
+[Illustration: 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 P^1. The cocks T^1 T^2 are normally open, allowing the
+ingress of steam and water respectively to the tube. Cock T^3 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 P^2
+P^3 are removed.
+
+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.
+
+[Illustration: FIG. 12.--Section of a water-gauge.]
+
+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.
+
+[Illustration: FIG. 13.--Showing the principle of the steam-gauge.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 15.--Diagram illustrating the principle of a
+steam-injector.]
+
+[Illustration: 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.
+
+
+Having 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 18.--Sectional plan of a horizontal engine.]
+
+
+DOUBLE-CYLINDER ENGINES.
+
+[Illustration: FIG. 19.]
+
+[Illustration: 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, C_1,
+of a double-cylinder engine is at a "dead point," the other, C_2, 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.
+
+[Illustration: 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_, G^1, 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.
+
+[Illustration: 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.
+
+
+[Illustration: 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 C^2,
+the centre of the shaft, and C^1, 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.
+
+[Illustration: FIG. 24.]
+
+[Illustration: 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.
+
+[Illustration: 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
+E^1 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.
+
+[Illustration: FIG. 27.--A slide-valve with "lap."]
+
+[Illustration: 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 E^2.
+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.
+
+[Illustration: 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.
+
+[Illustration: 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. E^1 and E^2 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 E^1 has pushed the lower end of the link fully
+back; E^2 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 E^1 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 E^2 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 34.]
+
+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 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.
+
+[Illustration: 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.
+
+
+More 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.
+
+[Illustration: FIG. 36.--The wheel and nozzles of a De Laval turbine.]
+
+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 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 8-1/2 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 44-1/2 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.
+
+[Illustration: 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, D^1, D^2,
+D^3, 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.
+
+[Illustration: FIG. 38.--Blades or vanes of a Parsons turbine.]
+
+[Illustration: 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 D^1 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
+D^3, 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, P^1, P^2, P^3.
+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 P^1 to the left with the same pressure as
+it pushes the blades of D^1 to the right. After completing the first
+stage it fills the passage C, which communicates with the second piston,
+P^2, and the pressure on that piston negatives the thrust on D^2.
+Similarly, the passage E causes the steam to press equally on P^3 and
+the vanes of D^3. 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.
+
+[Illustration: 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 1/2 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."
+
+In 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.
+
+[Illustration]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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, C^1, 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 25-1/3 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 1-1/2 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?
+
+Of 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.
+
+[Illustration: FIG. 50.--Permanent magnet, and the "lines of force"
+emanating from it.]
+
+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.
+
+[Illustration: FIG. 51.--Electro-magnet: A, armature; B, battery.]
+
+Having 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 must be put a crank to alter the direction of 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 T^1, 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.
+
+[Illustration: 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.
+
+[Illustration: 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 T^1. 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.
+
+
+Take 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.
+
+[Illustration: FIG. 54.--Sketch of the side elevation of a Wheatstone
+needle instrument.]
+
+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 right, and comes to rest in a
+normal position for a moment, the letter A is signified;
+right-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.
+
+[Illustration: FIGS. 55, 56.--The coils of a needle instrument. The
+arrows show the direction taken by the current.]
+
+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 are at the right end of
+the coils, the S. poles 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.
+
+[Illustration: 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, A^1
+B^1. Crossing these at right angles (under A A^1 and over B B^1)
+are the flat brass springs, L R, L^1 R^1, having buttons at their
+lower ends, and fixed at their upper ends to baseboards. When at rest
+they all press upwards against the plates A and A^1 respectively. R
+and L^1 are connected with the line circuit, in which are the coils of
+dials 1 and 2, one at each station. L and R^1 are connected with the
+earth-plates E E^1. 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 L^1 A^1 and
+R to earth-plate E^1, 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.
+
+[Illustration: FIG. 58.--Section of a telegraph wire insulator on its
+arm. The shaded circle is the line wire, the two blank circles indicate
+the wire which ties the line wire to the insulator.]
+
+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 produced. The
+ribbon-feeding apparatus is set in motion automatically by the current,
+and continues to pull the ribbon along until the message is completed.
+
+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.
+
+
+In 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]
+
+[Illustration: FIG. 59.--Sketch of the transmitter of a wireless
+telegraphy outfit.]
+
+[Illustration: 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.
+
+[Illustration: 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 R^1 R^2.
+
+
+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 1/2 mile.
+
+1885.--Sir William Preece signalled from Lavernock Point, near Cardiff,
+to Steep Holm, an island in the Bristol Channel. Distance about 5-1/2
+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; 17-1/2 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.
+
+
+For 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.
+
+[Illustration: 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.
+
+[Illustration: 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 T^1 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.
+
+[Illustration: 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 P_2 of induction coils. The
+transmitters are in the line circuit, which includes the secondary
+windings S S_2 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 B_2, 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 T_2 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 65.--The headdress of an operator at a telephone
+exchange. The receiver is fastened over one ear, and the transmitter to
+the chest.]
+
+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 telephonic
+connection, though three exchanges were in the 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 S_2 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 S_3, 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.
+
+
+In 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.
+
+[Illustration: 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 C^1. Two small
+brushes, B B^1, 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.
+
+[Illustration: FIG. 67.]
+
+[Illustration: 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 C^1 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.
+
+[Illustration: 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 C^1, 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 B^1 on to segment C^1. 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 71.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 77.--Sketch showing a "series" winding.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+
+In 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.
+
+[Illustration: FIG. 85.--Vacuum brake "off."]
+
+[Illustration: 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.
+
+[Illustration: FIG. 87.--Guard's valve for applying the Vacuum brake.]
+
+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 train pipe through the valve. It is thus 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_.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+
+Under 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.
+
+[Illustration: FIG. 91.--Distant and home signals.]
+
+
+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 reinforced by an "advance starting" signal 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 94.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 96.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 98.--Points open to main line.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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
+
+[Illustration: 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.
+
+
+Light 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.
+
+[Illustration: 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).
+
+[Illustration: FIG. 104.--Six forms of lenses.]
+
+
+THE CAMERA.
+
+[Illustration: FIG. 105.]
+
+[Illustration: 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.
+
+[Illustration: 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 A^1 and B^1. In reality a countless number
+of rays would be transmitted from every point of the object and
+collected to form the image.
+
+[Illustration: 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.
+
+[Illustration: 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,
+F^1, 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.
+
+[Illustration: 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 B^1, denotes the principal
+focal length of the lens, and A C, or A C^1, denotes twice the focal
+length. We will take the positions in order:--
+
+_Position I._ Object further away than 2_f_. Inverted image _smaller_
+than object, at distance somewhat exceeding _f_.
+
+_Position II._ Object at distance = 2_f_. Inverted image at distance =
+2_f_, and of size equal to that of object.
+
+_Position III_ Object nearer than 2_f_. Inverted image further away than
+2_f_; _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 2_f_ from the
+optical centre of the lens. And it follows from this that the further he
+can rack his camera out beyond 2_f_ 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.
+
+[Illustration: FIG. 111.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 113.]
+
+[Illustration: FIG. 114.]
+
+
+DISTORTION OF IMAGE.
+
+[Illustration: FIG. 115.--Section of a rectilinear lens.]
+
+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 this kind are called _rectilinear_,
+or 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?
+
+[Illustration: 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.
+
+[Illustration: FIG. 117_a_.]
+
+[Illustration: FIG. 117_b_.]
+
+[Illustration: FIG. 118_a_.]
+
+[Illustration: FIG. 118_b_.]
+
+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 117_a_, 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. 117_b_) 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. 118_a_ 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. 118_b_, 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.
+
+
+In 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_.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 121.]
+
+[Illustration: FIG. 122.]
+
+
+USE OF THE SIMPLE MICROSCOPE IN THE TELESCOPE.
+
+[Illustration: 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:--
+
+  Magnification = focal length of lens
+                  --------------------
+                      _d_
+
+[Illustration: FIG. 124.]
+
+In Fig. 124 we have interposed between the eye and the object a small
+magnifying glass of 2-1/2-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
+  2-1/2
+
+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.
+
+[Illustration: FIG. 125.]
+
+
+THE GALILEAN TELESCOPE.
+
+[Illustration: 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.
+
+[Illustration: FIG. 127.]
+
+
+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 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 A^1, in
+which the lines D _b_, E _c_ meet when produced backwards.
+
+[Illustration: 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.
+
+
+Sound 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.
+
+[Illustration: 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).
+
+[Illustration: 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.
+
+[Illustration: 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, B^1, 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.
+
+In 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.
+
+[Illustration: FIG 136.--Showing how the harmonics of a "stopped" pipe
+are formed.]
+
+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 the pipe. Blow very
+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 A^1 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.
+
+[Illustration: FIG. 137.--Showing how harmonics of an open pipe are
+formed, B, B^1, and C are "nodes." The arrows indicate the distance
+travelled by a sound impulse from a node to a node.]
+
+
+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.
+
+
+WHERE OVERTONES ARE USED.
+
+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.
+
+[Illustration: FIG. 138.--Section of an ordinary wooden "flue" pipe.]
+
+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.
+
+[Illustration: FIG. 139.--The table of a sound-board.]
+
+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_, one for each 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
+pipes. The grooves under the big pipes are wider than those under the
+small pipes, as they have to pass more 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.
+
+[Illustration: FIG. 140.]
+
+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 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.
+
+[Illustration: FIG. 141.]
+
+
+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.
+
+[Illustration: FIG. 142.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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, [Greek: akouein], "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.
+
+
+In 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 148.--Perspective view of a phonograph recorder.]
+
+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 much larger than others (Fig. 151, _a_). A 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.
+
+[Illustration: FIG. 149.--Section of the reproducer of an Edison Bell
+phonograph.]
+
+[Illustration: FIG. 150.--Perspective view of a phonograph reproducer.]
+
+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 note.
+
+
+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. 151_b_). 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.
+
+[Illustration: FIG. 151_a._]
+
+[Illustration: FIG. 151_b._]
+
+[Illustration: FIG. 151_c._--Section of a gramophone reproducer.]
+
+In Fig. 151_c_ 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.
+
+
+When 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 P^1, 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.
+
+[Illustration: 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 4-1/3 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 163.--Divers at work below water with pneumatic
+tools.]
+
+
+AIR-PUMPS.
+
+[Illustration: FIG. 164.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 166.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 7-1/4 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.
+
+[Illustration: 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."
+
+
+In 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.
+
+[Illustration: FIG. 172.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 174.]
+
+
+THE FORCE-PUMP.
+
+[Illustration: FIG. 175. Force-pump; suction stroke.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 179.]
+
+[Illustration: FIG. 180.--The cylinder and ram of a hydraulic press.]
+
+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.
+
+Among these, the most used is the tap, 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 182.--An automatic ball-valve.]
+
+
+THE WATER-METER.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 72-1/2 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 199.]
+
+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 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.
+
+[Illustration: FIG. 200.--Sketch of the bellows and chambers of a "dry"
+gas meter.]
+
+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 a certain volume of gas has been
+ejected either from them or from the chambers in which they 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.
+
+The 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.
+
+[Illustration: FIG. 201.]
+
+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 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.
+
+[Illustration: FIG. 202.]
+
+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 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 204.--"Cylinder" watch escapement.]
+
+
+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. When the
+tooth has passed the nearer edge it flies 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 P^1 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.
+
+[Illustration: FIG. 205.--"Lever" watch escapement.]
+
+In the illustration a tooth has just passed under the "impulse face" _b_
+of P^1. 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 1/3 of 1/4 = 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.
+
+[Illustration: 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.
+
+[Illustration: 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, S^1, which presses on the tail of the tumbler.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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!
+
+[Illustration: 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 A^1, B^1, C^1, D^1, E^1,
+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.
+
+[Illustration: 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 _un_lock 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.
+
+[Illustration: 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 = 2-1/2. One turn of the pedal turns the rear wheel 2-1/2 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 2-1/2 × 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.
+
+[Illustration: FIG. 222.]
+
+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.
+
+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.
+
+[Illustration: FIG. 223.]
+
+[Illustration: FIG. 224.]
+
+[Illustration: 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 = 1/4 of a revolution in a clockwise direction, and C 10/30
+= 1/3 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 1/3 of distance round C; A
+advances 1/3 + 1/4 = 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 = 4/5 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 228.]
+
+[Illustration: 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.)
+
+[Illustration: FIG. 230.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 232.--Relative positions of sun, moon, and earth at
+"spring" tides.]
+
+[Illustration: FIG. 233.--Relative positions of sun, 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.
+
+  Cylinder, hydraulic press, _363_;
+    steam, _49_.
+
+
+  Danes, 382.
+
+  Dead point, 47.
+
+  De Brouwer stoker, 401.
+
+  Detector in Chubb lock, 435.
+
+  Diving-bell, _332_;
+    simple, _333_, _334_.
+
+  Diving-dress, 335.
+
+  Direction of current in dynamo circuit, 163.
+
+  Diver's feats, 338;
+    helmet, _336_;
+    lamp, _338_.
+
+  Donkey-engines, 68.
+
+  Doorstop, self-closing, 344.
+
+  Double-cylinder engines, 47.
+
+  Draught, forced, 28, _29_;
+    induced, 29.
+
+  Drum and fusee, _414_.
+
+  Durability of motor-car engine, 96.
+
+  D-valve, 67.
+
+  Dynamo, alternating, 164, 174;
+    brushes, _172_;
+    compound, 174;
+    continuous-current, 165;
+    multipolar, 169;
+    series wound, _173_;
+    shunt wound, _173_;
+    simple, 161, _162_.
+
+
+  Ear, the, _271_, _273_;
+    a good, 274, 307;
+    sensitiveness of, 275.
+
+  Eccentric, _52_, 53;
+    setting of, 53.
+
+  Edison, Thomas, 310.
+
+  Edison-Bell phonograph, 310.
+
+  Electricity, current, 115;
+    forms of, 113;
+    nature of, 112;
+    static, 114.
+
+  Electric bell, 119, _120_;
+    signalling, 225;
+    slot, 226.
+
+  Electroplating, 185, _186_.
+
+  Electro-magnets, 117.
+
+  Endolymph, 272.
+
+  Engines, compound, 59;
+    donkey, 68;
+    double-cylinder, 47;
+    internal-combustion, 87, 95;
+    reciprocating, 44.
+
+  Escapement of timepieces, 416;
+    cylinder, _420_;
+    lever, 421, _422_.
+
+  Ether, 270.
+
+  Eustachian tube, 276.
+
+  Eye, human, 246, _247_;
+    self-accommodation of, 248.
+
+  Expansive working of steam, 56.
+
+
+  Faraday, Michael, 159.
+
+  Field, magnetic, 159;
+    magnets, 171;
+    ring, 174.
+
+  Filters, 374;
+    Maignen, _373_;
+    Berkefeld, 374.
+
+  Filtration beds, 372.
+
+  Flute, 308.
+
+  Flying-machines, 348.
+
+  Fly-wheel, use of, 48.
+
+  Focus, meaning of, 237;
+    principal, 238.
+
+  Foci, conjugate, 262.
+
+  Force, lines of, 116.
+
+  Forces, component, 345.
+
+  Free wheel, _440_.
+
+  Furring-up of pipes, 391.
+
+  Fusee, drum and, 414.
+
+
+  Galileo, 259, 325, 416.
+
+  Galilean telescope, _259_.
+
+  Gas, coal, 394;
+    governor, 402;
+    meter, 405;
+    traps, 374;
+    works, 394.
+
+  Gasometer, 397;
+    largest, _398_, 399.
+
+  Gauge, steam, 36, _38_;
+    water, 35, _36_.
+
+  Gear, compensating, 107, _108_.
+
+  Gear-box of motor car, 105.
+
+  Gearing of cycle, 439.
+
+  Glaisher, 348.
+
+  Gland, 50, 363.
+
+  Glass, flint and crown, 242.
+
+  Going-barrel for watches, 415.
+
+  Gooch reversing gear, 65.
+
+  Governors, speed, 67;
+    of motor car, 103, _104_.
+
+  Graham, 418.
+
+  Gramophone, 317;
+    records, 319, 321;
+    reproducer, _318_.
+
+
+  Hairspring, 412.
+
+  Hay-cutter, 451.
+
+  Heart, the, 355;
+    disease, 361;
+    rate of pulsation of, 361;
+    size of, 357.
+
+  Heat of sun, 451.
+
+  Hele, Peter, 412.
+
+  Helmet, diver's, _336_.
+
+  Helmholtz, 274, 308.
+
+  Hero of Alexandria, 74.
+
+  Herschel, 261.
+
+  Hertz, Dr., 138.
+
+  Hertzian waves, 138.
+
+  Hot-water supply, 386.
+
+  Hour-hand train in timepieces, _429_.
+
+  Household water supply, 364.
+
+  Hughes type-printer, 134.
+
+  Hydraulic press, 361, _362_.
+
+  Hydro, 385.
+
+
+  Ignition of charge in motor-car cylinder, 100, _101_.
+
+  Image and object, relative positions of, 239;
+    distortion of, 245.
+
+  Incandescent gas mantle, 407;
+    electric lamp, 179.
+
+  Incus, 272.
+
+  Index mechanism of water-meter, 37.
+
+  Indicator of electric bell, 119.
+
+  Induction coil, 121;
+    uses of, 125.
+
+  Injector, 39;
+    Giffard's, _41_;
+    principle of, 40;
+    self-starting, 42.
+
+  Interlocking of signals, 204, 222.
+
+  Internal-combustion engine, 87.
+
+  Iris of eye, 249;
+    stop, 249.
+
+
+  Kelvin, Lord, 158.
+
+  Keyless winding mechanism, 425, _426_, 428.
+
+  Kite, 345.
+
+
+  Lamp, arc, 182;
+    how it works, 392;
+    incandescent, 179;
+    manufacture of incandescent lamps, 180.
+
+  Lap of slide-valve, _57_, 59.
+
+  Larynx, 306.
+
+  Laxey wheel, _380_, 381.
+
+  Leads, 208.
+
+  Lenses, 231;
+    correction of for colour, 240, _241_;
+    focus of, 236;
+    rectilinear, _245_;
+    spherical aberration in, 243.
+
+  Levers, signal, colours of, 208.
+
+  Limit of error in cylinder, 52.
+
+  Light, electric, 179;
+    nature of, 230;
+    propagation of, 231.
+
+  Li Hung Chang, 157.
+
+  Lindsay, James Bowman, 145.
+
+  Lines of force, 116, 162.
+
+  "Linking up," 65.
+
+  Locks, 430;
+    Barron, 433;
+    Bramah, 437;
+    Chubb, 433, 434;
+    Hobbs, 437;
+    simplest, _431_;
+    tumbler, _432_;
+    Yale, _436_.
+
+  Locking gear for signals, 205.
+
+  Locomotive, electric, 178;
+    advantages of, 179.
+
+  Lungs, 359.
+
+
+  Magic-lantern, 263, _264_.
+
+  Magnet, 115;
+    permanent, 115, 116;
+    temporary, 115.
+
+  Magnetism, 115.
+
+  Magnetic needle, influence of current on, 129.
+
+  Mainspring, invention of, 412.
+
+  Malleus, 272.
+
+  Marconi, 140, 146.
+
+  Marine chronometers, 415;
+    delicacy of, 425.
+
+  Marine speed governor, 71.
+
+  Marine turbine, advantages of, 84.
+
+  Maudslay, Henry, 363.
+
+  Maxim, Sir Hiram, 348.
+
+  Micrometer free wheel, 441.
+
+  Micro-photography, 265.
+
+  Microscope, 254;
+    compound, 261, _263_;
+    in telescope, 257;
+    simple, _254_.
+
+  Mineral oil, 392.
+
+  Mirror, parabolic, 261, _262_;
+    plane, _267_.
+
+  Morse, 132, 145;
+    code, 128;
+    inker, 142;
+    sounder, 132.
+
+  Motor car, the, 92;
+    electric, 177.
+
+  Mouth, 307.
+
+  Mowing-machines, 450.
+
+  Musical sounds, 277.
+
+
+  Nerve, auditory, 272;
+    optic, 246.
+
+  Nodes on a string, 285;
+    column of air, 291.
+
+  Note, fundamental, 285;
+    quality of, 285.
+
+  Niagara Falls, power station at, 174.
+
+
+  Organ, the, 294, _300_;
+    bellows, 303;
+    console, 305;
+    echo, solo, swell, great, and choir, 301;
+    electric and pneumatic, 305;
+    largest in the world, 306;
+    pedals, 298;
+    pipes, 295;
+    pipes, arrangement of, 295;
+    sound-board, _296_;
+    wind-chest, 297.
+
+  Otto cycle, 91.
+
+  Overtones, 285.
+
+
+  Pallets of organ, 297.
+
+  Parallel arrangement of electric lamps, 184.
+
+  Paris, siege of, 265.
+
+  Pedals of organ, 298.
+
+  Pelton wheel, _377_.
+
+  Pendulum, 412;
+    compensating, 418, _419_.
+
+  Perilymph, 272.
+
+  Perry, Professor, 16.
+
+  Petrol, 98.
+
+  Phonograph, 310;
+    governor, _311_;
+    recorder, 312, _313_;
+    records, making of, 319;
+    reproducer, 315;
+    tracings on record of, _317_.
+
+  Pianoforte, 277;
+    sounding-board, 280;
+    striking mechanism, 281;
+    strings, 281.
+
+  Piccolo, 308.
+
+  Pipes, closed, 289;
+    flue, 301;
+    open, 292;
+    organ, 295;
+    reed, 301, _302_;
+    tuning, 302.
+
+  Piston valve, 67.
+
+  Pneumatic tyres, 341.
+
+  Poldhu, signalling station at, 138.
+
+  Points, railway, 208, _210_;
+    and signals in combination, 211.
+
+  Poles of a magnet, 115.
+
+  Popoff, Professor A., 138, 145.
+
+  Power, transmission of, 175.
+
+  Preece, Sir William, 145.
+
+  Primary winding of induction coil, 122.
+
+  Pump, air, 340;
+    bucket, 352, _353_;
+    force, 354;
+    most marvellous, 355;
+    Westinghouse air, 199.
+
+
+  Railway brakes, 187;
+    signalling, 200.
+
+  Rays, converging and diverging, _256_;
+    heat, concentrated by lens, _232_;
+    light, 232, 235, 236, 237.
+
+  Records, master, 319, 320.
+
+  Reciprocation, 51.
+
+  Reed, human, 306;
+    pipes, 301, _302_.
+
+  Reflecting telescope, 260.
+
+  Relays, telegraphic, 133, 141.
+
+  Retina, 247.
+
+  Retorts, 395.
+
+  Reversing gear, 62;
+    Allan, 65;
+    Gooch, 65;
+    radial, 66.
+
+  Rocking bar mechanism for watches, 425.
+
+  Rods of Corti, 274.
+
+  Ruhmkorff coil, 121, _122_.
+
+
+  Safety-valve, 32, _33_, 391.
+
+  Sand-glasses, 411.
+
+  Scissors, action of, _450_.
+
+  Secondary winding of induction coil, 122.
+
+  Series arrangement of electric lamps, 183.
+
+  Series winding of dynamo, _173_.
+
+  Shunt wound dynamo, _173_.
+
+  Sight, long and short, 250.
+
+  Signalling, automatic, 228;
+    electric, 225;
+    pneumatic, 225;
+    power, 225.
+
+  Signal levers, _206_.
+
+  Signals, interlocking of, 204;
+    position of, 202;
+    railway, 200;
+    single line, 215.
+
+  Silencer on motor cars, 109.
+
+  Siphon, _351_.
+
+  Slide-valve, 49, 50, 51;
+    setting of, 53.
+
+  Sliders, 297.
+
+  Sound, nature of, 270;
+    board of organ, 296;
+    board of piano, 280.
+
+  Spagnoletti disc instrument, 212.
+
+  Sparking-plug, _102_.
+
+  Spectacles, use of, 249.
+
+  Spectrum, colours of, 230.
+
+  Speed governors, 67, _68_, _69_;
+    Hartwell, 70;
+    marine, 71.
+
+  Speed of motor cars, 110.
+
+  Spot, blind, in eye, 251;
+    yellow, in eye, 251.
+
+  Spring balance for watches, 419;
+    compensating, 423, _424_.
+
+  Stapes, 272.
+
+  Steam, what it is, 13;
+    energy of, 14;
+    engines, 44;
+    engines, reciprocating, _45_;
+    expansive working of, 59, 81;
+    gauge, 36;
+    gauge, principle of, 37;
+    turbine, 74;
+    turbine, De Laval, 76, _77_;
+    turbine, Hero's, 74;
+    turbine, Parsons, 79, _80_;
+    volume of, as compared with water, 15.
+
+  Stephenson, George, 63, 375.
+
+  Stop, in lens, 244;
+    iris, 249;
+    use of, 244.
+
+  Sun-dial of Ahaz, 410.
+
+  Syntonic transmission of wireless messages, 143.
+
+
+  Talking-machines, 310.
+
+  Tapper in wireless telegraphy receiver, 141.
+
+  Tappet arm, 205.
+
+  Telegraph, electric, 127;
+    insulator, _133_;
+    needle, _128_;
+    recording, 133;
+    sounder, 132.
+
+  Telegraphy, high-speed, 135;
+    wireless, 137.
+
+  Telephone, 147;
+    Bell, _148_;
+    circuit, double-line, 155;
+    circuit, general arrangement, _152_, 153;
+    exchange, _154_, 155.
+
+  Telephony, submarine, 157.
+
+  Telescope, 257;
+    Galilean, _259_;
+    prismatic, _260_;
+    reflecting, 260;
+    terrestrial, _259_.
+
+  Threshing-machine, 447, _448_.
+
+  Thurston, Professor, 31.
+
+  Tides, 452;
+    high, 453;
+    neap and spring, 455.
+
+  Timbre, 285.
+
+  Tompion, Thomas, 412.
+
+  Torricelli, 325.
+
+  Trachea, 306.
+
+  Train staff signalling, 216;
+    single, 216;
+    and ticket, 217;
+    electric, 218.
+
+  Transformation of current, 124, 176.
+
+  Transmission of power, 174, _175_.
+
+  Transmitter, Edison telephone, 150;
+    granular carbon, 150, _151_.
+
+  Triple-valve, 196.
+
+  Trolley arm, 176.
+
+  Turbines, steam, 74.
+
+  _Turbinia_, the, 79.
+
+  Tympanum, 137, 271, 272.
+
+
+  Universal joint, 93.
+
+
+  Vacuum brake, 189, _190_, _191_.
+
+  Vacuum chamber of aneroid barometer, _330_.
+
+  Valve, piston, 67;
+    safety, 32;
+    of internal-combustion engine, 89.
+
+  Valves of the heart, 357.
+
+  Veins, 358;
+    capillary, 358;
+    pulmonary, 361.
+
+  Ventral segments, 291.
+
+  Ventricles, 357.
+
+  Vibration of columns of air, 288, 289;
+    of rods, 287;
+    of strings, 278;
+    of strings, conditions regulating, 278.
+
+  _Viper_, the, 86.
+
+  Virag, Pollak--high-speed telegraphy, 136.
+
+  Vitreous humour, 246.
+
+  Voltage, 121, 161.
+
+  Vowel sounds, 308.
+
+
+  Wasborough, Matthew, 51.
+
+  Watches, first, 412.
+
+  Water cock, _365_;
+    engines, 375;
+    gauge, 35, _36_;
+    jacket, 19, 95;
+    meter, _368_;
+    supply, 371;
+    turbines, 174, 376;
+    wheels, 375.
+
+  Watt, James, 51, 69, 375.
+
+  Welsbach incandescent mantle, 407.
+
+  Westinghouse air-brake, 194, _195_, _197_;
+    George, 194.
+
+  Wheatstone needle instrument, 128, 131;
+    automatic transmitter, 135.
+
+  Wind, why it blows, 323;
+    action of on kites, 345;
+    on sails, 346.
+
+  Windmills, 375.
+
+  Window, oval, in ear, 272;
+    round, in ear, 272.
+
+  Wireless telegraphy, 137;
+    advance of, 145;
+    receiver, 140, 141;
+    syntonic, 143;
+    transmitter, 138, _139_.
+
+
+  Yale lock, _436_, _437_.
+
+  Yellow spot, in eye, 251.
+
+
+  Zech, Jacob, 414.
+
+  Zeiss field-glasses, 260.
+
+
+THE END.
+
+
+
+
+
+End of the Project Gutenberg EBook of How it Works, by Archibald Williams
+
+*** END OF THIS PROJECT GUTENBERG EBOOK HOW IT WORKS ***
+
+***** This file should be named 28553-8.txt or 28553-8.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/2/8/5/5/28553/
+
+Produced by Steven Gibbs, Greg Bergquist and the Online
+Distributed Proofreading Team at https://www.pgdp.net
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions means that no
+one owns a United States copyright in these works, so the Foundation
+(and you!) can copy and distribute it in the United States without
+permission and without paying copyright royalties.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need are critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
diff --git a/28553-8.zip b/28553-8.zip
new file mode 100644
index 0000000..b8ca2fa
--- /dev/null
+++ b/28553-8.zip
diff --git a/28553-h.zip b/28553-h.zip
new file mode 100644
index 0000000..cbaef45
--- /dev/null
+++ b/28553-h.zip
diff --git a/28553-h/28553-h.htm b/28553-h/28553-h.htm
new file mode 100644
index 0000000..7cf4455
--- /dev/null
+++ b/28553-h/28553-h.htm
@@ -0,0 +1,10616 @@
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
+    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1" />
+    <title>
+      The Project Gutenberg eBook of How It Works, by Archibald Williams.
+    </title>
+    <style type="text/css">
+<!--
+    p {  margin-top: .75em; text-align: justify;
+         margin-bottom: .75em; text-indent: 1.25em;
+         line-height: 130%;}
+    p.t1 {text-indent: 0em; text-align: center; font-size: 400%; font-family: garamond, serif;}
+    p big {font-size: 140%;}
+
+    h1  {text-align: center; clear: both; font-weight: normal;}
+    h2  {text-align: center; clear: both; font-weight: normal;
+         letter-spacing: 0.2em;}
+    h3,h4 {
+         text-align: center;
+         clear: both;
+         }
+    hr { width: 33%;
+	 margin-top: 2em;
+	 margin-bottom: 2em;
+         margin-left: auto;
+         margin-right: auto;
+         clear: both;
+       }
+
+    table {margin-left: auto; margin-right: auto;}
+    .tr1 td {padding-top: 1.5em;}
+    .tr2 td {vertical-align: bottom;}
+
+    body {margin-left: 10%;
+         margin-right: 10%;}
+
+    a {text-decoration: none;}
+
+    .pagenum  {display: inline;	font-size: 0.9em; text-align: right;
+               position: absolute; right: 2%; text-indent: 0em;
+               padding: 1px 2px; font-style: normal; font-family: garamond, serif;
+               font-variant: normal; font-weight: normal; text-decoration: none;
+               color: #444; background-color: #FF99CC;}
+
+
+    .blockquot{margin-left: 5%; margin-right: 10%; font-size: 90%;}
+
+    .center        {text-align: center; text-indent: 0em;}
+    .right         {text-align: right; padding-right: 2em;}
+    .smcap         {font-variant: small-caps;}
+    .above, .below {font-size: 70%;}
+    .above         {vertical-align: 0.7ex;}
+    .below         {vertical-align: -0.3ex;}
+    .ampm     {text-transform: lowercase; font-variant: small-caps;}
+    .noin          {text-indent: 0em;}
+    .hang     {text-indent: -1.5em; margin-left: 2em;}
+    .caption       {font-weight: normal; text-indent: 0em;}
+    .index    {text-indent: 0em; margin-left: 6em; line-height: 110%}
+    .tn {background-color: #EEE; padding: 0.5em 1em 0.5em 1em;}
+    .bigletter     {font-size: 120%; font-weight: bold; font-family: Arial, sans-serif;}
+    .dcap     {float: left; padding-right: 3px; font-size: 300%; line-height: 80%;
+               width: auto;}
+    .section    {text-align: center; text-indent: 0em; font-size: 90%; margin-top: 1.5em;}
+    .caps   {text-transform: uppercase;}
+    .u        {text-decoration: underline;}
+
+    .caption  {font-weight: bold; font-family: garamond, serif;}
+
+    .figcenter   {margin: auto; text-align: center;}
+
+    .figleft     {float: left; clear: left; margin-left: 0; margin-bottom: 1em; margin-top:
+                 1em; margin-right: 1em; padding: 0; text-align: center; width: auto;}
+
+    .figright    {float: right; clear: right; margin-left: 1em; margin-bottom: 1em;
+                 margin-top: 1em; margin-right: 0; padding: 0; text-align: center; width: auto;}
+
+    .footnotes        {border: dashed 1px;}
+    .footnote         {margin-left: 10%; margin-right: 10%; font-size: 0.9em;}
+    .footnote .label  {position: absolute; right: 84%; text-align: right;}
+    .fnanchor         {vertical-align: 0.2ex; font-size: .8em; text-decoration: none;}
+-->
+    </style>
+  </head>
+<body>
+
+
+<pre>
+
+The Project Gutenberg EBook of How it Works, by Archibald Williams
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: How it Works
+       Dealing in simple language with steam, electricity, light,
+              heat, sound, hydraulics, optics, etc., and with their
+              applications to
+
+Author: Archibald Williams
+
+Release Date: April 10, 2009 [EBook #28553]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK HOW IT WORKS ***
+
+
+
+
+Produced by Steven Gibbs, Greg Bergquist and the Online
+Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+
+<div class="tn">
+
+<p class="center"><big><b>Transcriber&#8217;s Note</b></big></p>
+
+<p class="noin">The punctuation and spelling from the original text have been faithfully preserved. Only obvious
+typographical errors have been corrected.</p>
+
+</div>
+<hr />
+
+
+
+<h1>HOW IT WORKS</h1>
+
+
+
+<hr />
+<h2>AUTHOR'S NOTE.</h2>
+<hr style="width: 10%;" />
+
+<p class="noin"><span class="smcap">I beg</span> 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"&mdash;</p>
+
+<p>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.</p>
+
+<hr />
+
+<div class="figcenter" style="width: 518px;">
+<img src="images/image1.jpg" width="518" height="328" alt="ON THE FOOTPLATE OF A LOCOMOTIVE." title="" />
+<span class="caption">ON THE FOOTPLATE OF A LOCOMOTIVE.</span>
+</div>
+
+<hr />
+
+<p class="t1">
+How It Works
+</p>
+
+<p class="center">
+<big>Dealing in Simple Language with Steam, Electricity,<br />
+Light, Heat, Sound, Hydraulics, Optics, etc.<br />
+and with their applications to Apparatus<br />
+in Common Use</big><br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<big>By</big><br /><br />
+<big>ARCHIBALD WILLIAMS</big><br />
+<br />
+Author of "The Romance of Modern Invention,"<br />
+"The Romance of Mining," etc., etc.<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<big>THOMAS NELSON AND SONS</big><br />
+<br />
+London, Edinburgh, Dublin, and New York<br />
+</p>
+
+
+
+<hr />
+<h2>PREFACE.</h2>
+<hr style="width: 10%;" />
+
+<p class="noin"><span class="smcap">How</span> 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.</p>
+
+<p>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&mdash;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 <i>all</i> 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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>Four organs of the body&mdash;the eye, the ear, the larynx, and the
+heart&mdash;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.</p>
+
+<p class="right">
+A.W.
+</p>
+
+<p><span class="smcap">Uplands, Stoke Poges, Bucks.</span></p>
+
+
+
+<hr />
+<h2>CONTENTS.</h2>
+<hr style="width: 15%;" />
+
+<div class='center'>
+<table border="0" width="65%" cellpadding="4" cellspacing="0" summary="Contents">
+
+<tr class='tr1'>
+<td align='center'><b>Chapter I.&mdash;THE STEAM-ENGINE.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+What is steam?&mdash;The mechanical energy of steam&mdash;The boiler&mdash;The
+circulation of water in a boiler&mdash;The enclosed furnace&mdash;The
+multitubular boiler&mdash;Fire-tube boilers&mdash;Other types of boilers&mdash;Aids
+to combustion&mdash;Boiler fittings&mdash;The safety-valve&mdash;The
+water-gauge&mdash;The steam-gauge&mdash;The water supply to a
+boiler</div></td>
+<td align='right'><a href="#Chapter_I">13</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter II.&mdash;THE CONVERSION OF HEAT ENERGY<br />
+INTO MECHANICAL MOTION.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+Reciprocating engines&mdash;Double-cylinder engines&mdash;The function of
+the fly-wheel&mdash;The cylinder&mdash;The slide-valve&mdash;The eccentric&mdash;"Lap"
+of the valve: expansion of steam&mdash;How the cut-off is
+managed&mdash;Limit of expansive working&mdash;Compound engines&mdash;Arrangement
+of expansion engines&mdash;Compound locomotives&mdash;Reversing
+gears&mdash;"Linking-up"&mdash;Piston-valves&mdash;Speed governors&mdash;Marine-speed
+governors&mdash;The condenser</div></td>
+<td align='right'><a href="#Chapter_II">44</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter III.&mdash;THE STEAM TURBINE.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+How a turbine works&mdash;The De Laval turbine&mdash;The Parsons turbine&mdash;Description
+of the Parsons turbine&mdash;The expansive action of
+steam in a Parsons turbine&mdash;Balancing the thrust&mdash;Advantages
+of the marine turbine</div></td>
+<td align='right'><a href="#Chapter_III">74</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter IV.&mdash;THE INTERNAL-COMBUSTION ENGINE.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+The meaning of the term&mdash;Action of the internal-combustion engine&mdash;The
+motor car&mdash;The starting-handle&mdash;The engine&mdash;The carburetter&mdash;Ignition
+of the charge&mdash;Advancing the spark&mdash;Governing
+the engine&mdash;The clutch&mdash;The gear-box&mdash;The compensating
+gear&mdash;The silencer&mdash;The brakes&mdash;Speed of cars</div></td>
+<td align='right'><a href="#Chapter_IV">87</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter V.&mdash;ELECTRICAL APPARATUS.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+What is electricity?&mdash;Forms of electricity&mdash;Magnetism&mdash;The permanent
+magnet&mdash;Lines of force&mdash;Electro-magnets&mdash;The electric
+bell&mdash;The induction coil&mdash;The condenser&mdash;Transformation of
+current&mdash;Uses of the induction coil</div></td>
+<td align='right'><a href="#Chapter_V">112</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter VI.&mdash;THE ELECTRIC TELEGRAPH.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+Needle instruments&mdash;Influence of current on the magnetic needle&mdash;Method
+of reversing the current&mdash;Sounding instruments&mdash;Telegraphic
+relays&mdash;Recording telegraphs&mdash;High-speed telegraphy</div></td>
+<td align='right'><a href="#Chapter_VI">127</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter VII.&mdash;WIRELESS TELEGRAPHY.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+The transmitting apparatus&mdash;The receiving apparatus&mdash;Syntonic<br />
+transmission&mdash;The advance of wireless telegraphy</div></td>
+<td align='right'><a href="#Chapter_VII">137</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter VIII.&mdash;THE TELEPHONE.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+The Bell telephone&mdash;The Edison transmitter&mdash;The granular carbon
+transmitter&mdash;General arrangement of a telephone circuit&mdash;Double-line
+circuits&mdash;Telephone exchanges&mdash;Submarine telephony</div></td>
+<td align='right'><a href="#Chapter_VIII">147</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter IX.&mdash;DYNAMOS AND ELECTRIC MOTORS.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+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</div></td>
+<td align='right'><a href="#Chapter_IX">159</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter X.&mdash;RAILWAY BRAKES.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+The Vacuum Automatic brake&mdash;The Westinghouse air-brake</div></td>
+<td align='right'><a href="#Chapter_X">187</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XI.&mdash;RAILWAY SIGNALLING.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+The block system&mdash;Position of signals&mdash;Interlocking the signals&mdash;Locking
+gear&mdash;Points&mdash;Points and signals in combination&mdash;Working
+the block system&mdash;Series of signalling operations&mdash;Single
+line signals&mdash;The train staff&mdash;Train staff and ticket&mdash;Electric
+train staff system&mdash;Interlocking&mdash;Signalling operations&mdash;Power
+signalling&mdash;Pneumatic signalling&mdash;Automatic
+signalling</div></td>
+<td align='right'><a href="#Chapter_XI">200</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XII.&mdash;OPTICS.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+Lenses&mdash;The image cast by a convex lens&mdash;Focus&mdash;Relative position
+of object and lens&mdash;Correction of lenses for colour&mdash;Spherical
+aberration&mdash;Distortion of image&mdash;The human eye&mdash;The use of
+spectacles&mdash;The blind spot</div></td>
+<td align='right'><a href="#Chapter_XII">230</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XIII.&mdash;THE MICROSCOPE, THE TELESCOPE,<br />
+AND THE MAGIC-LANTERN.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+The simple microscope&mdash;Use of the simple microscope in the telescope&mdash;The
+terrestrial telescope&mdash;The Galilean telescope&mdash;The
+prismatic telescope&mdash;The reflecting telescope&mdash;The parabolic
+mirror&mdash;The compound microscope&mdash;The magic-lantern&mdash;The
+bioscope&mdash;The plane mirror</div></td>
+<td align='right'><a href="#Chapter_XIII">253</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XIV.&mdash;SOUND AND MUSICAL INSTRUMENTS.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+Nature of sound&mdash;The ear&mdash;Musical instruments&mdash;The vibration of
+strings&mdash;The sounding-board and the frame of a piano&mdash;The
+strings&mdash;The striking mechanism&mdash;The quality of a note</div></td>
+<td align='right'><a href="#Chapter_XIV">270</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XV.&mdash;WIND INSTRUMENTS.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+Longitudinal vibration&mdash;Columns of air&mdash;Resonance of columns of
+air&mdash;Length and tone&mdash;The open pipe&mdash;The overtones of an
+open pipe&mdash;Where overtones are used&mdash;The arrangement of the
+pipes and pedals&mdash;Separate sound-boards&mdash;Varieties of stops&mdash;Tuning
+pipes and reeds&mdash;The bellows&mdash;Electric and pneumatic
+actions&mdash;The largest organ in the world&mdash;Human reeds</div></td>
+<td align='right'><a href="#Chapter_XV">287</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XVI.&mdash;TALKING-MACHINES.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+The phonograph&mdash;The recorder&mdash;The reproducer&mdash;The gramophone&mdash;The
+making of records&mdash;Cylinder records&mdash;Gramophone
+records</div></td>
+<td align='right'><a href="#Chapter_XVI">310</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XVII.&mdash;WHY THE WIND BLOWS.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+Why the wind blows&mdash;Land and sea breezes&mdash;Light air and moisture&mdash;The
+barometer&mdash;The column barometer&mdash;The wheel barometer&mdash;A
+very simple barometer&mdash;The aneroid barometer&mdash;Barometers
+and weather&mdash;The diving-bell&mdash;The diving-dress&mdash;Air-pumps&mdash;Pneumatic
+tyres&mdash;The air-gun&mdash;The self-closing door-stop&mdash;The
+action of wind on oblique surfaces&mdash;The balloon&mdash;The
+flying-machine</div></td>
+<td align='right'><a href="#Chapter_XVII">322</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XVIII.&mdash;HYDRAULIC MACHINERY.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+The siphon&mdash;The bucket pump&mdash;The force-pump&mdash;The most marvellous
+pump&mdash;The blood channels&mdash;The course of the blood&mdash;The
+hydraulic press&mdash;Household water-supply fittings&mdash;The
+ball-cock&mdash;The water-meter&mdash;Water-supply systems&mdash;The household
+filter&mdash;Gas traps&mdash;Water engines&mdash;The cream separator&mdash;The
+"hydro"</div></td>
+<td align='right'><a href="#Chapter_XVIII">350</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XIX.&mdash;HEATING AND LIGHTING.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+The hot-water supply&mdash;The tank system&mdash;The cylinder system&mdash;How
+a lamp works&mdash;Gas and gasworks&mdash;Automatic stoking&mdash;A
+gas governor&mdash;The gas meter&mdash;Incandescent gas lighting</div></td>
+<td align='right'><a href="#Chapter_XIX">386</a></td>
+</tr>
+
+<tr class='tr1'>
+<td align='center'><b>Chapter XX.&mdash;VARIOUS MECHANISMS.</b></td></tr>
+
+<tr class='tr2'>
+<td align='left'><div class="hang">
+<span class="smcap">Clocks and Watches:</span>&mdash;A short history of timepieces&mdash;The construction
+of timepieces&mdash;The driving power&mdash;The escapement&mdash;Compensating
+pendulums&mdash;The spring balance&mdash;The cylinder
+escapement&mdash;The lever escapement&mdash;Compensated balance-wheels&mdash;Keyless
+winding mechanism for watches&mdash;The hour hand
+train. <span class="smcap">Locks</span>:&mdash;The Chubb lock&mdash;The Yale lock. <span class="smcap">The Cycle</span>:&mdash;The
+gearing of a cycle&mdash;The free wheel&mdash;The change-speed gear.
+<span class="smcap">Agricultural Machines</span>:&mdash;The threshing-machine&mdash;Mowing-machines.
+<span class="smcap">Some Natural Phenomena</span>:&mdash;Why sun-heat varies
+in intensity&mdash;The tides&mdash;Why high tide varies daily</div></td>
+<td align='right'><a href="#Chapter_XX">410</a></td>
+</tr>
+</table></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_13" id="Page_13">[Pg 13]</a></span></p>
+<h2>HOW IT WORKS.</h2>
+
+
+
+<hr />
+<h3><a name="Chapter_I" id="Chapter_I"></a>Chapter I.</h3>
+
+<h4>THE STEAM-ENGINE.</h4>
+
+<div class="blockquot"><p class="hang">What is steam?&mdash;The mechanical energy of steam&mdash;The boiler&mdash;The
+circulation of water in a boiler&mdash;The enclosed furnace&mdash;The
+multitubular boiler&mdash;Fire-tube boilers&mdash;Other types of
+boilers&mdash;Aids to combustion&mdash;Boiler fittings&mdash;The safety-valve&mdash;The
+water-gauge&mdash;The steam-gauge&mdash;The water supply to a boiler.</p></div>
+
+
+<p class="section">WHAT IS STEAM?</p>
+
+<p class="noin"><span class="dcap">I</span><span class="caps">f</span> ice be heated above 32&deg; Fahrenheit, its molecules lose their
+cohesion, and move freely round one another&mdash;the ice is turned into
+water. Heat water above 212&deg; 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<span class='pagenum'><a name="Page_14" id="Page_14">[Pg 14]</a></span> 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.</p>
+
+
+<p class="section">THE MECHANICAL ENERGY OF STEAM.</p>
+
+<p>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&mdash;that is, pressure&mdash;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 <i>is</i> energy.</p>
+
+
+<p class="section">THE BOILER.</p>
+
+<p>The combustion of fuel in a furnace causes the<span class='pagenum'><a name="Page_15" id="Page_15">[Pg 15]</a></span> walls of the furnace to
+become <i>hot</i>, 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&mdash;that is, if not confined in any way&mdash;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
+<i>pressure</i> 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.</p>
+
+<p>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<span class='pagenum'><a name="Page_16" id="Page_16">[Pg 16]</a></span> 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.</p>
+
+<p>A good boiler must be&mdash;(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.</p>
+
+<p>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<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a> 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.</p>
+
+
+<p class="section">CIRCULATION OF WATER IN A BOILER.</p>
+
+<p>If you place a pot filled with water on an open fire, and watch it when
+it boils, you will notice<span class='pagenum'><a name="Page_17" id="Page_17">[Pg 17]</a></span> 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.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 1 and 2">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image2.jpg" width="300" height="425" alt="Fig. 1." title="" />
+<span class="caption"><span class="smcap">Fig. 1.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image3.jpg" width="300" height="392" alt="Fig. 2." title="" />
+<span class="caption"><span class="smcap">Fig. 2.</span></span>
+</div></td></tr>
+</table></div>
+
+<p>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."</p>
+
+<p>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<span class='pagenum'><a name="Page_18" id="Page_18">[Pg 18]</a></span> 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.</p>
+
+<p>We can easily follow out the process of development. In Fig. 3 we see a
+simple <span class="bigletter">U</span>-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.</p>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/image4.jpg" width="300" height="484" alt="Fig. 3." title="" />
+<span class="caption"><span class="smcap">Fig. 3.</span></span>
+</div>
+
+<p>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.</p>
+
+
+<p><span class='pagenum'><a name="Page_19" id="Page_19">[Pg 19]</a></span></p><p class="section">THE ENCLOSED FURNACE.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 4 and 5">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image5.jpg" width="300" height="299" alt="Fig. 4." title="" />
+<span class="caption"><span class="smcap">Fig. 4.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image6.jpg" width="300" height="330" alt="Fig. 5." title="" />
+<span class="caption"><span class="smcap">Fig. 5.</span></span>
+</div></td></tr>
+</table></div>
+
+<p>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 (<a href="#Fig_6">see Fig. 6</a>). 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.</p>
+
+<p><span class='pagenum'><a name="Page_20" id="Page_20">[Pg 20]</a></span></p>
+<div class="figcenter" style="width: 600px;"><a name="Fig_6" id="Fig_6"></a>
+<img src="images/image7.jpg" width="600" height="313" alt="Fig. 6." title="" />
+<span class="caption"><span class="smcap">Fig. 6.</span>&mdash;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.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_21" id="Page_21">[Pg 21]</a></span></p><p class="section">THE MULTITUBULAR BOILER.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/image8.jpg" width="400" height="316" alt="Fig. 7." title="" />
+<span class="caption"><span class="smcap">Fig. 7.</span>&mdash;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.</span>
+</div>
+
+<p>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&mdash;(1) <i>Water</i>-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.<span class='pagenum'><a name="Page_22" id="Page_22">[Pg 22]</a></span> (2)
+<i>Fire</i>-tube boilers; in which the hot gases pass through tubes
+surrounded by water. The ordinary locomotive boiler (Fig. 6) illustrates
+this form.</p>
+
+<p>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&mdash;(1) A drum, <span class="ampm">H</span>, 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
+<span class="ampm">F</span> the flames and hot gases rise round the upper end of the sloping tubes
+<span class="ampm">TT</span> into the space <span class="ampm">A</span>, where they play upon the under surface of <span class="ampm">H</span> before
+plunging downward again among the tubes into the space <span class="ampm">B</span>. Here the
+temperature is lower. The arrows indicate further journeys upwards into
+the space <span class="ampm">C</span> on the right of a fire-brick division, and past the down
+tubes <span class="ampm">SS</span> into <span class="ampm">D</span>, whence the hot gases find an escape into the chimney
+through the opening <span class="ampm">E</span>. It will be noticed that the greatest heat is
+brought to bear on <span class="ampm">TT</span> near their junction with <span class="ampm">UU</span>, the "uptake" tubes;
+and that every succeeding passage of the pipes brings the gradually
+cooling gases nearer to the "downtake" tubes <span class="ampm">SS</span>.</p>
+
+<p><span class='pagenum'><a name="Page_23" id="Page_23">[Pg 23]</a></span></p><p>The pipes <span class="ampm">TT</span> are easily brushed and scraped after the removal of plugs
+from the "headers" into which the tube ends are expanded.</p>
+
+<p>Other well-known water-tube boilers are the Yarrow, Belleville,
+Stirling, and Thorneycroft, all used for driving marine engines.</p>
+
+
+<p class="section">FIRE-TUBE BOILERS.</p>
+
+<p>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 &frac12; to &frac34; inch thick. The shells of the jacket are braced
+together by a large number of rivets, <span class="ampm">RR</span>; 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<span class='pagenum'><a name="Page_24" id="Page_24">[Pg 24]</a></span> 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.</p>
+
+<p>The <i>fire-brick arch</i> 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.</p>
+
+<p>The water circulation in a locomotive boiler is&mdash;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.</p>
+
+
+<p class="section">OTHER TYPES OF BOILERS.</p>
+
+<p>For small stationary land engines the <i>vertical</i><span class='pagenum'><a name="Page_25" id="Page_25">[Pg 25]</a></span> boiler is much used.
+In Fig. 8 we have three forms of this type&mdash;<span class="ampm">A</span> and <span class="ampm">B</span> with cross
+water-tubes; <span class="ampm">C</span> with vertical fire-tubes. The furnace in every case is
+surrounded by water, and fed through a door at one side.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image9.jpg" width="500" height="290" alt="Fig. 8." title="" />
+<span class="caption"><span class="smcap">Fig. 8.</span>&mdash;Diagrammatic representation of three types of
+vertical boilers.</span>
+</div>
+
+<p>The <i>Lancashire</i> 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, <span class="ampm">A</span> and <span class="ampm">BB</span>, shown in cross section in Fig. 9.
+The furnace gases, after leaving the two furnace flues, are deflected
+downwards into the channel <span class="ampm">A</span>, by which they pass underneath the boiler
+to a point<span class='pagenum'><a name="Page_26" id="Page_26">[Pg 26]</a></span> almost under the furnace, where they divide right and left
+and travel through cross passages into the side channels <span class="ampm">BB</span>, to be led
+along the boiler's flanks to the chimney exit <span class="ampm">C</span>. 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image10.jpg" width="500" height="142" alt="Fig. 9." title="" />
+<span class="caption"><span class="smcap">Fig. 9.</span>&mdash;Cross and longitudinal sections of a Lancashire
+boiler.</span>
+</div>
+
+<p>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.</p>
+
+
+<p class="section">AIDS TO COMBUSTION.</p>
+
+<p>We may now turn our attention more particularly to the chemical process
+called <i>combustion</i>, upon<span class='pagenum'><a name="Page_27" id="Page_27">[Pg 27]</a></span> 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.</p>
+
+<p>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.</p>
+
+<p>Now, carbon may unite with oxygen, atom for atom, and form <i>carbon
+monoxide</i> (CO); or in the proportion of one atom of carbon to <i>two</i> of
+<span class='pagenum'><a name="Page_28" id="Page_28">[Pg 28]</a></span>oxygen, and form <i>carbon dioxide</i> (CO<sub>2</sub>). The former gas is
+combustible&mdash;that is, will admit another atom of carbon to the
+molecule&mdash;but the latter is saturated with oxygen, and will not burn,
+or, to put it otherwise, is the product of <i>perfect</i> 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 CO<sub>2</sub> 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 <i>pounds</i> of
+air for every pound of fuel. There are two methods of creating a violent
+draught through the furnace. The first is&mdash;</p>
+
+<p>The <i>forced draught</i>; very simply exemplified by the ordinary bellows
+used in every house. On a ship (Fig. 10) the principle is developed as
+follows:&mdash;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"&mdash;namely,
+through the furnace<span class='pagenum'><a name="Page_29" id="Page_29">[Pg 29]</a></span> 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&deg; Fahrenheit or more, and the poor stokers have a
+very bad time of it.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image11.jpg" width="500" height="238" alt="Fig. 10." title="" />
+<span class="caption"><span class="smcap">Fig. 10.</span>&mdash;Sketch showing how the "forced draught" is
+produced in a stokehold and how it affects the furnaces.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_30" id="Page_30">[Pg 30]</a></span></p>
+<div class="figcenter" style="width: 522px;"><br />
+<img src="images/image12.jpg" width="522" height="337" alt="SCENE IN THE STOKEHOLD OF A BATTLE-SHIP." title="" />
+<span class="caption">SCENE IN THE STOKEHOLD OF A BATTLE-SHIP.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_31" id="Page_31">[Pg 31]</a></span></p><p>The second system is that of the <i>induced draught</i>. Here air is
+<i>sucked</i> 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&mdash;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 (<a href="#Page_72">p. 72</a>).</p>
+
+
+<p class="section">BOILER FITTINGS.</p>
+
+<p>The most important fittings on a boiler are:&mdash;(1) the safety-valve; (2)
+the water-gauge; (3) the steam-gauge; (4) the mechanisms for feeding it
+with water.</p>
+
+
+<p class="section">THE SAFETY-VALVE.</p>
+
+<p>Professor Thurston, an eminent authority on the steam-engine, has
+estimated that a plain cylindrical<span class='pagenum'><a name="Page_32" id="Page_32">[Pg 32]</a></span> boiler carrying 100 lbs. pressure to
+the square inch contains sufficient stored energy to project it into the
+air a vertical distance of 3&frac12; miles. In the case of a Lancashire
+boiler at equal pressure the distance would be 2&frac12; miles; of a
+locomotive boiler, at 125 lbs., 1&frac12; 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 <i>about
+the same energy as one pound of gunpowder</i>.</p>
+
+<p>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 <i>safety-valve</i>.
+It usually blows off at less than half the greatest pressure that the
+boiler has been proved by experiment to be capable of withstanding.</p>
+
+<p>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<span class='pagenum'><a name="Page_33" id="Page_33">[Pg 33]</a></span> of the weight
+or spring, the plug rises, and steam escapes until equilibrium of the
+opposing forces is restored.</p>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image13.jpg" width="500" height="186" alt="Fig. 11." title="" />
+<span class="caption"><span class="smcap">Fig. 11.</span>&mdash;<span class="smcap">A Lever Safety-Valve.</span> <span class="ampm">V</span>, valve; <span class="ampm">S</span>, seating; <span class="ampm">P</span>,
+pin; <span class="ampm">L</span>, lever; <span class="ampm">F</span>, fulcrum; <span class="ampm">W</span>, 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.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_34" id="Page_34">[Pg 34]</a></span> would not be attended by an explosion, as water is very
+inelastic.</p>
+
+<p>The days when an engineer could "sit on the valves"&mdash;that is, screw them
+down&mdash;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.</p>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_35" id="Page_35">[Pg 35]</a></span></p><p>The following is taken from a journal, dated December 22, 1895:
+"<i>Providence</i> (<i>Rhode Island</i>).&mdash;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."</p>
+
+
+<p class="section">THE WATER-GAUGE.</p>
+
+<p>No fitting of a boiler is more important than the <i>water-gauge</i>, 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, <span class="ampm">G</span>, to make a steam-tight
+joint round the glass tube, which is inserted through the hole covered
+by the plug <span class="ampm">P<sup>1</sup></span>. The cocks <span class="ampm">T<sup>1</sup> T<sup>2</sup></span> are normally open, allowing the
+ingress of steam and water respectively to the tube. Cock <span class="ampm">T<sup>3</sup></span> is kept
+closed unless for any reason it is necessary to blow steam or water
+<span class='pagenum'><a name="Page_36" id="Page_36">[Pg 36]</a></span>through the gauge. The holes <span class="ampm">C C</span> can be cleaned out if the plugs <span class="ampm">P<sup>2</sup>
+P<sup>3</sup></span> are removed.</p>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/image14.jpg" width="300" height="569" alt="Fig. 12." title="" />
+<span class="caption"><span class="smcap">Fig. 12.</span>&mdash;Section of a water-gauge.</span>
+</div>
+
+<p>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.</p>
+
+<p>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.</p>
+
+
+<p class="section">THE STEAM-GAUGE.</p>
+
+<p>It is of the utmost importance that a person in charge of a boiler
+should know what pressure the<span class='pagenum'><a name="Page_37" id="Page_37">[Pg 37]</a></span> steam has reached. Every boiler is
+therefore fitted with one <i>steam-gauge</i>; many with two, lest one might
+be unreliable. There are two principal types of steam-gauge:&mdash;(1) The
+Bourdon; (2) the Sch&auml;ffer-Budenberg. The principle of the Bourdon is
+illustrated by Fig. 13, in which <span class="ampm">A</span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image15.jpg" width="500" height="126" alt="Fig. 13." title="" />
+<span class="caption"><span class="smcap">Fig. 13.</span>&mdash;Showing the principle of the steam-gauge.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_38" id="Page_38">[Pg 38]</a></span></p>
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image16.jpg" width="400" height="568" alt="Fig. 14." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 14.</span>&mdash;Bourdon steam-gauge. Part of dial removed to
+show mechanism.</span>
+</div>
+
+<p>In Fig. 14 we have a Bourdon gauge, with part of the dial face broken
+away to show the internal mechanism. <span class="ampm">T</span> 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,
+<span class="ampm">L</span>, which works an arm of a quadrant rack, <span class="ampm">R</span>, engaging with a small
+pinion, <span class="ampm">P</span>, actuating the pointer. As the steam pressure rises,<span class='pagenum'><a name="Page_39" id="Page_39">[Pg 39]</a></span> the tube
+<span class="ampm">T</span> 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.</p>
+
+<p>The Sch&auml;ffer-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 (<a href="#Page_329">p.
+329</a>).</p>
+
+
+<p class="section">THE WATER SUPPLY TO A BOILER.</p>
+
+<p>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 <i>injector</i>, 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<span class='pagenum'><a name="Page_40" id="Page_40">[Pg 40]</a></span> an injector. Steam is led from the boiler through pipe <span class="ampm">A</span>,
+which terminates in a nozzle surrounded by a cone, <span class="ampm">E</span>, connected by the
+pipe <span class="ampm">B</span> with the water tank. When steam is turned on it rushes with
+immense velocity from the nozzle, and creates a partial vacuum in cone
+<span class="ampm">E</span>, which soon fills with water. On meeting the water the steam
+condenses, but not before it has imparted some of its <i>velocity</i> to the
+water, which thus gains sufficient momentum to force down the valve and
+find its way to the boiler. The overflow space <span class="ampm">O O</span> between <span class="ampm">E</span> and <span class="ampm">C</span>
+allows steam and water to escape until the water has gathered the
+requisite momentum.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image17.jpg" width="400" height="300" alt="Fig. 15." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 15.</span>&mdash;Diagram illustrating the principle of a
+steam-injector.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_41" id="Page_41">[Pg 41]</a></span></p>
+<div class="figcenter" style="width: 400px;"><br />
+<img src="images/image18.jpg" width="400" height="638" alt="Fig. 16." title="" />
+<span class="caption"><span class="smcap">Fig. 16.</span>&mdash;The Giffard injector.</span>
+</div>
+
+<p>A form of injector very commonly used is Giffard's (Fig. 16). Steam is
+allowed to enter by screwing up the valve <span class="ampm">V</span>. As it rushes through the
+nozzle of the cone <span class="ampm">A</span> it takes up water and projects it into the "mixing
+cone" <span class="ampm">B</span>, which can be raised or lowered by the pinion <span class="ampm">D</span> (worked by the
+hand-wheel wheel shown) so as to regulate the amount of water admitted
+to <span class="ampm">B</span>.<span class='pagenum'><a name="Page_42" id="Page_42">[Pg 42]</a></span> At the centre of <span class="ampm">B</span> is an aperture, <span class="ampm">O</span>, communicating with the
+overflow. The water passes to the boiler through the valve on the left.
+It will be noticed that the cone <span class="ampm">A</span> and the part of <span class="ampm">B</span> above the orifice <span class="ampm">O</span>
+contract downward. This is to convert the <i>pressure</i> of the steam into
+<i>velocity</i>. Below <span class="ampm">O</span> is a cone, the diameter of which increases
+downwards. Here the <i>velocity</i> of the water is converted back into
+<i>pressure</i> in obedience to a well-known hydromechanic law.</p>
+
+<p>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.<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> Some injectors have flap-valves covering the overflow orifice,
+to prevent air being sucked in and carried to the boiler.</p>
+
+<p>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"&mdash;that is,
+stop momentarily&mdash;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 <i>self-starting</i> variety,
+which automatically<span class='pagenum'><a name="Page_43" id="Page_43">[Pg 43]</a></span> controls the admission of water to the
+"mixing-cone," and allows the injector to "pick up" of itself.</p>
+
+<p>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.</p>
+
+
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> "The Steam-Engine," p. 3.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> 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.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_44" id="Page_44">[Pg 44]</a></span></p>
+<h3><a name="Chapter_II" id="Chapter_II"></a>Chapter II.</h3>
+
+<h4>THE CONVERSION OF HEAT ENERGY INTO MECHANICAL MOTION.</h4>
+
+<div class="blockquot"><p class="hang">Reciprocating engines&mdash;Double-cylinder engines&mdash;The function of the
+fly-wheel&mdash;The cylinder&mdash;The slide-valve&mdash;The eccentric&mdash;"Lap" of
+the valve: expansion of steam&mdash;How the cut-off is managed&mdash;Limit of
+expansive working&mdash;Compound engines&mdash;Arrangement of expansion
+engines&mdash;Compound locomotives&mdash;Reversing
+gears&mdash;"Linking-up"&mdash;Piston-valves&mdash;Speed governors&mdash;Marine-speed
+governors&mdash;The condenser.</p></div>
+
+
+<p class="noin"><span class="dcap">H</span><span class="caps">aving</span> 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
+<i>work</i>.</p>
+
+<p>Steam-engines are of two kinds:&mdash;(1) <i>reciprocating</i>, employing
+cylinders and cranks; (2) <i>rotary</i>, called turbines.</p>
+
+
+<p class="section">RECIPROCATING ENGINES.</p>
+
+<p><span class='pagenum'><a name="Page_45" id="Page_45">[Pg 45]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image19.jpg" width="500" height="268" alt="Fig. 17." title="" />
+<span class="caption"><span class="smcap">Fig. 17.</span>&mdash;Sketch showing parts of a horizontal
+steam-engine.</span>
+</div>
+
+<p>Fig. 17 is a skeleton diagram of the simplest form of reciprocating
+engine. <span class="ampm">C</span> is a <i>cylinder</i> to which steam is admitted through the
+<i>steam-ways</i><a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a> <span class="ampm">W W</span>, first on one side of the piston <span class="ampm">P</span>, then on the
+other. The pressure on the piston pushes it along the cylinder, and the
+force is transmitted through the piston rod <span class="ampm">P R</span> to the <i>connecting rod</i>
+<span class="ampm">C R</span>, which causes the <i>crank</i> <span class="ampm">K</span> to revolve. At the point where the two
+rods meet there is a "crosshead," <span class="ampm">H</span>, 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 <span class="ampm">C R</span> to the crank <span class="ampm">K</span>. The latter is keyed
+to a <i>shaft</i> <span class="ampm">S</span> 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 <i>bore</i>. The travel of the piston is
+called its <i>stroke</i>. The distance from the centre of the shaft to the
+centre of the crank pin is called the crank's <i>throw</i>, which is half of
+the piston's <i>stroke</i>. 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&mdash;that
+is, when the piston is fully out, or fully in&mdash;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.</p>
+
+<p><span class='pagenum'><a name="Page_46" id="Page_46">[Pg 46]</a></span></p>
+<div class="figcenter" style="width: 600px;">
+<img src="images/image20.jpg" width="600" height="380" alt="Fig. 18." title="" />
+<span class="caption"><span class="smcap">Fig. 18.</span>&mdash;Sectional plan of a horizontal engine.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_47" id="Page_47">[Pg 47]</a></span></p><p class="section">DOUBLE-CYLINDER ENGINES.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image21.jpg" width="500" height="107" alt="Fig. 19." title="" />
+<span class="caption"><span class="smcap">Fig. 19.</span></span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image22.jpg" width="500" height="107" alt="Fig. 20." title="" />
+<span class="caption"><span class="smcap">Fig. 20.</span></span>
+</div>
+
+<p>Locomotive, marine, and all other engines which must be started in any
+position have at least <i>two</i> cylinders, and as many cranks set at an
+<span class='pagenum'><a name="Page_48" id="Page_48">[Pg 48]</a></span>angle to one another. Fig. 19 demonstrates that when one crank, <span class="ampm">C<sub>1</sub></span>,
+of a double-cylinder engine is at a "dead point," the other, <span class="ampm">C<sub>2</sub></span>, has
+reached a position at which the piston exerts the maximum of turning
+power. In Fig. 20 each crank is at 45&deg; 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 <i>single</i>-action
+cylinders are used, at least <i>three</i> of these are needed to produce a
+perpetual turning movement, independently of a fly-wheel.</p>
+
+
+<p class="section">THE FUNCTION OF THE FLY-WHEEL.</p>
+
+<p>A fly-wheel acts as a <i>reservoir of energy</i>, 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 <i>his</i> dead points&mdash;that is, those
+parts of the circle described by the handle in which he can do little
+work.</p>
+
+
+<p class="section">THE CYLINDER.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image23.jpg" width="500" height="427" alt="Fig. 21." title="" />
+<span class="caption"><span class="smcap">Fig. 21.</span>&mdash;Diagrammatic section of a cylinder and its
+slide-valve.</span>
+</div>
+
+<p>The cylinders of an engine take the place of the<span class='pagenum'><a name="Page_49" id="Page_49">[Pg 49]</a></span> 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 <span class="ampm">P</span>, the piston. Round it are white
+grooves, <span class="ampm">R R</span>, 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"&mdash;that is, wear to a true fit in the
+cylinder. Each end of<span class='pagenum'><a name="Page_50" id="Page_50">[Pg 50]</a></span> 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 <i>gland</i>, <span class="ampm">G<sup>1</sup></span>, 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, <span class="ampm">W W</span>, were it not for the</p>
+
+
+<p class="section">SLIDE-VALVE,</p>
+
+<p class="noin">a hollow box open at the bottom, and long enough for its edges to cover
+both steam-ways at once. Between <span class="ampm">W W</span> is <span class="ampm">E</span>, 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<span class='pagenum'><a name="Page_51" id="Page_51">[Pg 51]</a></span>
+on the right begins to open. The steam-way on the left is now in
+communication with the exhaust port <span class="ampm">E</span>, so that the steam that has done
+its duty is released and pressed from the cylinder by the piston.
+<i>Reciprocation</i> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image24.jpg" width="500" height="383" alt="Fig. 22." title="" />
+<span class="caption"><span class="smcap">Fig. 22.</span>&mdash;Perspective section of cylinder.</span>
+</div>
+
+<p>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&mdash;that is, the high-pressure&mdash;steam-engine; and that, owing to
+the imperfection of the existing<span class='pagenum'><a name="Page_52" id="Page_52">[Pg 52]</a></span> 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 <span class="above">1</span>&#8260;<span class="below">500</span> of
+an inch out of truth; and in small petrol-engines <span class="above">1</span>&#8260;<span class="below">5000</span> of an inch is
+sometimes the greatest "limit of error" allowed.</p>
+
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image25.jpg" width="500" height="154" alt="Fig. 23." title="" />
+<span class="caption"><span class="smcap">Fig. 23.</span>&mdash;The eccentric and its rod.</span>
+</div>
+
+<p class="section">THE ECCENTRIC</p>
+
+<p class="noin">is used to move the slide-valve to and fro over the steam ports (Fig.
+23). It consists of three main parts&mdash;the <i>sheave</i>, or circular plate <span class="ampm">S</span>,
+mounted on the crank shaft; and the two <i>straps</i> 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, <span class="ampm">B B</span>, passing
+through lugs, or thickenings at the ends of the semicircles. The sheave
+has a deep groove all round the edges,<span class='pagenum'><a name="Page_53" id="Page_53">[Pg 53]</a></span> in which the straps ride. The
+"eccentricity" or "throw" of an eccentric is the distance between <span class="ampm">C<sup>2</sup></span>,
+the centre of the shaft, and <span class="ampm">C<sup>1</sup></span>, 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, <span class="ampm">K</span>, 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.</p>
+
+<p>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 <span class="bigletter">V</span>-shaped
+bend in the shaft itself&mdash;in which case its position is of course
+permanent.</p>
+
+
+<p class="section">SETTING OF THE SLIDE-VALVE AND ECCENTRIC.</p>
+
+<p>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<span class='pagenum'><a name="Page_54" id="Page_54">[Pg 54]</a></span> will glance at some of
+the more elementary principles.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image26.jpg" width="500" height="201" alt="Fig. 24." title="" />
+<span class="caption"><span class="smcap">Fig. 24.</span></span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image27.jpg" width="500" height="183" alt="Fig. 25." title="" />
+<span class="caption"><span class="smcap">Fig. 25.</span></span>
+</div>
+
+<p>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&mdash;the
+thicker, <span class="ampm">C</span>, representing the position of the crank; the thinner, <span class="ampm">E</span>, 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&mdash;that is, they have no
+<i>lap</i>. The piston is about to commence its stroke towards the left; and
+the eccentric,<span class='pagenum'><a name="Page_55" id="Page_55">[Pg 55]</a></span> which is set at an angle of 90&deg; in <i>advance</i> of the
+crank, is about to begin opening the left-hand port. By the time that <span class="ampm">C</span>
+has got to the position originally occupied by <span class="ampm">E</span>, <span class="ampm">E</span> will be horizontal
+(Fig. 25)&mdash;that is, the eccentric will have finished its stroke towards
+the left; and while <span class="ampm">C</span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image28.jpg" width="500" height="195" alt="Fig. 26." title="" />
+<span class="caption"><span class="smcap">Fig. 26.</span></span>
+</div>
+
+<p>It must be noticed here&mdash;(1) that steam is admitted at full pressure
+<i>all through</i> 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
+<i>cushion</i> 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<span class='pagenum'><a name="Page_56" id="Page_56">[Pg 56]</a></span> than 90&deg; in advance&mdash;that is, more than what the engineers call
+<i>square</i>. Fig. 26 shows such an arrangement. The angle between <span class="ampm">E</span> and
+<span class="ampm">E<sup>1</sup></span> is called the <i>angle of advance</i>. 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.</p>
+
+
+<p class="section">"LAP" OF THE VALVE&mdash;EXPANSION OF STEAM.</p>
+
+<p>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
+<i>square</i> with the crank, the admission of steam lasts until the very end
+of the stroke; if set a little in advance&mdash;that is, given <i>lead</i>&mdash;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 <i>expansion</i> 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<span class='pagenum'><a name="Page_57" id="Page_57">[Pg 57]</a></span> 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 <i>average</i> 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 <i>work</i>.</p>
+
+
+<p class="section">HOW THE CUT-OFF IS MANAGED.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image29.jpg" width="500" height="170" alt="Fig. 27." title="" />
+<span class="caption"><span class="smcap">Fig. 27.</span>&mdash;A slide-valve with "lap."</span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image30.jpg" width="500" height="185" alt="Fig. 28." title="" />
+<span class="caption"><span class="smcap">Fig. 28.</span></span>
+</div>
+
+<p>Look at Fig. 27. Here we have a slide-valve, with faces much wider than
+the steam ports. The parts marked black, <span class="ampm">P P</span>, are those corresponding to
+the faces of the valves shown in previous diagrams (<a href="#Page_54">p. 54</a>). The shaded
+parts, <span class="ampm">L L</span>, are called the <i>lap</i>. 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.<span class='pagenum'><a name="Page_58" id="Page_58">[Pg 58]</a></span> 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:&mdash;Travel of valve = 2 &times; (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 <span class="ampm">A</span>, is now very
+considerable. By the time that the crank <span class="ampm">C</span> has assumed the position of
+the line <span class="ampm">S</span>, 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.</p>
+
+<p>If the valve has to have "lead" to admit steam <i>before</i> the end of the
+stroke to the other side of the piston, the <i>angle of advance</i> must be
+increased, and the eccentric centre line would lie on the line <span class="ampm">E<sup>2</sup></span>.
+Therefore&mdash;total angle of advance = angle for <i>lap</i> and angle for
+<i>lead</i>.</p>
+
+
+<p><span class='pagenum'><a name="Page_59" id="Page_59">[Pg 59]</a></span></p><p class="section">LIMIT OF EXPANSIVE WORKING.</p>
+
+<p>Theoretically, by increasing the <i>lap</i> 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&mdash;namely, that <i>as the
+steam expands its temperature falls</i>. 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?</p>
+
+
+<p class="section">COMPOUND ENGINES.</p>
+
+<p>In the year 1853, John Elder, founder of the shipping firm of Elder and
+Co., Glasgow, introduced the <i>compound</i> 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.</p>
+
+<p><span class='pagenum'><a name="Page_60" id="Page_60">[Pg 60]</a></span></p>
+<div class="figcenter" style="width: 600px;">
+<img src="images/image31.jpg" width="600" height="381" alt="Fig. 29." title="" />
+<span class="caption"><span class="smcap">Fig. 29.</span>&mdash;Sketch of the arrangement of a
+triple-expansion marine engine. No valve gear or supports, etc., shown.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_61" id="Page_61">[Pg 61]</a></span></p><p>In Fig. 29 we have a triple-expansion marine engine. Steam enters the
+high-pressure cylinder<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a> 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 <span class="ampm">C</span>. In fact, the condenser exerts a <i>sucking</i> power on the
+exhaust side of <span class="ampm">C</span>'s piston.</p>
+
+
+<p class="section">ARRANGEMENT OF EXPANSION ENGINES.</p>
+
+<p>In the illustration the cranks are set at angles of 120&deg;, 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<span class='pagenum'><a name="Page_62" id="Page_62">[Pg 62]</a></span> cylinders are
+often arranged <i>tandem</i>, 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.</p>
+
+
+<p class="section">COMPOUND LOCOMOTIVES.</p>
+
+<p>In 1876 Mr. A. Mallet introduced <i>compounding</i> in locomotives; and the
+practice has been largely adopted. The various types of "compounds" may
+be classified as follows:&mdash;(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.</p>
+
+
+<p class="section">REVERSING GEARS.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image32.jpg" width="500" height="342" alt="Figs. 30, 31, 32." title="" />
+<span class="caption"><span class="smcap">Figs. 30, 31, 32.</span>&mdash;Showing how a reversing gear alters
+the position of the slide-valve.</span>
+</div>
+
+<p>The engines of a locomotive or steamship must be reversible&mdash;that is,
+when steam is admitted to the<span class='pagenum'><a name="Page_63" id="Page_63">[Pg 63]</a></span> 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. <span class="ampm">E<sup>1</sup></span> and <span class="ampm">E<sup>2</sup></span> are two
+eccentrics set square with the crank at opposite ends of a diameter.
+Their rods are connected to the ends of a link, <span class="ampm">L</span>, which can be raised
+and lowered by means of levers (not shown). <span class="ampm">B</span> is a block which can
+partly revolve on a pin projecting<span class='pagenum'><a name="Page_64" id="Page_64">[Pg 64]</a></span> from the valve rod, working through
+a guide, <span class="ampm">G</span>. In Fig. 31 the link is half raised, or in "mid-gear," as
+drivers say. Eccentric <span class="ampm">E<sup>1</sup></span> has pushed the lower end of the link fully
+back; <span class="ampm">E<sup>2</sup></span> 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.</p>
+
+<p>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 <i>raises</i> the link,
+which brings the rod of <span class="ampm">E<sup>1</sup></span> into line with the valve rod and presses
+the block <i>backwards</i> 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
+<i>drop</i> the link, bringing the rod of <span class="ampm">E<sup>2</sup></span> into line with the valve rod,
+and drawing <span class="ampm">V</span> <i>forward</i> to uncover the rear port (Fig. 32). In either
+case the eccentric working the end of the link remote<span class='pagenum'><a name="Page_65" id="Page_65">[Pg 65]</a></span> from <span class="ampm">B</span> has no
+effect, since it merely causes that end to describe arcs of circles of
+which <span class="ampm">B</span> is the centre.</p>
+
+
+<p class="section">"LINKING UP."</p>
+
+<p>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
+<i>full</i>, or <i>end</i>, 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.</p>
+
+
+<p class="section">OTHER GEARS.</p>
+
+<p>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<span class='pagenum'><a name="Page_66" id="Page_66">[Pg 66]</a></span> that when the link is raised the block is lowered, and <i>vice
+vers&acirc;</i>. These are really only modifications of Stephenson's
+principle&mdash;namely, the employment of <i>two</i> 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 <i>radial</i>
+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.</p>
+
+<p>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<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a> 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<span class='pagenum'><a name="Page_67" id="Page_67">[Pg 67]</a></span> 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."<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a> One would
+imagine that in modern shunting yards such a device would somewhat delay
+operations!</p>
+
+
+<p class="section">PISTON VALVES.</p>
+
+<p>In marine engines, and on many locomotives and some stationary engines,
+the <span class="bigletter">D</span>-valve (shown in Figs. 30&ndash;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 <span class="bigletter">D</span>-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 <span class="bigletter">D</span>-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.</p>
+
+
+<p class="section">SPEED GOVERNORS.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image33.jpg" width="500" height="519" alt="Fig. 33." title="" />
+<span class="caption"><span class="smcap">Fig. 33.</span>&mdash;A speed governor.</span>
+</div>
+
+<p>Practically all engines except locomotives and those<span class='pagenum'><a name="Page_68" id="Page_68">[Pg 68]</a></span> known as
+"donkey-engines"&mdash;used on cranes&mdash;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, <span class="ampm">P</span>, at the foot of the
+governor. This transmits motion through two bevel-wheels, <span class="ampm">G</span>, to a
+vertical shaft, from the top of which hang two heavy balls on links, <span class="ampm">K
+K</span>.<span class='pagenum'><a name="Page_69" id="Page_69">[Pg 69]</a></span> Two more links, <span class="ampm">L L</span>, connect the balls with a weight, <span class="ampm">W</span>, 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 <span class="ampm">K K</span> and <span class="ampm">L L</span> become less and
+less obtuse, and the weight <span class="ampm">W</span> is drawn upwards, bringing with it the
+fork <span class="ampm">C</span> of the rod <span class="ampm">A</span>, which has ends engaging with the groove. As <span class="ampm">C</span>
+rises, the other end of the rod is depressed, and the rod <span class="ampm">B</span> depresses
+rod <span class="ampm">O</span>, 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<span class='pagenum'><a name="Page_70" id="Page_70">[Pg 70]</a></span> the crank
+shaft without the intervention of bevel gearing.</p>
+
+<div class="figright" style="width: 300px;">
+<img src="images/image34.jpg" width="300" height="308" alt="Fig. 34." title="" />
+<span class="caption"><span class="smcap">Fig. 34.</span></span>
+</div>
+
+<p>The Hartwell governor employs a link motion. You must here picture the
+balls raising and lowering the <i>free end</i> 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 the
+governor, diminishing the movement of the valve, and so shortening the
+period of steam admission to the cylinder.</p>
+
+<p>Governors are of special importance where the <i>load</i> 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.&mdash;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 <i>very</i> 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.</p>
+
+
+<p><span class='pagenum'><a name="Page_71" id="Page_71">[Pg 71]</a></span></p><p class="section">MARINE GOVERNORS.</p>
+
+<p>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.</p>
+
+
+<p class="section">CONDENSERS.</p>
+
+<p>The <i>condenser</i> serves two purposes:&mdash;(1) It makes it possible to use
+the same water over and over<span class='pagenum'><a name="Page_72" id="Page_72">[Pg 72]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image35.jpg" width="500" height="312" alt="Fig. 35." title="" />
+<span class="caption"><span class="smcap">Fig. 35.</span>&mdash;The marine condenser.</span>
+</div>
+
+<p>Fig. 35 is a sectional illustration of a marine condenser. Steam enters
+the condenser through the large pipe <span class="ampm">E</span>, 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 <span class="ampm">A</span> into the lower part of a large cap covering
+one end of the condenser and divided<span class='pagenum'><a name="Page_73" id="Page_73">[Pg 73]</a></span> transversely by a diaphragm, <span class="ampm">D</span>.
+Passing through the pipes, it reaches the cap attached to the other end,
+and flows back through the upper tubes to the outlet <span class="ampm">C</span>. 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 <span class="ampm">F</span>. In some condensers the positions of steam and water are
+reversed, steam going through the tubes outside which cold water
+circulates.</p>
+
+
+<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a> Also called <i>ports</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> The bores of the cylinders are in the proportion of 4: 6:
+9. The stroke of all three is the same.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> The ends furthest from the eccentric.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> "The Locomotive of To-day," p. 87.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_74" id="Page_74">[Pg 74]</a></span></p>
+<h3><a name="Chapter_III" id="Chapter_III"></a>Chapter III.</h3>
+
+<h4>THE STEAM TURBINE.</h4>
+
+<div class="blockquot"><p class="hang">How a turbine works&mdash;The De Laval turbine&mdash;The Parsons
+turbine&mdash;Description of the Parsons turbine&mdash;The expansive action
+of steam in a Parsons turbine&mdash;Balancing the thrust&mdash;Advantages of
+the marine turbine. </p></div>
+
+
+<p class="noin"><span class="dcap">M</span><span class="caps">ore</span> 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 <span class="bigletter">L</span>-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<span class='pagenum'><a name="Page_75" id="Page_75">[Pg 75]</a></span> cauldron, steam
+was generated. It passed up through the upright, through the pivots, and
+into the globe, from which it escaped by the two <span class="bigletter">L</span>-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.</p>
+
+
+<p class="section">HOW A TURBINE WORKS.</p>
+
+<p>In reciprocating&mdash;that is, cylinder&mdash;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&mdash;that is, the
+piston&mdash;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 <i>reaction</i>&mdash;namely, the
+Hero-type&mdash;turbine the nozzle from which the steam or water issues<span class='pagenum'><a name="Page_76" id="Page_76">[Pg 76]</a></span>
+moves, along with bodies to which it may be attached. In <i>action</i>
+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.</p>
+
+
+<p class="section">THE DE LAVAL TURBINE.</p>
+
+<p>In its earliest form this turbine was a modification of Hero's. The
+wheel was merely a pipe bent in <span class="bigletter">S</span> 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 <span class="bigletter">S</span>,
+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.</p>
+
+<div class="figleft" style="width: 400px;">
+<img src="images/image36.jpg" width="364" height="352" alt="Fig. 36." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 36.</span>&mdash;The wheel and nozzles of a De Laval turbine.</span>
+</div>
+
+<p>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 is of curved section, the concave side
+directed towards the nozzles, which, as will be<span class='pagenum'><a name="Page_77" id="Page_77">[Pg 77]</a></span> 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<span class='pagenum'><a name="Page_78" id="Page_78">[Pg 78]</a></span> 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 <i>minute</i>&mdash;that
+is, at a speed which would take it right round the world in 8&frac12; hours!
+The wheel itself would not move at more than about one-third of this
+speed as a maximum.<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a> But even so, it may make as many as 30,000
+revolutions per minute. A mechanical difficulty is now
+encountered&mdash;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&mdash;perhaps only a tiny fraction of an ounce&mdash;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<span class='pagenum'><a name="Page_79" id="Page_79">[Pg 79]</a></span> in which it
+revolves to pieces, if&mdash;and this is the point&mdash;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.</p>
+
+<p>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.</p>
+
+
+<p class="section">THE PARSONS TURBINE.</p>
+
+<p>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
+<i>Turbinia</i> of 44&frac12; 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 <i>Turbinia</i> 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<span class='pagenum'><a name="Page_80" id="Page_80">[Pg 80]</a></span> not far distant when reciprocating engines
+will be abandoned on all high-speed craft.</p>
+
+
+<p class="section">DESCRIPTION OF THE PARSONS TURBINE.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image37.jpg" width="500" height="197" alt="Fig. 37." title="" />
+<span class="caption"><span class="smcap">Fig. 37.</span>&mdash;Section of a Parsons turbine.</span>
+</div>
+
+<p>The essential parts of a Parsons turbine are:&mdash;(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, <span class="ampm">D<sup>1</sup></span>, <span class="ampm">D<sup>2</sup></span>,
+<span class="ampm">D<sup>3</sup></span>, towards the right. From end to end it is studded with little
+vanes, <span class="ampm">M M</span>, set in parallel rings small distances apart. Each vane has a
+curved section (<a href="#Fig_38">see Fig. 38</a>), the hollow side facing towards the left.
+The vanes stick out from the drum like<span class='pagenum'><a name="Page_81" id="Page_81">[Pg 81]</a></span> short spokes, and their outer
+ends almost touch the casing. To the latter are attached equally-spaced
+rings of fixed vanes, <span class="ampm">F F</span>, 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 <span class="ampm">M M</span>. Steam enters the casing at <span class="ampm">A</span>,
+and at once rushes through the vanes towards the outlet at <span class="ampm">B</span>. 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
+<span class="ampm">F F</span>, so that it may treat the next row of <span class="ampm">M M</span> in a similar fashion.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_38" id="Fig_38"></a>
+<img src="images/image38.jpg" width="500" height="192" alt="Fig. 38." title="" />
+<span class="caption"><span class="smcap">Fig. 38.</span>&mdash;Blades or vanes of a Parsons turbine.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_82" id="Page_82">[Pg 82]</a></span></p>
+<div class="figcenter" style="width: 519px;"><br />
+<img src="images/image39.jpg" width="519" height="332" alt="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." title="" />
+<span class="caption">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.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_83" id="Page_83">[Pg 83]</a></span></p><p class="section">THE EXPANSIVE ACTION OF STEAM IN A TURBINE.</p>
+
+<p>On reaching the end of <span class="ampm">D<sup>1</sup></span> 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
+<span class="ampm">D<sup>3</sup></span>, the low-pressure drum. The steam then escapes to the condenser
+through <span class="ampm">B</span>, 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.</p>
+
+<p>The vanes are made of brass. In the turbines of the <i>Carmania</i>, 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.</p>
+
+
+<p class="section">BALANCING OF THRUST.</p>
+
+<p>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, <span class="ampm">P<sup>1</sup></span>, <span class="ampm">P<sup>2</sup></span>, <span class="ampm">P<sup>3</sup></span>.
+Each dummy consists of a number of discs revolving between rings
+projecting from the casing, the distance<span class='pagenum'><a name="Page_84" id="Page_84">[Pg 84]</a></span> between discs and rings being
+so small that but little steam can pass. In the high-pressure
+compartment the steam pushes <span class="ampm">P<sup>1</sup></span> to the left with the same pressure as
+it pushes the blades of <span class="ampm">D<sup>1</sup></span> to the right. After completing the first
+stage it fills the passage <span class="ampm">C</span>, which communicates with the second piston,
+<span class="ampm">P<sup>2</sup></span>, and the pressure on that piston negatives the thrust on <span class="ampm">D<sup>2</sup></span>.
+Similarly, the passage <span class="ampm">E</span> causes the steam to press equally on <span class="ampm">P<sup>3</sup></span> and
+the vanes of <span class="ampm">D<sup>3</sup></span>. 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.</p>
+
+<p><span class='pagenum'><a name="Page_85" id="Page_85">[Pg 85]</a></span></p>
+<div class="figcenter" style="width: 536px;">
+<img src="images/image40.jpg" width="536" height="330" alt="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.
+" title="" />
+<span class="caption">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.
+</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_86" id="Page_86">[Pg 86]</a></span></p><p class="section">ADVANTAGES OF THE MARINE TURBINE.</p>
+
+<p>(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&mdash;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.</p>
+
+<p>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.</p>
+
+<p>The highest speed ever attained on the sea was the forty-two miles per
+hour of the unfortunate <i>Viper</i>, 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.</p>
+
+
+<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> 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 &frac12; oz.) exerts under these conditions a
+centrifugal pull of 15 cwt. on the wheel!</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_87" id="Page_87">[Pg 87]</a></span></p>
+<h3><a name="Chapter_IV" id="Chapter_IV"></a>Chapter IV.</h3>
+
+<h4>THE INTERNAL-COMBUSTION ENGINE.</h4>
+
+<div class="blockquot"><p class="hang">The meaning of the term&mdash;Action of the internal-combustion
+engine&mdash;The motor car&mdash;The starting-handle&mdash;The engine&mdash;The
+carburetter&mdash;Ignition of the charge&mdash;Advancing the spark&mdash;Governing
+the engine&mdash;The clutch&mdash;The gear-box&mdash;The compensating gear&mdash;The
+silencer&mdash;The brakes&mdash;Speed of cars. </p></div>
+
+
+<p class="section">THE MEANING OF THE TERM "INTERNAL-COMBUSTION ENGINE."</p>
+
+<p class="noin"><span class="dcap">I</span><span class="caps">n</span> 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 <i>compressed</i> 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.</p>
+
+<p><span class='pagenum'><a name="Page_88" id="Page_88">[Pg 88]</a></span></p>
+<div class="figcenter" style="width: 529px;">
+<img src="images/image41.jpg" width="529" height="336" alt="" title="Car" />
+</div>
+
+<p><span class='pagenum'><a name="Page_89" id="Page_89">[Pg 89]</a></span></p>
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image42.jpg" width="500" height="878" alt="Fig. 39." title="" />
+<span class="caption"><span class="smcap">Fig. 39.</span>&mdash;Showing the four strokes that the piston of a
+gas-engine makes during one "cycle."</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_90" id="Page_90">[Pg 90]</a></span></p><p class="section">ACTION OF THE ENGINE.</p>
+
+<p>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<span class='pagenum'><a name="Page_91" id="Page_91">[Pg 91]</a></span> 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&mdash;(1) The piston moves
+from left to right, and just as the movement commences valves <span class="ampm">G</span> (gas)
+and <span class="ampm">A</span> (air) open to admit the explosive mixture. By the time that <span class="ampm">P</span> 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 <span class="ampm">I</span> (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 <span class="ampm">E</span> (exhaust) opens,
+and (4) the piston flies back under the momentum of the fly-wheel,
+driving out the burnt gases through the still open <span class="ampm">E</span>. 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<span class='pagenum'><a name="Page_92" id="Page_92">[Pg 92]</a></span>
+develops an impulse every half-turn&mdash;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.</p>
+
+
+<p class="section">THE MOTOR CAR.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image43.jpg" width="500" height="234" alt="Fig. 40." title="" />
+<span class="caption"><span class="smcap">Fig. 40.</span>&mdash;Plan of the chassis of a motor car.</span>
+</div>
+
+<p>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.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a> Fig. 40 is a bird's-eye view of the
+<i>chassis</i> (or "works" and wheels) of a car, from which the body has been
+removed. Starting at the<span class='pagenum'><a name="Page_93" id="Page_93">[Pg 93]</a></span> 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.</p>
+
+<p>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)<span class='pagenum'><a name="Page_94" id="Page_94">[Pg 94]</a></span> 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).</p>
+
+<p>Several parts&mdash;the carburetter, tanks, governor, and pump&mdash;are not shown
+in the general plan. These will be referred to in the more detailed
+account that follows.</p>
+
+
+<p class="section">THE STARTING-HANDLE.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/image44.jpg" width="400" height="239" alt="Fig. 41." title="" />
+<span class="caption"><span class="smcap">Fig. 41.</span>&mdash;The starting-handle.</span>
+</div>
+
+<p>Fig. 41 gives the starting-handle in part section. The handle <span class="ampm">H</span> is
+attached to a tube which terminates in a clutch, <span class="ampm">C</span>. A powerful spring
+keeps <span class="ampm">C</span> normally apart from a second clutch, <span class="ampm">C<sup>1</sup></span>, 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<span class='pagenum'><a name="Page_95" id="Page_95">[Pg 95]</a></span> direction. As soon as the engine begins to fire, the faces
+of the clutches slip over one another.</p>
+
+
+<p class="section">THE ENGINE.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image45.jpg" width="500" height="329" alt="Fig. 42." title="" />
+<span class="caption"><span class="smcap">Fig. 42.</span>&mdash;End and cross sections of a two-cylinder motor.</span>
+</div>
+
+<p>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<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a> (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<span class='pagenum'><a name="Page_96" id="Page_96">[Pg 96]</a></span> carried away. The pistons are of "trunk" form&mdash;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 <span class="above">1</span>&#8260;<span class="below">5000</span> 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 <i>continuously for several
+months</i>, and at the end of the trial was in absolutely perfect
+condition.</p>
+
+<p>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, <span class="ampm">P P</span>, through a slot in which the
+piston rod works, prevents an excess of oil being flung up. Channels are
+provided for leading oil<span class='pagenum'><a name="Page_97" id="Page_97">[Pg 97]</a></span> into the bearings. The cranks are 180&deg; 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image46.jpg" width="500" height="237" alt="Fig. 43." title="" />
+<span class="caption"><span class="smcap">Fig. 43.</span>&mdash;Showing how the water which cools the cylinders
+is circulated.</span>
+</div>
+
+<p>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&mdash;on all powerful engines&mdash;the inlet, like the
+exhaust valve, is lifted by a cam, lest it should stick or work
+irregularly. Three dotted circles show <span class="ampm">A</span>, a cog on the crank shaft; <span class="ampm">B</span>, a
+"lay" cog, which transmits motion to <span class="ampm">C</span>, on a short shaft rotating the
+cam that lifts the exhaust valve. <span class="ampm">C</span>,<span class='pagenum'><a name="Page_98" id="Page_98">[Pg 98]</a></span> having twice as many teeth as <span class="ampm">A</span>,
+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.</p>
+
+
+<p class="section">THE CARBURETTER.</p>
+
+<p>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&mdash;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 <i>carburetter</i> 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)&mdash;the <i>float chamber</i> and the <i>jet
+chamber</i>. In the former is a contrivance for regulating the petrol
+supply. A float&mdash;a cork, or air-tight metal box&mdash;is arranged to move
+freely up and down the stem of a needle-valve,<span class='pagenum'><a name="Page_99" id="Page_99">[Pg 99]</a></span> which closes the inlet
+from the tank. At the bottom of the chamber are two pivoted levers, <span class="ampm">W W</span>,
+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 <span class="ampm">G</span>.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image47.jpg" width="500" height="389" alt="Fig. 44." title="" />
+<span class="caption"><span class="smcap">Fig. 44.</span>&mdash;Section of a carburetter.</span>
+</div>
+
+<p>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, <span class="ampm">A</span>.<span class='pagenum'><a name="Page_100" id="Page_100">[Pg 100]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image48.jpg" width="500" height="400" alt="Fig. 45." title="" />
+<span class="caption"><span class="smcap">Fig. 45.</span>&mdash;Sketch of the electrical ignition arrangements
+on a motor car.</span>
+</div>
+
+
+<p class="section">IGNITION OF THE CHARGE.</p>
+
+<p>All petrol-cars now use electrical ignition. There are two main
+systems&mdash;(1) by an accumulator and induction coil; (2) <i>magneto
+ignition</i>, by means of a small dynamo driven by the engine. A general
+arrangement of the first is shown in Fig. 45. A disc, <span class="ampm">D</span>, of some
+insulating material&mdash;fibre or vulcanite&mdash;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, <span class="ampm">M P</span>, which can be rotated concentrically with <span class="ampm">D</span> through
+part of a circle, carries a "wipe" block at the end of a spring, which
+presses it against <span class="ampm">D</span>. The spring itself is attached to an insulated
+plate. When the revolution<span class='pagenum'><a name="Page_101" id="Page_101">[Pg 101]</a></span> of <span class="ampm">D</span> brings the wipe and contact together,
+current flows from the accumulator through switch <span class="ampm">S</span> to the wipe; through
+the contact-piece to <span class="ampm">C</span>; from <span class="ampm">C</span> to <span class="ampm">M P</span> and the induction coil; and back
+to the accumulator. This is the <i>primary, or low-tension, circuit</i>. A
+<i>high-tension</i> current is induced by the coil in the <i>secondary</i>
+circuit, indicated by dotted lines.<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a> In this circuit is the
+sparking-plug (<a href="#Fig_46">see Fig. 46</a>), having a central insulated rod in
+connection with one terminal of the secondary coil. Between it<span class='pagenum'><a name="Page_102" id="Page_102">[Pg 102]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_46" id="Fig_46"></a>
+<img src="images/image49.jpg" width="500" height="368" alt="Fig. 46." title="" />
+<span class="caption"><span class="smcap">Fig. 46.</span>&mdash;Section of a sparking-plug.</span>
+</div>
+
+
+<p class="section">ADVANCING THE SPARK.</p>
+
+<p>We will assume that the position of <span class="ampm">W</span> (in Fig. 45) is such that the
+contact touches <span class="ampm">W</span> at the moment when the piston has just completed the
+compression<span class='pagenum'><a name="Page_103" id="Page_103">[Pg 103]</a></span> 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 <span class="ampm">L</span>, the
+position of <span class="ampm">W</span> may be so altered that contact is made slightly <i>before</i>
+the compression stroke is complete, so that the charge is fairly alight
+by the time the piston has altered its direction. This is called
+<i>advancing</i> the spark.</p>
+
+
+<p class="section">GOVERNING THE ENGINE.</p>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image50.jpg" width="500" height="415" alt="Fig. 47." title="" />
+<span class="caption"><span class="smcap">Fig. 47.</span>&mdash;One form of governor used on motor cars.</span>
+</div>
+
+<p>A sketch of a neat governor, with regulating attachment, is given in
+Fig. 47. The governor shaft<span class='pagenum'><a name="Page_104" id="Page_104">[Pg 104]</a></span> is driven from the engine. As the balls, <span class="ampm">B
+B</span>, increase their velocity, they fly away from the shaft and move the
+arms, <span class="ampm">A A</span>, and a sliding tube, <span class="ampm">C</span>, towards the right. This rocks the
+lever <span class="ampm">R</span>, and allows the valves in the inlet pipe to close and reduce the
+supply of air and gas. A wedge, <span class="ampm">W</span>, which can be raised or lowered by
+lever <span class="ampm">L</span>, intervenes between the end of <span class="ampm">R</span> 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<span class='pagenum'><a name="Page_105" id="Page_105">[Pg 105]</a></span> the driver depresses <span class="ampm">L</span>, forces the
+wedge down, and so minimizes the effect of the governor.</p>
+
+
+<p class="section">THE CLUTCH.</p>
+
+<p>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
+<i>metal to metal</i> clutches.</p>
+
+
+<p class="section">THE GEAR-BOX.</p>
+
+<p>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<span class='pagenum'><a name="Page_106" id="Page_106">[Pg 106]</a></span> 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 <i>loose</i> 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<span class='pagenum'><a name="Page_107" id="Page_107">[Pg 107]</a></span>
+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.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_48" id="Fig_48"></a>
+<img src="images/image51.jpg" width="500" height="380" alt="Fig. 48." title="" />
+<span class="caption"><span class="smcap">Fig. 48.</span>&mdash;The gear-box of a motor car.</span>
+</div>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image52.jpg" width="500" height="355" alt="Fig. 49." title="" />
+<span class="caption"><span class="smcap">Fig. 49.</span></span>
+</div>
+
+
+<p class="section">THE COMPENSATING GEAR.</p>
+
+<p>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<span class='pagenum'><a name="Page_108" id="Page_108">[Pg 108]</a></span> 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
+<i>compensating gear</i> 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, <span class="ampm">C</span> and <span class="ampm">D</span>. Between these are small bevels mounted
+on a shaft supported by the driving drum. If the latter be rotated, the
+bevels would turn <span class="ampm">C</span> and <span class="ampm">D</span> at equal speeds, assuming that<span class='pagenum'><a name="Page_109" id="Page_109">[Pg 109]</a></span> 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 <i>average</i> speed
+remains 50. It should be mentioned that drum <span class="ampm">A</span> has incorporated with it
+on the outside a bevel-wheel (not shown) rotated by a smaller bevel on
+the end of the propeller shaft.</p>
+
+
+<p class="section">THE SILENCER.</p>
+
+<p>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 <i>silencers</i> 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<span class='pagenum'><a name="Page_110" id="Page_110">[Pg 110]</a></span> "pop." There are numerous types of
+silencers, but all employ this principle in one form or another.</p>
+
+
+<p class="section">THE BRAKES.</p>
+
+<p>Every car carries at least two brakes of band pattern&mdash;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 (<a href="#Fig_48">see Fig. 48</a>). 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 25&#8531; and 70 feet respectively for
+the distance in which the speed could be reduced from sixteen miles per
+hour to zero.</p>
+
+
+<p class="section">SPEED OF CARS.</p>
+
+<p>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<span class='pagenum'><a name="Page_111" id="Page_111">[Pg 111]</a></span> an hour has actually been reached. Engines of 150 h.p. can now be
+packed into a vehicle scaling less than 1&frac12; 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.</p>
+
+<p><small><i>Note.</i>&mdash;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.</small></p>
+
+
+<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> Steam-driven cars are not considered in this chapter, as
+their principle is much the same as that of the ordinary locomotive.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> 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.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span></a> For explanation of the induction coil, <a href="#Page_122">see p. 122</a></p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_112" id="Page_112">[Pg 112]</a></span></p>
+<h3><a name="Chapter_V" id="Chapter_V"></a>Chapter V.</h3>
+
+<h4>ELECTRICAL APPARATUS.</h4>
+
+<div class="blockquot"><p class="hang">What is electricity?&mdash;Forms of electricity&mdash;Magnetism&mdash;The
+permanent magnet&mdash;Lines of force&mdash;Electro-magnets&mdash;The electric
+bell&mdash;The induction coil&mdash;The condenser&mdash;Transformation of
+current&mdash;Uses of the induction coil. </p></div>
+
+
+<p class="section">WHAT IS ELECTRICITY?</p>
+
+<p class="noin"><span class="dcap">O</span><span class="caps">f</span> 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<span class='pagenum'><a name="Page_113" id="Page_113">[Pg 113]</a></span> 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 <i>what</i> 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&mdash;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.</p>
+
+
+<p class="section">FORMS OF ELECTRICITY.</p>
+
+<p>Rub a vulcanite rod and hold one end near some<span class='pagenum'><a name="Page_114" id="Page_114">[Pg 114]</a></span> tiny pieces of paper.
+They fly to it, stick to it for a time, and then fall off. The rod was
+electrified&mdash;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 <i>static</i>.</p>
+
+<p>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 <i>flow</i> 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,<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a> "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<span class='pagenum'><a name="Page_115" id="Page_115">[Pg 115]</a></span> 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 <i>current</i>
+electricity can be very easily confined to its conductor by means of
+some insulating or nonconducting envelope.</p>
+
+
+<p class="section">MAGNETISM.</p>
+
+<p>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-<i>seeking</i> pole of a
+magnet is marked N., though it is in reality the <i>south</i> pole; for
+unlike poles are mutually attractive, and like poles repellent.)</p>
+
+<p>There are two forms of magnet&mdash;<i>permanent</i> and <i>temporary</i>. 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<span class='pagenum'><a name="Page_116" id="Page_116">[Pg 116]</a></span> closely compacted than iron,
+and the molecules therefore would be able to turn about more easily.<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a>
+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.</p>
+
+
+<p class="section">THE PERMANENT MAGNET.</p>
+
+<p>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.</p>
+
+
+<p class="section">LINES OF FORCE.</p>
+
+<p>In Fig. 50 are seen a number of dotted lines. These are called <i>lines of
+magnetic force</i>. 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 <i>proved</i>) that magnetic force
+streams away from the N. pole and describes a<span class='pagenum'><a name="Page_117" id="Page_117">[Pg 117]</a></span> circular course through
+the air back to the S. pole. The same remark applies to the bar magnet.</p>
+
+
+<p class="section">ELECTRICAL MAGNETS.</p>
+
+<div class="figleft" style="width: 300px;">
+<img src="images/image53.jpg" width="300" height="344" alt="Fig. 50." title="" />
+<span class="caption"><span class="smcap">Fig. 50.</span>&mdash;Permanent magnet, and the "lines of force"
+emanating from it.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_118" id="Page_118">[Pg 118]</a></span> 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.</p>
+
+<p>In Fig. 51 you will notice that some of the "lines of force" are
+deflected through the iron bar <span class="ampm">A</span>. 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 <span class="ampm">A</span> 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.</p>
+
+<div class="figright" style="width: 300px;">
+<img src="images/image54.jpg" width="300" height="343" alt="Fig. 51." title="" />
+<span class="caption"><span class="smcap">Fig. 51.</span>&mdash;Electro-magnet: <span class="ampm">A</span>, armature;
+<span class="ampm">B</span>, battery.</span>
+</div>
+
+<p>Having now considered electricity in three of its forms&mdash;static,
+current, and rotatory&mdash;we will pass to some of its applications.</p>
+
+
+<p><span class='pagenum'><a name="Page_119" id="Page_119">[Pg 119]</a></span></p><p class="section">THE ELECTRIC BELL.</p>
+
+<p>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 must be put a crank to alter the direction of 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 <i>indicator</i> 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.</p>
+
+<p>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
+<span class='pagenum'><a name="Page_120" id="Page_120">[Pg 120]</a></span>terminal <span class="ampm">T<sup>1</sup></span>, round the electro-magnet <span class="ampm">M</span>, through the pillar <span class="ampm">P</span> and
+flat steel springs <span class="ampm">S</span> and <span class="ampm">B</span>, through the platinum-pointed screw, and back
+to the battery through the push. The circulation of current magnetizes
+<span class="ampm">M</span>, which attracts the iron armature <span class="ampm">A</span> attached to the spring <span class="ampm">S</span>, and
+draws the hammer <span class="ampm">H</span> towards the gong. Just before the stroke occurs, the
+spring <span class="ampm">B</span> leaves the tip of the screw, and the circuit is broken, so that
+the magnet no longer attracts. <span class="ampm">H</span> is carried by its momentum against the
+gong, and is withdrawn by the spring, until <span class="ampm">B</span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image55.jpg" width="500" height="381" alt="Fig. 52." title="" />
+<span class="caption"><span class="smcap">Fig. 52.</span>&mdash;Sketch of an electric-bell circuit.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_121" id="Page_121">[Pg 121]</a></span></p><p>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.</p>
+
+
+<p class="section">THE INDUCTION OR RUHMKORFF COIL.</p>
+
+<p>Reference was made in connection with the electrical ignition of
+internal-combustion engines (<a href="#Page_101">p. 101</a>) to the <i>induction coil</i>. This is a
+device for increasing the <i>voltage</i>, 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&middot;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.</p>
+
+<p><span class='pagenum'><a name="Page_122" id="Page_122">[Pg 122]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image56.jpg" width="500" height="409" alt="Fig. 53." title="" />
+<span class="caption"><span class="smcap">Fig. 53.</span>&mdash;Sketch of an induction coil.</span>
+</div>
+
+<p>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
+<i>primary</i> coil is attached to the battery, the other to the base of a
+hammer, <span class="ampm">H</span>, vibrating between the end of the core and a screw, <span class="ampm">S</span>, passing
+through an upright, <span class="ampm">T</span>, 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<span class='pagenum'><a name="Page_123" id="Page_123">[Pg 123]</a></span> primary coil. The ends of
+this <i>secondary</i> 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 <span class="ampm">H</span> touches <span class="ampm">S</span> 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. <span class="ampm">H</span> 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.</p>
+
+
+<p class="section">THE CONDENSER.</p>
+
+<p>The sudden parting of <span class="ampm">H</span> and <span class="ampm">S</span> 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
+<span class='pagenum'><a name="Page_124" id="Page_124">[Pg 124]</a></span>the "odd" sheets are connected with <span class="ampm">T</span>, all the "even" with <span class="ampm">T<sup>1</sup></span>. 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.</p>
+
+
+<p class="section">TRANSFORMATION OF CURRENT.</p>
+
+<p>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&ndash;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.</p>
+
+<p>It must not be supposed that an induction coil increases the <i>amount</i> of
+current given off by a battery. It merely increases its pressure at the
+expense of its volume&mdash;stores up its energy, as it<span class='pagenum'><a name="Page_125" id="Page_125">[Pg 125]</a></span> 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.</p>
+
+<p>Any intermittent current can be transformed as regards its intensity.
+You may either increase its pressure while decreasing its rate of flow,
+or <i>amperage</i>; 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&mdash;that is, is already intermittent&mdash;the
+contact-breaker is not needed. There will be more to say about
+transformation of current in later paragraphs.</p>
+
+
+<p class="section">USES OF THE INDUCTION COIL.</p>
+
+<p>The induction coil is used&mdash;(1.) For passing currents through glass
+tubes almost exhausted of air<span class='pagenum'><a name="Page_126" id="Page_126">[Pg 126]</a></span> or containing highly rarefied gases. The
+luminous effects of these "Geissler" tubes are very beautiful. (2.) For
+producing the now famous X or R&ouml;ntgen 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 R&ouml;ntgen 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.</p>
+
+
+<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span></a> "What is Electricity?" <a href="#Page_46">p. 46</a>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span></a> 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.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_127" id="Page_127">[Pg 127]</a></span></p>
+<h3><a name="Chapter_VI" id="Chapter_VI"></a>Chapter VI.</h3>
+
+<h4>THE ELECTRIC TELEGRAPH.</h4>
+
+<div class="blockquot"><p class="hang">Needle instruments&mdash;Influence of current on the magnetic
+needle&mdash;Method of reversing the current&mdash;Sounding
+instruments&mdash;Telegraphic relays&mdash;Recording telegraphs&mdash;High-speed
+telegraphy. </p></div>
+
+
+<p class="noin"><span class="dcap">T</span><span class="caps">ake</span> 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&mdash;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.</p>
+
+
+<p><span class='pagenum'><a name="Page_128" id="Page_128">[Pg 128]</a></span></p><p class="section">NEEDLE INSTRUMENTS.</p>
+
+<div class="figleft" style="width: 300px;">
+<img src="images/image57.jpg" width="300" height="478" alt="Fig. 54." title="" />
+<span class="caption"><span class="smcap">Fig. 54.</span>&mdash;Sketch of the side elevation of a Wheatstone
+needle instrument.</span>
+</div>
+
+<p>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, <span class="ampm">B</span>, and an
+upright front, <span class="ampm">A</span>, 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, <span class="ampm">N</span>, outside the front. The wires <span class="ampm">W W</span> are connected to the
+telegraph line and to the commutator, a device which, when the operator
+moves the handle <span class="ampm">H</span> 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 right, and comes to rest in a
+normal position for a moment,<span class='pagenum'><a name="Page_129" id="Page_129">[Pg 129]</a></span> the letter A is signified;
+right-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.</p>
+
+
+<p class="section">INFLUENCE OF CURRENT ON A MAGNETIC NEEDLE.</p>
+
+<div class="figright" style="width: 300px;">
+<img src="images/image58.jpg" width="300" height="364" alt="Figs. 55, 56." title="" />
+<span class="caption"><span class="smcap">Figs. 55, 56.</span>&mdash;The coils of a needle instrument. The
+arrows show the direction taken by the current.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_130" id="Page_130">[Pg 130]</a></span> 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 are at the right end of
+the coils, the S. poles 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.</p>
+
+
+<p class="section">METHOD OF REVERSING THE CURRENT.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image59.jpg" width="500" height="213" alt="Fig. 57." title="" />
+<span class="caption"><span class="smcap">Fig. 57.</span>&mdash;General arrangement of needle-instrument
+circuit. The shaded plates on the left (<span class="ampm">B</span> and <span class="ampm">R</span>) are in contact.</span>
+</div>
+
+<p>A simple method of changing the direction of the current in a
+two-instrument circuit is shown diagrammatically in Fig. 57. The
+<i>principle</i> is used in the<span class='pagenum'><a name="Page_131" id="Page_131">[Pg 131]</a></span> Wheatstone needle instrument. The battery
+terminals at each station are attached to two brass plates, <span class="ampm">A B</span>, <span class="ampm">A<sup>1</sup>
+B<sup>1</sup></span>. Crossing these at right angles (under <span class="ampm">A A<sup>1</sup></span> and over <span class="ampm">B B<sup>1</sup></span>)
+are the flat brass springs, <span class="ampm">L R</span>, <span class="ampm">L<sup>1</sup> R<sup>1</sup></span>, having buttons at their
+lower ends, and fixed at their upper ends to baseboards. When at rest
+they all press upwards against the plates <span class="ampm">A</span> and <span class="ampm">A<sup>1</sup></span> respectively. <span class="ampm">R</span>
+and <span class="ampm">L<sup>1</sup></span> are connected with the line circuit, in which are the coils of
+dials 1 and 2, one at each station. <span class="ampm">L</span> and <span class="ampm">R<sup>1</sup></span> are connected with the
+earth-plates <span class="ampm">E E<sup>1</sup></span>. An operator at station 1 depresses <span class="ampm">R</span> so as to
+touch <span class="ampm">B</span>. Current now flows from the battery to <span class="ampm">B</span>, thence through <span class="ampm">R</span> to
+the line circuit, round the coils of both dials through <span class="ampm">L<sup>1</sup> A<sup>1</sup></span> and
+<span class="ampm">R</span> to earth-plate <span class="ampm">E<sup>1</sup></span>, through the earth to <span class="ampm">E</span>, and then back to the
+battery through <span class="ampm">L</span> and <span class="ampm">A</span>. The needles assume the position shown. To
+reverse the current the operator allows <span class="ampm">R</span> to rise into contact with <span class="ampm">A</span>,
+and depresses <span class="ampm">L</span> to touch <span class="ampm">B</span>. The course can be traced out easily.</p>
+
+<p>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<span class='pagenum'><a name="Page_132" id="Page_132">[Pg 132]</a></span> similar communication
+with the &ndash; 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 <i>vice vers&acirc;</i>
+when moved in the opposite direction.</p>
+
+
+<p class="section">SOUNDING INSTRUMENTS.</p>
+
+<p>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<span class='pagenum'><a name="Page_133" id="Page_133">[Pg 133]</a></span> duration of
+its stay decides whether a "long" or "short" is meant.</p>
+
+
+<p class="section">TELEGRAPHIC RELAYS.</p>
+
+<div class="figright" style="width: 300px;">
+<img src="images/image60.jpg" width="300" height="516" alt="Fig. 58." title="" />
+<span class="caption"><span class="smcap">Fig. 58.</span>&mdash;Section of a telegraph wire insulator on its
+arm. The shaded circle is the line wire, the two blank circles indicate
+the wire which ties the line wire to the insulator.</span>
+</div>
+
+<p>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 <i>relay</i>, which pulls down a light bar and closes a second "local"
+circuit&mdash;that is, one at the receiver end&mdash;worked by a separate battery,
+which has sufficient power to operate the sounder.</p>
+
+
+<p class="section">RECORDING TELEGRAPHS.</p>
+
+<p>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<span class='pagenum'><a name="Page_134" id="Page_134">[Pg 134]</a></span> ribbon slowly along for the wheel to press against
+when it rises, a self-recording Morse inker is produced. The
+ribbon-feeding apparatus is set in motion automatically by the current,
+and continues to pull the ribbon along until the message is completed.</p>
+
+<p>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&mdash;that is, in
+time&mdash;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.</p>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_135" id="Page_135">[Pg 135]</a></span></p><p>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.</p>
+
+
+<p class="section">HIGH-SPEED TELEGRAPHY.</p>
+
+<p>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
+<i>automatic transmitter</i> 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.</p>
+
+<p>It has been found possible to send several messages<span class='pagenum'><a name="Page_136" id="Page_136">[Pg 136]</a></span> simultaneously over
+a single line. To effect this a <i>distributer</i> 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.</p>
+
+<p>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.</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_137" id="Page_137">[Pg 137]</a></span></p>
+<h3><a name="Chapter_VII" id="Chapter_VII"></a>Chapter VII.</h3>
+
+<h4>WIRELESS TELEGRAPHY.</h4>
+
+<div class="blockquot"><p class="hang">The transmitting apparatus&mdash;The receiving apparatus&mdash;Syntonic
+transmission&mdash;The advance of wireless telegraphy. </p></div>
+
+
+<p class="noin"><span class="dcap">I</span><span class="caps">n</span> 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.</p>
+
+<p>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<span class='pagenum'><a name="Page_138" id="Page_138">[Pg 138]</a></span> "hear a noise." The hammer is here the
+transmitter, the air the conductor, the ear the receiver.</p>
+
+<p>In wireless telegraphy we use the ether as the conductor of electrical
+disturbances.<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a> Marconi, Slaby, Branly, Lodge, De Forest, Popoff, and
+others have invented apparatus for causing disturbances of the requisite
+kind, and for detecting their presence.</p>
+
+<p>The main features of a wireless telegraphy outfit are shown in Figs. 59
+and 61.</p>
+
+
+<p class="section">THE TRANSMITTER APPARATUS.</p>
+
+<p>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 a&euml;rial
+wires hang. At their upper and lower ends respectively the earth and
+a&euml;rial wires terminate in brass balls separated by a gap. When the
+operator depresses the key the induction coil<span class='pagenum'><a name="Page_139" id="Page_139">[Pg 139]</a></span> 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 a&euml;rial 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.<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a></p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image61.jpg" width="500" height="260" alt="Fig. 59." title="" />
+<span class="caption"><span class="smcap">Fig. 59.</span>&mdash;Sketch of the transmitter of a wireless
+telegraphy outfit.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_140" id="Page_140">[Pg 140]</a></span></p>
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image62.jpg" width="500" height="128" alt="Fig. 60." title="" />
+<span class="caption"><span class="smcap">Fig. 60.</span>&mdash;A Marconi coherer.</span>
+</div>
+
+
+<p class="section">RECEIVING APPARATUS.</p>
+
+<p>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 <i>coherer</i>. A
+Marconi coherer is seen in Fig. 60. Inside a small glass tube exhausted
+of air are two silver plugs, <span class="ampm">P P</span>, carrying terminals, <span class="ampm">T T</span>, 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&mdash;that is, pack tightly together&mdash;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.</p>
+
+<p><span class='pagenum'><a name="Page_141" id="Page_141">[Pg 141]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image63.jpg" width="500" height="340" alt="Fig. 61." title="" />
+<span class="caption"><span class="smcap">Fig. 61.</span>&mdash;Sketch of the receiving apparatus in a
+wireless telegraphy outfit.</span>
+</div>
+
+<p>We are now in a position to examine the apparatus of which a coherer
+forms part (Fig. 61). First, we notice the a&euml;rial and earth wires, to
+which are attached other wires from battery <span class="ampm">A</span>. This battery circuit
+passes round the relay magnet <span class="ampm">R</span> and through two choking coils, whose
+function is to prevent the Hertzian waves entering the battery. The
+relay, when energized, brings contact <span class="ampm">D</span> against <span class="ampm">E</span> and closes the circuit
+of battery <span class="ampm">B</span>, which is much more powerful than battery <span class="ampm">A</span>, and operates
+the magnet <span class="ampm">M</span> as well as the <i>tapper</i>, which is practically an electric
+bell minus the gong. (The tapper circuit is indicated by the dotted
+lines.)</p>
+
+<p><span class='pagenum'><a name="Page_142" id="Page_142">[Pg 142]</a></span></p><p>We will suppose the transmitter of a distant station to be at work. The
+electric waves strike the a&euml;rial 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 <span class="ampm">A</span> 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 <span class="ampm">M</span> the armature is pulled
+towards it, the end <span class="ampm">P</span>, carrying an inked wheel, rises, and a mark is
+made on the tape <span class="ampm">W</span>, which is moved continuously<span class='pagenum'><a name="Page_143" id="Page_143">[Pg 143]</a></span> being drawn forward off
+reel <span class="ampm">R</span> by the clockwork&mdash;or electrically-driven rollers <span class="ampm">R<sup>1</sup> R<sup>2</sup></span>.</p>
+
+
+<p class="section">SYNTONIC TRANSMISSION.</p>
+
+<p>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.</p>
+
+<p>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<span class='pagenum'><a name="Page_144" id="Page_144">[Pg 144]</a></span> 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.</p>
+
+<p>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 a&euml;rial rod or wire of a receiving station with a
+transmitter. A vertical wire about 200 feet in length, says Professor
+J.A. Fleming,<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a> 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<span class='pagenum'><a name="Page_145" id="Page_145">[Pg 145]</a></span> transmitter, or
+<i>vice vers&acirc;</i>, selective wireless telegraphy becomes possible.</p>
+
+
+<p class="section">ADVANCE OF WIRELESS TELEGRAPHY.</p>
+
+<p>The history of wireless telegraphy may be summed up as follows:&mdash;</p>
+
+<p>1842.&mdash;Professor Morse sent a&euml;rial 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.</p>
+
+<p>1859.&mdash;James Bowman Lindsay transmitted messages across the Tay at
+Glencarse in a somewhat similar way. Distance about &frac12; mile.</p>
+
+<p>1885.&mdash;Sir William Preece signalled from Lavernock Point, near Cardiff,
+to Steep Holm, an island in the Bristol Channel. Distance about 5&frac12;
+miles.</p>
+
+<p>In all these electrical <i>induction</i> of current was employed.</p>
+
+<p>1886.&mdash;Hertzian waves discovered.</p>
+
+<p>1895.&mdash;Professor A. Popoff sent Hertzian wave messages over a distance
+of 3 miles.</p>
+
+<p><span class='pagenum'><a name="Page_146" id="Page_146">[Pg 146]</a></span></p><p>1897.&mdash;Marconi signalled from the Needles Hotel, Isle of Wight, to
+Swanage; 17&frac12; miles.</p>
+
+<p>1901.&mdash;Messages sent at sea for 380 miles.</p>
+
+<p>1901, Dec. 17.&mdash;Messages transmitted from Poldhu, Cornwall, to Hospital
+Point, Newfoundland; 2,099 miles.</p>
+
+<p>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.</p>
+
+
+<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span></a> Named after their first discoverer, Dr. Hertz of
+Carlsruhe, "Hertzian waves."</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_14" id="Footnote_14_14"></a><a href="#FNanchor_14_14"><span class="label">[14]</span></a> For long-distance transmission powerful dynamos take the
+place of the induction coil and battery.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_15" id="Footnote_15_15"></a><a href="#FNanchor_15_15"><span class="label">[15]</span></a> "Technics," vol. ii. p. 566.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_147" id="Page_147">[Pg 147]</a></span></p>
+<h3><a name="Chapter_VIII" id="Chapter_VIII"></a>Chapter VIII.</h3>
+
+<h4>THE TELEPHONE.</h4>
+
+<div class="blockquot"><p class="hang">The Bell telephone&mdash;The Edison transmitter&mdash;The granular carbon
+transmitter&mdash;General arrangement of a telephone
+circuit&mdash;Double-line circuits&mdash;Telephone exchanges&mdash;Submarine
+telephony. </p></div>
+
+
+<p class="noin"><span class="dcap">F</span><span class="caps">or</span> 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.</p>
+
+<p>Wonderful as the transmission of signals over great<span class='pagenum'><a name="Page_148" id="Page_148">[Pg 148]</a></span> 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.</p>
+
+
+<p class="section">THE BELL TELEPHONE.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image64.jpg" width="500" height="203" alt="Fig. 62." title="" />
+<span class="caption"><span class="smcap">Fig. 62.</span>&mdash;Section of a Bell telephone.</span>
+</div>
+
+<p>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, <span class="ampm">M</span>. 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, <span class="ampm">D</span>, of very thin iron plate, clamped between
+the concave mouthpiece and the casing, almost touches the end of the
+magnet.</p>
+
+<p><span class='pagenum'><a name="Page_149" id="Page_149">[Pg 149]</a></span></p><p>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 <span class="above">1</span>&#8260;<span class="below">10,000,000</span>
+of an inch! Its movements distort the shape of the "lines of force" (<a href="#Page_118">see
+p. 118</a>) 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.</p>
+
+<p>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.</p>
+
+
+<p><span class='pagenum'><a name="Page_150" id="Page_150">[Pg 150]</a></span></p><p class="section">THE EDISON TRANSMITTER.</p>
+
+<p>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.</p>
+
+<p>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 <i>microphone</i> 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.</p>
+
+<p><span class='pagenum'><a name="Page_151" id="Page_151">[Pg 151]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image65.jpg" width="500" height="497" alt="Fig. 63." title="" />
+<span class="caption"><span class="smcap">Fig. 63.</span>&mdash;Section of a granular carbon transmitter.</span>
+</div>
+
+
+<p class="section">THE GRANULAR CARBON TRANSMITTER.</p>
+
+<p>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, <span class="ampm">C</span>, with a face moulded into a number of pyramidal
+projections, <span class="ampm">P P</span>. The space between <span class="ampm">C</span> and a carbon diaphragm, <span class="ampm">D</span>, is
+packed with carbon granules, <span class="ampm">G G</span>. <span class="ampm">C</span> has direct contact with line
+terminal <span class="ampm">T</span>, which screws into it; <span class="ampm">D</span> with <span class="ampm">T<sup>1</sup></span> through the brass casing,
+screw <span class="ampm">S</span>, and a small plate at the back of the transmitter. Voice
+vibrations compress <span class="ampm">G G</span>, and allow current to pass<span class='pagenum'><a name="Page_152" id="Page_152">[Pg 152]</a></span> more freely from <span class="ampm">D</span>
+to <span class="ampm">C</span>. This form of microphone is very delicate, and unequalled for
+long-distance transmission.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/image66.jpg" width="600" height="238" alt="Fig. 64." title="" />
+<span class="caption"><span class="smcap">Fig. 64.</span>&mdash;A diagrammatic representation of a telephonic
+circuit.</span>
+</div>
+
+
+<p class="section">GENERAL ARRANGEMENT OF A TELEPHONE CIRCUIT.</p>
+
+<p>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<span class='pagenum'><a name="Page_153" id="Page_153">[Pg 153]</a></span>
+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 <span class="ampm">P P<sub>2</sub></span> of induction coils. The
+transmitters are in the line circuit, which includes the secondary
+windings <span class="ampm">S S<sub>2</sub></span> of the coils.</p>
+
+<p>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:&mdash;Line <span class="ampm">A</span>, <span class="ampm">E B<sub>2</sub></span>, contact 10, the switch 9;
+line <span class="ampm">B</span>, 4, the other switch, contact 5, and <span class="ampm">E B</span>. 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 <span class="ampm">T</span>
+in primary coil <span class="ampm">P</span> are magnified by secondary coil <span class="ampm">S</span> for transmission
+through the line circuit, and affect both receivers. The same thing
+happens when <span class="ampm">T<sub>2</sub></span> 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.</p>
+
+<p><span class='pagenum'><a name="Page_154" id="Page_154">[Pg 154]</a></span></p>
+<div class="figcenter" style="width: 514px;">
+<img src="images/image67.jpg" width="514" height="327" alt="A TELEPHONE EXCHANGE." title="" />
+<span class="caption">A TELEPHONE EXCHANGE.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_155" id="Page_155">[Pg 155]</a></span></p><p class="section">DOUBLE-LINE CIRCUITS.</p>
+
+<p>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.</p>
+
+
+<p class="section">TELEPHONE EXCHANGES.</p>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_156" id="Page_156">[Pg 156]</a></span></p><p>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.</p>
+
+<div class="figleft" style="width: 400px;">
+<img src="images/image68.jpg" width="400" height="521" alt="Fig. 65." title="" />
+<span class="caption"><span class="smcap">Fig. 65.</span>&mdash;The headdress of an operator at a telephone
+exchange. The receiver is fastened over one ear, and the transmitter to
+the chest.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_157" id="Page_157">[Pg 157]</a></span> 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 telephonic
+connection, though three exchanges were in the circuit.</p>
+
+
+<p class="section">SUBMARINE TELEPHONY.</p>
+
+<p>Though telegraphic messages are transmitted easily through thousands of
+miles of cable,<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a> 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<span class='pagenum'><a name="Page_158" id="Page_158">[Pg 158]</a></span> 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 (<a href="#Page_128">p. 128</a>).</p>
+
+<p>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 <span class="ampm">S</span> to be
+on shore at the English end, and <span class="ampm">S<sub>2</sub></span> to be the <i>primary</i> winding of an
+induction coil a hundred miles away in the sea, which magnifies the
+enfeebled vibrations for a journey to <span class="ampm">S<sub>3</sub></span>, 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.</p>
+
+
+<div class="footnote"><p><a name="Footnote_16_16" id="Footnote_16_16"></a><a href="#FNanchor_16_16"><span class="label">[16]</span></a> In 1896 the late Li Hung Chang sent a cablegram from China
+to England (12,608 miles), and received a reply, in <i>seven minutes</i>.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_159" id="Page_159">[Pg 159]</a></span></p>
+<h3><a name="Chapter_IX" id="Chapter_IX"></a>Chapter IX.</h3>
+
+<h4>DYNAMOS AND ELECTRIC MOTORS.</h4>
+
+<div class="blockquot"><p class="hang">A simple dynamo&mdash;Continuous-current dynamos&mdash;Multipolar
+dynamos&mdash;Exciting the field magnets&mdash;Alternating current
+dynamos&mdash;The transmission of power&mdash;The electric motor&mdash;Electric
+lighting&mdash;The incandescent lamp&mdash;Arc lamps&mdash;"Series" and "parallel"
+arrangement of lamps&mdash;Current for electric lamps&mdash;Electroplating. </p></div>
+
+
+<p class="noin"><span class="dcap">I</span><span class="caps">n</span> previous chapters we have incidentally referred to the conversion of
+mechanical work into electrical energy. In this we shall examine how it
+is done&mdash;how the silently spinning dynamo develops power, and why the
+motor spins when current is passed through it.</p>
+
+<p>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
+<i>magnetic field</i> of the magnet.</p>
+
+<p>Many years ago (1831) the great Michael Faraday discovered that if a
+loop of wire were moved up<span class='pagenum'><a name="Page_160" id="Page_160">[Pg 160]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image69.jpg" width="500" height="342" alt="Fig. 66." title="" />
+<span class="caption"><span class="smcap">Fig. 66.</span></span>
+</div>
+
+<p>The strength of the current induced in a circuit<span class='pagenum'><a name="Page_161" id="Page_161">[Pg 161]</a></span> 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.</p>
+
+<p>The voltage depends on three things:&mdash;(1.) The <i>strength</i> of the magnet:
+the stronger it is, the greater the number of lines of force coming from
+it. (2.) The <i>length</i> of the conductor cutting the lines of force: the
+longer it is, the more lines it will cut. (3.) The <i>speed</i> 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.</p>
+
+
+<p class="section">A SIMPLE DYNAMO.</p>
+
+<p>In Fig. 67 we have the simplest possible form of dynamo&mdash;a single turn
+of wire, <i>w x y z</i>, mounted on a spindle, and having one end attached to
+an insulated ring <span class="ampm">C</span>, the other to an insulated ring <span class="ampm">C<sup>1</sup></span>. Two small
+brushes, <span class="ampm">B B<sup>1</sup></span>, of wire gauze or carbon, rubbing continuously against
+these collecting rings,<span class='pagenum'><a name="Page_162" id="Page_162">[Pg 162]</a></span> 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 <i>supposed</i> to stream from the N. to the S. pole.</p>
+
+<p>In Fig. 67 the armature has reached a position in which <i>y z</i> and <i>w x</i>
+are cutting no, or very few, lines of force, as they move practically
+parallel to the lines. This is called the <i>zero</i> position.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image70.jpg" width="500" height="332" alt="Fig. 67." title="" />
+<span class="caption"><span class="smcap">Fig. 67.</span></span>
+</div>
+
+<p><span class='pagenum'><a name="Page_163" id="Page_163">[Pg 163]</a></span></p>
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image71.jpg" width="500" height="350" alt="Fig. 68." title="" />
+<span class="caption"><span class="smcap">Fig. 68.</span></span>
+</div>
+
+<p>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 <i>y z</i>, therefore, is
+moving downwards. Now, suppose that you rest your <i>left</i> 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 <i>direction of
+the lines of force</i>. Bend your other three fingers downwards over the
+edge of the N. pole. They will indicate the <i>direction in which the
+conductor is moving</i> across the magnetic field. Stick<span class='pagenum'><a name="Page_164" id="Page_164">[Pg 164]</a></span> out the thumb at
+right angles to the forefinger. It points in the direction in which the
+<i>induced</i> 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.</p>
+
+<p>While current travels from <i>z</i> to <i>y</i>&mdash;that is, <i>from</i> the ring <span class="ampm">C<sup>1</sup></span> to
+<i>y</i>&mdash;it also travels from <i>x</i> to <i>w</i>, because <i>w x</i> rises while <i>y z</i>
+descends. So that a current circulates through the coil and the exterior
+part of the circuit, including the lamp. After <i>z y</i> has passed the
+lowest possible point of the circle it begins to ascend, <i>w x</i> 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 <i>alternator</i> 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.</p>
+
+<p><span class='pagenum'><a name="Page_165" id="Page_165">[Pg 165]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image72.jpg" width="500" height="319" alt="Fig. 69." title="" />
+<span class="caption"><span class="smcap">Fig. 69.</span></span>
+</div>
+
+
+<p class="section">CONTINUOUS-CURRENT DYNAMOS.</p>
+
+<p>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 <span class="ampm">C C<sup>1</sup></span>, we now have a single ring split longitudinally
+into two portions, one of which is connected to each end of the coil <i>w
+x y z</i>. In Fig. 69 brush <span class="ampm">B</span> has just passed the gap on to segment <span class="ampm">C</span>,
+brush <span class="ampm">B<sup>1</sup></span> on to segment <span class="ampm">C<sup>1</sup></span>. For half a revolution these remain
+respectively in contact; then, just as <i>y z</i> begins to rise and <i>w x</i> to
+descend, the brushes cross the gaps again and exchange segments, so that
+the current is perpetually flowing one way<span class='pagenum'><a name="Page_166" id="Page_166">[Pg 166]</a></span> through the circuit. The
+effect of the commutator<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a> is, in fact, equivalent to transposing the
+brushes of the collecting rings of the alternator every time the coil
+reaches a zero position.</p>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image73.jpg" width="500" height="317" alt="Fig. 70." title="" />
+<span class="caption"><span class="smcap">Fig. 70.</span></span>
+</div>
+
+
+<p class="section">PRACTICAL CONTINUOUS-CURRENT DYNAMOS.</p>
+
+<p>The electrical output of our simple dynamo would<span class='pagenum'><a name="Page_167" id="Page_167">[Pg 167]</a></span> 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.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 71 and 72">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image74.jpg" width="300" height="215" alt="Fig. 71." title="" />
+<span class="caption"><span class="smcap">Fig. 71.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image75.jpg" width="300" height="212" alt="Fig. 72." title="" />
+<span class="caption"><span class="smcap">Fig. 72.</span></span>
+</div></td></tr>
+</table></div>
+
+<p>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.<a name="FNanchor_18_18" id="FNanchor_18_18"></a><a href="#Footnote_18_18" class="fnanchor">[18]</a> Sometimes there are openings through the core from
+end to end to ventilate and cool it.</p>
+
+<p><span class='pagenum'><a name="Page_168" id="Page_168">[Pg 168]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image76.jpg" width="500" height="329" alt="Fig. 73." title="" />
+<span class="caption"><span class="smcap">Fig. 73.</span></span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_169" id="Page_169">[Pg 169]</a></span> 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&deg; 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.</p>
+
+<p>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.</p>
+
+
+<p class="section">MULTIPOLAR DYNAMOS.</p>
+
+<p>Hitherto we have considered the magnetic field produced by one bi-polar
+magnet only. Large dynamos<span class='pagenum'><a name="Page_170" id="Page_170">[Pg 170]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 321px;">
+<img src="images/image77.jpg" width="321" height="235" alt="Fig. 74." title="" />
+<span class="caption"><span class="smcap">Fig. 74.</span>&mdash;A Holmes continuous current dynamo: <span class="ampm">A</span>,
+armature; <span class="ampm">C</span>, commutator; <span class="ampm">M</span>, field magnets.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_171" id="Page_171">[Pg 171]</a></span></p><p class="section">EXCITING THE FIELD MAGNETS.</p>
+
+<p>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,<a name="FNanchor_19_19" id="FNanchor_19_19"></a><a href="#Footnote_19_19" class="fnanchor">[19]</a> owing to
+the residual magnetism left in the magnet cores.</p>
+
+<div class="figcenter" style="width: 325px;">
+<img src="images/image78.jpg" width="325" height="214" alt="Fig. 75." title="" />
+<span class="caption"><span class="smcap">Fig. 75.</span>&mdash;Partly finished commutator.</span>
+</div>
+
+<p>Look carefully at Figs. 77 and 78. In the first of these you will
+observe that part of the wire forming<span class='pagenum'><a name="Page_172" id="Page_172">[Pg 172]</a></span> the external circuit is wound
+round the arms of the field magnet. This is called a <i>series</i> winding.
+In this case <i>all</i> 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.</p>
+
+<div class="figcenter" style="width: 276px;">
+<img src="images/image79.jpg" width="276" height="247" alt="Fig. 76." title="" />
+<span class="caption"><span class="smcap">Fig. 76.</span>&mdash;The brushes of a Holmes dynamo.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_173" id="Page_173">[Pg 173]</a></span></p><p>Fig. 78 shows another method of winding&mdash;the <i>shunt</i>. 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.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 77 and 78">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 375px;">
+<img src="images/image80.jpg" width="300" height="602" alt="Fig. 77." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 77.</span>&mdash;Sketch showing a "series" winding.</span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image81.jpg" width="300" height="556" alt="Fig. 78." title="" />
+<span class="caption"><span class="smcap">Fig. 78.</span>&mdash;"Shunt" winding.</span>
+</div></td></tr>
+</table></div>
+
+<p>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<span class='pagenum'><a name="Page_174" id="Page_174">[Pg 174]</a></span> winding as in Fig. 77. This compound method is adapted
+more especially for electric traction.</p>
+
+
+<p class="section">ALTERNATING DYNAMOS.</p>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_175" id="Page_175">[Pg 175]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image82.jpg" width="500" height="276" alt="Fig. 79." title="" />
+<span class="caption"><span class="smcap">Fig. 79</span>.</span>
+</div>
+
+
+<p class="section">TRANSMISSION OF POWER.</p>
+
+<p>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 <i>minus</i>
+a contact-breaker (<a href="#Page_122">see p. 122</a>). 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, <span class="ampm">B</span>, reduces the pressure to 500 volts to
+drive an alternating motor, <span class="ampm">C</span>, which in turn operates a direct current
+dynamo, <span class="ampm">D</span>. This dynamo has its + terminal connected with the insulated
+or "live" rail of an electric railway, and its &ndash; terminal with the wheel
+rails, which are metallically united at the joints to act as<span class='pagenum'><a name="Page_176" id="Page_176">[Pg 176]</a></span> 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.</p>
+
+<p>To return for a moment to the question of transformation of current.
+"Why," it may be asked, "should we not send low-pressure <i>direct</i>
+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 <i>that</i>?" The answer is, that
+to transmit a large amount of electrical energy at low pressure (or
+voltage) would necessitate large volume (or <i>amperage</i>) 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<span class='pagenum'><a name="Page_177" id="Page_177">[Pg 177]</a></span> a small and inexpensive conductor with little loss.
+Also its voltage can be transformed by apparatus having no revolving
+parts.</p>
+
+
+<p class="section">THE ELECTRIC MOTOR.</p>
+
+<p>Anybody who understands the dynamo will also be able to understand the
+electric motor, which is merely a reversed dynamo.</p>
+
+<p>Imagine in Fig. 70 a dynamo taking the place of the lamp and passing
+current through the brushes and commutator into the coil <i>w x y z</i>. 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 <i>vice vers&acirc;</i>. 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 <i>ad infinitum</i>. The rotation of the
+motor is therefore merely a question of repulsion and attraction of like
+and unlike poles. An ordinary<span class='pagenum'><a name="Page_178" id="Page_178">[Pg 178]</a></span> 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.</p>
+
+<p>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.</p>
+
+<p>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:&mdash;(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.</p>
+
+
+<p><span class='pagenum'><a name="Page_179" id="Page_179">[Pg 179]</a></span></p><p class="section">ELECTRIC LIGHTING.</p>
+
+<p>Dynamos are used to generate current for two main purposes&mdash;(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.</p>
+
+
+<p class="section">INCANDESCENT LAMP.</p>
+
+<p>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&mdash;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.</p>
+
+<p>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<span class='pagenum'><a name="Page_180" id="Page_180">[Pg 180]</a></span> speed of travel
+is much greater than elsewhere in the circuit, most heat will be
+produced.</p>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/image83.jpg" width="300" height="361" alt="Fig. 80." title="" />
+<span class="caption"><span class="smcap">Fig. 80.</span>&mdash;Diagram to show circulation of water through a pipe.</span>
+</div>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/image84.jpg" width="300" height="411" alt="Fig. 81." title="" />
+<span class="caption"><span class="smcap">Fig. 81.</span>&mdash;The electrical counterpart of Fig. 80. The
+filament takes the place of the contraction in the pipe.</span>
+</div>
+
+<p>The manufacture of glow-lamps is now an important industry. One brand of
+lamp<a name="FNanchor_20_20" id="FNanchor_20_20"></a><a href="#Footnote_20_20" class="fnanchor">[20]</a> is made as follows:&mdash;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<span class='pagenum'><a name="Page_181" id="Page_181">[Pg 181]</a></span> 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.</p>
+
+
+<p><span class='pagenum'><a name="Page_182" id="Page_182">[Pg 182]</a></span></p><p class="section">ARC LAMPS.</p>
+
+<p>In <i>arc</i> 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 <i>arc</i> 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<span class='pagenum'><a name="Page_183" id="Page_183">[Pg 183]</a></span> when no current flows. As soon as current circulates through
+the coil the bar is drawn upwards against the spring.</p>
+
+
+<p class="section">SERIES AND PARALLEL ARRANGEMENT OF LAMPS.</p>
+
+<p>When current passes from one lamp to another, as in Fig. 82, the lamps
+are said to be in <i>series</i>. 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image85.jpg" width="500" height="296" alt="Fig. 82." title="" />
+<span class="caption"><span class="smcap">Fig. 82.</span>&mdash;Incandescent lamps connected in "series."</span>
+</div>
+
+<p>Fig. 83 shows a number of lamps set <i>in parallel</i>. One terminal of each
+is attached to the positive conductor, the other to the negative
+conductor. Each<span class='pagenum'><a name="Page_184" id="Page_184">[Pg 184]</a></span> lamp therefore forms an independent bridge, and does
+not affect the efficiency of the rest. <i>Parallel series</i> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image86.jpg" width="500" height="228" alt="Fig. 83." title="" />
+<span class="caption"><span class="smcap">Fig. 83.</span>&mdash;Incandescent lamps connected in "parallel."</span>
+</div>
+
+
+<p class="section">CURRENT FOR ELECTRIC LAMPS.</p>
+
+<p>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.</p>
+
+<p>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<span class='pagenum'><a name="Page_185" id="Page_185">[Pg 185]</a></span> 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.</p>
+
+<p>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&mdash;that is, are surrounded by globes perforated by a
+single small hole, which renders combustion very slow, though preventing
+a vacuum.</p>
+
+
+<p class="section">ELECTROPLATING.</p>
+
+<p>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. <span class="ampm">A</span> is a battery; <span class="ampm">B</span> a vessel containing, say, an acidulated
+solution of sulphate of copper. A spoon, <span class="ampm">S</span>, hanging in this from a glass
+rod, <span class="ampm">R</span>, is connected with the zinc or negative element, <span class="ampm">Z</span>, of the
+battery, and a plate of copper, <span class="ampm">P</span>, with the positive element, <span class="ampm">C</span>. Current
+flows in the direction shown by the arrows, from <span class="ampm">Z</span> to <span class="ampm">C</span>, <span class="ampm">C</span> to <span class="ampm">P</span>, <span class="ampm">P</span> to
+<span class="ampm">S</span>,<span class='pagenum'><a name="Page_186" id="Page_186">[Pg 186]</a></span> <span class="ampm">S</span> to <span class="ampm">Z</span>. 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image87.jpg" width="500" height="276" alt="Fig. 84." title="" />
+<span class="caption"><span class="smcap">Fig. 84.</span>&mdash;An electroplating outfit.</span>
+</div>
+
+<p>In silver plating, <span class="ampm">P</span> 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.</p>
+
+
+<div class="footnote"><p><a name="Footnote_17_17" id="Footnote_17_17"></a><a href="#FNanchor_17_17"><span class="label">[17]</span></a> From the Latin <i>commuto</i>, "I exchange."</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_18" id="Footnote_18_18"></a><a href="#FNanchor_18_18"><span class="label">[18]</span></a> Only the "drum" type of armature is treated here.</p></div>
+
+<div class="footnote"><p><a name="Footnote_19_19" id="Footnote_19_19"></a><a href="#FNanchor_19_19"><span class="label">[19]</span></a> This refers to continuous-current dynamos only.</p></div>
+
+<div class="footnote"><p><a name="Footnote_20_20" id="Footnote_20_20"></a><a href="#FNanchor_20_20"><span class="label">[20]</span></a> The Robertson.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_187" id="Page_187">[Pg 187]</a></span></p>
+<h3><a name="Chapter_X" id="Chapter_X"></a>Chapter X.</h3>
+
+<h4>RAILWAY BRAKES.</h4>
+
+<div class="blockquot"><p class="hang">The Vacuum Automatic brake&mdash;The Westinghouse air-brake. </p></div>
+
+
+<p class="noin"><span class="dcap">I</span><span class="caps">n</span> 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.</p>
+
+<p>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<span class='pagenum'><a name="Page_188" id="Page_188">[Pg 188]</a></span>
+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.</p>
+
+<p>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:&mdash;(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<span class='pagenum'><a name="Page_189" id="Page_189">[Pg 189]</a></span> 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.</p>
+
+<p>At the present day one or other of two systems is used on practically
+all automatically-braked cars and coaches. These are known as&mdash;(1) The
+<i>vacuum automatic</i>, using the pressure of the atmosphere on a piston
+from the other side of which air has been mechanically exhausted; and
+(2) the <i>Westinghouse automatic</i>, using compressed air. The action of
+these brakes will now be explained as simply as possible.</p>
+
+
+<p class="section">THE VACUUM AUTOMATIC BRAKE.</p>
+
+<p>Under each carriage is a vacuum chamber (Fig. 85) riding on trunnions, <span class="ampm">E
+E</span>, 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<span class='pagenum'><a name="Page_190" id="Page_190">[Pg 190]</a></span> upwards with a
+maximum pressure of 15 lbs. to the square inch. The ball-valve ensures
+that while air can be sucked from <i>both</i> sides of the piston, it can be
+admitted to the lower side only.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image88.jpg" width="500" height="491" alt="Fig. 85." title="" />
+<span class="caption"><span class="smcap">Fig. 85.</span>&mdash;Vacuum brake "off."</span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image89.jpg" width="500" height="442" alt="Fig. 86." title="" />
+<span class="caption"><span class="smcap">Fig. 86.</span>&mdash;Vacuum brake "on."</span>
+</div>
+
+<p>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,<a name="FNanchor_21_21" id="FNanchor_21_21"></a><a href="#Footnote_21_21" class="fnanchor">[21]</a>
+which<span class='pagenum'><a name="Page_191" id="Page_191">[Pg 191]</a></span> 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 <span class="ampm">D</span>; and from the space <span class="ampm">A A</span> and the cylinder (open at the
+top) through the channel <span class="ampm">C</span>, 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 <span class="ampm">D</span> and into the space <span class="ampm">B</span> (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<span class='pagenum'><a name="Page_192" id="Page_192">[Pg 192]</a></span> is to be noted: If there
+is a leak, as in the case of the train parting, <i>the brakes go on at
+once</i>, since the vacuum below the piston is automatically broken.</p>
+
+<div class="figleft" style="width: 300px;">
+<img src="images/image90.jpg" width="300" height="414" alt="Fig. 87." title="" />
+<span class="caption"><span class="smcap">Fig. 87.</span>&mdash;Guard's valve for applying the Vacuum brake.</span>
+</div>
+
+<p>For ordinary stops the vacuum is only partially broken&mdash;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 <i>both</i> 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, <i>A</i>, connected by a bolt, <span class="ampm">B</span>, to an elastic diaphragm, <span class="ampm">C</span>, sealing
+the bottom of the chamber <span class="ampm">D</span>.<span class='pagenum'><a name="Page_193" id="Page_193">[Pg 193]</a></span> The bolt <span class="ampm">B</span> has a very small hole bored
+through it from end to end. When the vacuum is broken slowly, the
+pressure falls in <span class="ampm">D</span> as fast as in the pipe; but a sudden inrush of air
+causes the valve <span class="ampm">A</span> to be pulled off its seat by the diaphragm <span class="ampm">C</span>, as the
+vacuum in <span class="ampm">D</span> has not been broken to any appreciable extent. Air then
+rushes into the train pipe through the valve. It is thus 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.</p>
+
+<p>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 <span class="ampm">E</span>, 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.</p>
+
+<p>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<span class='pagenum'><a name="Page_194" id="Page_194">[Pg 194]</a></span> it to leave the station unprepared to make a sudden
+stop if necessary.</p>
+
+
+<p class="section">THE WESTINGHOUSE AIR-BRAKE.</p>
+
+<p>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.</p>
+
+<p>The air-brake, as first evolved by Mr. George Westinghouse, was a very
+simple affair&mdash;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<span class='pagenum'><a name="Page_195" id="Page_195">[Pg 195]</a></span> 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 <i>automatic</i>
+brake, now so largely used all over the world. The brake ensures
+practically instantaneous and simultaneous action on all the vehicles of
+<i>a train of any length</i>.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image91.jpg" width="500" height="275" alt="Fig. 88." title="" />
+<span class="caption"><span class="smcap">Fig. 88.</span>&mdash;Diagrammatic sketch of the details of the
+Westinghouse air-brake. Brake "off."</span>
+</div>
+
+<p>The principle of the brake will be gathered from Figs. 88 and 89. <span class="ampm">P</span> is a
+steam-driven air-pump on the engine, which compresses air into a
+reservoir, <span class="ampm">A</span>, situated below the engine or tender, and maintains a<span class='pagenum'><a name="Page_196" id="Page_196">[Pg 196]</a></span>
+pressure of from 80 to 90 lbs. per square inch. A three-way cock, <span class="ampm">C</span>,
+puts the train pipe into communication with <span class="ampm">A</span> or the open air at the
+wish of the driver. Under each coach is a triple-valve, <span class="ampm">T</span>, an auxiliary
+reservoir, <span class="ampm">B</span>, and a brake cylinder, <span class="ampm">D</span>. The triple-valve is the most
+noteworthy feature of the whole system. The reader must remember that
+the valve shown in the section is <i>only diagrammatic</i>.</p>
+
+<p>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 <span class="ampm">C</span> to the position shown in Fig. 89), the train-pipe pressure is
+reduced, the triple-valve at once shifts, putting <span class="ampm">B</span> in connection with
+the brake cylinder <span class="ampm">D</span>, and cutting off the connection between <span class="ampm">D</span> 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 <span class="ampm">B</span>, when the valve will assume its
+original position, allowing the air in <span class="ampm">D</span> to escape.</p>
+
+<p>The force with which the brake is applied depends<span class='pagenum'><a name="Page_197" id="Page_197">[Pg 197]</a></span> upon the reduction of
+pressure in the train pipe. A slight reduction would admit air very
+slowly from <span class="ampm">B</span> to <span class="ampm">D</span>, 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image92.jpg" width="500" height="259" alt="Fig. 89." title="" />
+<span class="caption"><span class="smcap">Fig. 89.</span>&mdash;Brake "on."</span>
+</div>
+
+<p>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 <i>directly</i> into <span class="ampm">D</span>. It will easily be understood that
+a double advantage is hereby gained&mdash;first, in utilizing a considerable
+portion of the air in the train pipe to increase the<span class='pagenum'><a name="Page_198" id="Page_198">[Pg 198]</a></span> 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.</p>
+
+<p>It may be added that this secondary communication is kept open only
+until the pressure in <span class="ampm">D</span> is equal to that in the train pipe. Then it is
+cut off, to prevent a return of air from <span class="ampm">B</span> to the pipe.</p>
+
+<p>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 <span class="ampm">F</span>, flows
+through valve 14 and passes by <span class="ampm">D</span> 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 <span class="ampm">C</span>. 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<span class='pagenum'><a name="Page_199" id="Page_199">[Pg 199]</a></span> 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 <span class="ampm">C</span> escapes
+through the small port <i>a</i> 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.</p>
+
+<div class="figcenter" style="width: 268px;">
+<img src="images/image93.jpg" width="268" height="301" alt="Fig. 90." title="" />
+<span class="caption"><span class="smcap">Fig. 90.</span>&mdash;Air-pump of Westinghouse brake.</span>
+</div>
+
+
+<div class="footnote"><p><a name="Footnote_21_21" id="Footnote_21_21"></a><a href="#FNanchor_21_21"><span class="label">[21]</span></a> This resembles the upper part of the rudimentary water
+injector shown in Fig. 15. The reader need only imagine pipe <span class="ampm">B</span> to be
+connected with the train pipe. A rush of steam through pipe <span class="ampm">A</span> creates a
+partial vacuum in the cone <span class="ampm">E</span>, causing air from the train pipe to rush
+into it and be expelled by the steam blast.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_200" id="Page_200">[Pg 200]</a></span></p>
+<h3><a name="Chapter_XI" id="Chapter_XI"></a>Chapter XI.</h3>
+
+<h4>RAILWAY SIGNALLING.</h4>
+
+<div class="blockquot"><p class="hang">The block system&mdash;Position of signals&mdash;Interlocking the
+signals&mdash;Locking gear&mdash;Points&mdash;Points and signals in
+combination&mdash;Working the block system&mdash;Series of signalling
+operations&mdash;Single line signals&mdash;The train staff&mdash;Train staff and
+ticket&mdash;Electric train staff system&mdash;Interlocking&mdash;Signalling
+operations&mdash;Power signalling&mdash;Pneumatic signalling&mdash;Automatic
+signalling. </p></div>
+
+
+<p class="noin"><span class="dcap">U</span><span class="caps">nder</span> certain conditions&mdash;namely, at sharp curves or in darkness&mdash;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.</p>
+
+<p>In the early days of the railway it was customary to allow a time
+interval between the passings of<span class='pagenum'><a name="Page_201" id="Page_201">[Pg 201]</a></span> 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.</p>
+
+
+<p class="section">THE BLOCK SYSTEM.</p>
+
+<p>Time limits were abolished and distance limits substituted. A line was
+divided into <i>blocks</i>, or lengths, and two trains going in the same
+direction were never allowed on any one block at the same time.</p>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_202" id="Page_202">[Pg 202]</a></span></p><p>The distant and other signal arms vary slightly in shape (Fig. 91). A
+distant signal has a forked end and a <span class="bigletter">V</span>-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&mdash;so that a driver is kept as fully informed at night as
+during the day, provided the lamp remains alight.</p>
+
+<div class="figright" style="width: 300px;">
+<img src="images/image94.jpg" width="300" height="389" alt="Fig. 91." title="" />
+<span class="caption"><span class="smcap">Fig. 91.</span>&mdash;Distant and home signals.</span>
+</div>
+
+
+<p class="section">POSITION OF SIGNALS.</p>
+
+<p>On double lines each set of rails has its own separate signals, and
+drivers travelling on the "up" line<span class='pagenum'><a name="Page_203" id="Page_203">[Pg 203]</a></span> take no notice of signals meant for
+the "down" line. Each signal-box usually controls three signals on each
+set of rails&mdash;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 reinforced by an "advance starting" signal some distance
+farther on.</p>
+
+<p>It should be noted that the distant is only a <i>caution</i> signal, whereas
+both home and starting are <i>stop</i> 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<span class='pagenum'><a name="Page_204" id="Page_204">[Pg 204]</a></span> box. That point is called the <i>standard
+clearing point</i>.</p>
+
+<p>Technically described, a <i>block</i> 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.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/image95.jpg" width="600" height="114" alt="Fig. 92." title="" />
+<span class="caption"><span class="smcap">Fig. 92.</span>&mdash;Showing position of signals. Those at the top are "off."</span>
+</div>
+
+
+<p class="section">INTERLOCKING SIGNALS.</p>
+
+<p>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:&mdash;Starting and home; <i>then</i> distant. And restore them&mdash;distant;
+<i>then</i> 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<span class='pagenum'><a name="Page_205" id="Page_205">[Pg 205]</a></span> from restoring the home or starting until the distant is
+at danger; and, <i>vice vers&acirc;</i>, he cannot lower the last until the other
+two are off. This mechanism is called <i>locking gear</i>.</p>
+
+
+<p class="section">LOOKING GEAR.</p>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image96.jpg" width="500" height="602" alt="Fig. 93." title="" />
+<span class="caption"><span class="smcap">Fig. 93.</span>&mdash;A signal lever and its connections. To move the
+lever, <span class="ampm">C</span> is pressed towards <span class="ampm">B</span> raising the catch-rod from its nick in the
+rack, <span class="ampm">G G G</span>, guides; <span class="ampm">R R</span>, anti-friction rollers; <span class="ampm">S</span>, sockets for
+catch-rod to work in.</span>
+</div>
+
+<p>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, <span class="ampm">D</span>, <span class="ampm">H</span>, and <span class="ampm">S</span>, crossed by a bar, <span class="ampm">B</span>, from which
+project these studs. The levers are all forward and the signals all
+"on." If the signalman<span class='pagenum'><a name="Page_206" id="Page_206">[Pg 206]</a></span> tried to pull the lever attached to <span class="ampm">D</span> down the
+page, as it were, he would fail to move it on account of the stud <i>a</i>,
+which engages with a notch in <span class="ampm">D</span>. Before this stud can be got free of the
+notch the tappets <span class="ampm">H</span> and <span class="ampm">S</span> must be pulled over, so as to bring their
+notches in line with studs <i>b</i> and <i>c</i> (Fig. 95). The signalman can now
+move <span class="ampm">D</span>, since the notch easily pushes the stud <i>a</i> to the<span class='pagenum'><a name="Page_207" id="Page_207">[Pg 207]</a></span> left (Fig.
+96). The signals must be restored to danger. As <span class="ampm">H</span> and <span class="ampm">S</span> are back-locked
+by <span class="ampm">D</span>&mdash;that is, prevented by <span class="ampm">D</span> from being put back into their normal
+positions&mdash;<span class="ampm">D</span> must be moved first. The interlocking of the three signals
+described is merely repeated in the interlocking of a large number of
+signals.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image97.jpg" width="500" height="319" alt="Fig. 94." title="" />
+<span class="caption"><span class="smcap">Fig. 94.</span></span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image98.jpg" width="500" height="346" alt="Fig. 95." title="" />
+<span class="caption"><span class="smcap">Fig. 95.</span></span>
+</div>
+
+<p>On entering a signal-box a visitor will notice that<span class='pagenum'><a name="Page_208" id="Page_208">[Pg 208]</a></span> the levers have
+different colours:&mdash;<i>Green</i>, signifying distant signals; <i>red</i>,
+signifying home and starting signals; <i>blue</i>, signifying facing points;
+<i>black</i>, signifying trailing points; <i>white</i>, signifying spare levers.
+These different colours help the signalman to pick out the right levers
+easily.</p>
+
+<p>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 <i>leads</i>, refer to levers which must be pulled
+before that particular lever can be released.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image99.jpg" width="500" height="346" alt="Fig. 96." title="" />
+<span class="caption"><span class="smcap">Fig. 96.</span></span>
+</div>
+
+<p><span class='pagenum'><a name="Page_209" id="Page_209">[Pg 209]</a></span></p>
+<div class="figcenter" style="width: 525px;"><br />
+<img src="images/image100.jpg" width="525" height="335" alt="Fig. 97." title="" />
+<span class="caption"><span class="smcap">Fig. 97.</span>&mdash;Model signal equipment in a signalling school.
+(By permission of the "G.W.R. Magazine").</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_210" id="Page_210">[Pg 210]</a></span></p><p class="section">POINTS.</p>
+
+<p>Mention was made, in connection with the lever, of <i>points</i>. 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 <span class="bigletter">V</span>-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), <span class="ampm">A A</span>.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image101.jpg" width="500" height="165" alt="Fig. 98." title="" />
+<span class="caption"><span class="smcap">Fig. 98.</span>&mdash;Points open to main line.</span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image102.jpg" width="500" height="165" alt="Fig. 99." title="" />
+<span class="caption"><span class="smcap">Fig. 99.</span>&mdash;Points open to branch line.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_211" id="Page_211">[Pg 211]</a></span></p><p>It might be thought that the wheels would bump badly when they reach
+the point <span class="ampm">B</span>, where there is a gap. This is prevented, however, by the
+bent ends <span class="ampm">E E</span> (Fig. 98), on which the tread of the wheel rests until it
+has reached some distance along the point of <span class="ampm">V</span>. The safety rails <span class="ampm">S R</span>
+keep the outer wheel up against its rail until the <span class="bigletter">V</span> has been passed.</p>
+
+
+<p class="section">POINTS AND SIGNALS IN COMBINATION.</p>
+
+<p>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<span class='pagenum'><a name="Page_212" id="Page_212">[Pg 212]</a></span> 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.</p>
+
+
+<p class="section">WORKING OF BLOCK SYSTEM.</p>
+
+<p>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:&mdash;(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&mdash;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<span class='pagenum'><a name="Page_213" id="Page_213">[Pg 213]</a></span> opening,
+behind which moves a disc carrying two "flags"&mdash;"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.</p>
+
+
+<p class="section">SERIES OF SIGNALLING OPERATIONS.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/image103.jpg" width="600" height="126" alt="Fig. 100." title="" />
+<span class="caption"><span class="smcap">Fig. 100.</span>&mdash;The signaling instruments in three adjacent
+cabins. The featherless arrows show the connection of the instruments.</span>
+</div>
+
+<p>We may now watch the doings<span class='pagenum'><a name="Page_214" id="Page_214">[Pg 214]</a></span> 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?"&mdash;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.<span class='pagenum'><a name="Page_215" id="Page_215">[Pg 215]</a></span> 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.</p>
+
+
+<p class="section">THE WORKING OF SINGLE LINES.</p>
+
+<p>We have dealt with the signalling arrangements pertaining to double
+lines of railway, showing that<span class='pagenum'><a name="Page_216" id="Page_216">[Pg 216]</a></span> 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 <i>at the same time</i>.
+This is effected in several ways, the essence of each being that the
+engine-driver shall have in his possession <i>visible</i> evidence of the
+permission accorded him by the signalman to enter a section of single
+line.</p>
+
+
+<p class="section">A SINGLE TRAIN STAFF.</p>
+
+<p>The simplest form of working is to allocate to the length of line a
+"train staff"&mdash;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.</p>
+
+
+<p class="section">TRAIN STAFF AND TICKET.</p>
+
+<p>On long lengths of single line where more than one<span class='pagenum'><a name="Page_217" id="Page_217">[Pg 217]</a></span> 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
+<i>ticket</i> used in conjunction with the staff.</p>
+
+<p>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 <i>see</i> the staff. If a
+second train is required to follow, the staff is <i>shown</i> 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 <i>handed</i> to the driver.</p>
+
+<p>To render this system as safe as possible, train staff<span class='pagenum'><a name="Page_218" id="Page_218">[Pg 218]</a></span> 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.</p>
+
+
+<p class="section">ELECTRIC TRAIN STAFF AND TABLET SYSTEMS.</p>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_219" id="Page_219">[Pg 219]</a></span></p>
+<div class="figcenter" style="width: 324px;">
+<img src="images/image104.jpg" width="324" height="471" alt="Fig. 101." title="" />
+<span class="caption"><span class="smcap">Fig. 101.</span>&mdash;An electric train staff holder: <span class="ampm">S S</span>, staffs
+in the slot of the instrument. Leaning against the side of the cabin is
+a staff showing the key <span class="ampm">K</span> 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.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_220" id="Page_220">[Pg 220]</a></span></p><p>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
+<i>all</i> the staffs are in the instruments, a staff may be withdrawn at
+<i>either</i> 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.</p>
+
+<p>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<span class='pagenum'><a name="Page_221" id="Page_221">[Pg 221]</a></span> 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&mdash;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"&mdash;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&mdash;an odd number, and, what
+is more important, <i>raising</i> 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<span class='pagenum'><a name="Page_222" id="Page_222">[Pg 222]</a></span> 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.</p>
+
+<p>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.</p>
+
+
+<p class="section">INTERLOCKING.</p>
+
+<p>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&mdash;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.</p>
+
+
+<p><span class='pagenum'><a name="Page_223" id="Page_223">[Pg 223]</a></span></p><p class="section">SIGNALLING OPERATIONS.</p>
+
+<p>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</p>
+
+<p><span class='pagenum'><a name="Page_224" id="Page_224">[Pg 224]</a></span></p>
+<div class="figcenter" style="width: 470px;">
+<img src="images/image105.jpg" width="470" height="336" alt="Fig. 102." title="" />
+<span class="caption"><span class="smcap">Fig. 102.</span>&mdash;An electric lever-frame in a signalling cabin
+at Didcot.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_225" id="Page_225">[Pg 225]</a></span></p><p class="section">"POWER" SIGNALLING.</p>
+
+<p>In a power system of signalling the signalman is provided with some
+auxiliary means&mdash;electricity, compressed air, etc.&mdash;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.</p>
+
+
+<p class="section">ELECTRIC SIGNALLING.</p>
+
+<p>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<span class='pagenum'><a name="Page_226" id="Page_226">[Pg 226]</a></span> 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."</p>
+
+<p>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<span class='pagenum'><a name="Page_227" id="Page_227">[Pg 227]</a></span> 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.</p>
+
+
+<p class="section">PNEUMATIC SIGNALLING.</p>
+
+<p>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.</p>
+
+
+<p><span class='pagenum'><a name="Page_228" id="Page_228">[Pg 228]</a></span></p><p class="section">AUTOMATIC SIGNALLING.</p>
+
+<p>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.</p>
+
+<p>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<span class='pagenum'><a name="Page_229" id="Page_229">[Pg 229]</a></span> 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.</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_230" id="Page_230">[Pg 230]</a></span></p>
+<h3><a name="Chapter_XII" id="Chapter_XII"></a>Chapter XII.</h3>
+
+<h4>OPTICS.</h4>
+
+<div class="blockquot"><p class="hang">Lenses&mdash;The image cast by a convex lens&mdash;Focus&mdash;Relative position
+of object and lens&mdash;Correction of lenses for colour&mdash;Spherical
+aberration&mdash;Distortion of image&mdash;The human eye&mdash;The use of
+spectacles&mdash;The blind spot. </p></div>
+
+
+<p class="noin"><span class="dcap">L</span><span class="caps">ight</span> is a third form of that energy of which we have already treated
+two manifestations&mdash;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.</p>
+
+<p>If a beam of white light be passed through a prism it is resolved into
+the seven visible colours of the spectrum&mdash;violet, indigo, blue, green,
+yellow, orange, and red&mdash;in this order. The human eye is most sensitive
+to the yellow-red rays, a photographic plate to the green-violet rays.</p>
+
+<p>All bodies fall into one of two classes&mdash;(1)<span class='pagenum'><a name="Page_231" id="Page_231">[Pg 231]</a></span> <i>Luminous</i>&mdash;that is, those
+which are a <i>source</i> of light, such as the sun, a candle flame, or a
+red-hot coal; and (2) <i>non-luminous</i>, which become visible only by
+virtue of light which they receive from other bodies and reflect to our
+eyes.</p>
+
+
+<p class="section">THE PROPAGATION OF LIGHT.</p>
+
+<p>Light naturally travels in a straight line. It is deflected only when it
+passes from one transparent medium into another&mdash;for example, from air
+to water&mdash;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.</p>
+
+
+<p class="section">LENSES.</p>
+
+<p>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<span class='pagenum'><a name="Page_232" id="Page_232">[Pg 232]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image106.jpg" width="500" height="145" alt="Fig. 103." title="" />
+<span class="caption"><span class="smcap">Fig. 103.</span>&mdash;Showing how a burning-glass concentrates the
+heat rays which fall upon it.</span>
+</div>
+
+<p>It should be noted here that <i>sunlight</i>, as we call it, is accompanied
+by heat. A burning-glass is used to concentrate the <i>heat</i> rays, not the
+<i>light</i> rays, which, though they are collected too, have no igniting
+effect.</p>
+
+<p>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&mdash;that is, the curve from the
+centre to the circumference must be the same in all directions.</p>
+
+<p><span class='pagenum'><a name="Page_233" id="Page_233">[Pg 233]</a></span></p><p>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 <i>convex</i> lens; when thinner, a <i>concave</i> lens. The
+names of the various shapes are as follows:&mdash;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), <i>concentrate</i> a pencil of rays passing through
+them; while the thin-centre lenses (Nos. 2, 4, 6) <i>scatter</i> the rays
+(<a href="#Fig_105">see Fig. 105</a>).</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image107.jpg" width="500" height="182" alt="Fig. 104." title="" />
+<span class="caption"><span class="smcap">Fig. 104.</span>&mdash;Six forms of lenses.</span>
+</div>
+
+
+<p class="section">THE CAMERA.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_105" id="Fig_105"></a>
+<img src="images/image108.jpg" width="500" height="189" alt="Fig. 105." title="" />
+<span class="caption"><span class="smcap">Fig. 105.</span></span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image109.jpg" width="500" height="163" alt="Fig. 106." title="" />
+<span class="caption"><span class="smcap">Fig. 106.</span></span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_234" id="Page_234">[Pg 234]</a></span> 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, <span class="ampm">B</span>, to catch the image. Suppose that <span class="ampm">A</span> 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 <span class="ampm">B</span>, <i>above</i> the centre of the
+screen, though <span class="ampm">A</span> is below the axis of the<span class='pagenum'><a name="Page_235" id="Page_235">[Pg 235]</a></span> 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, <span class="ampm">A</span>, and focussing them on a point, <span class="ampm">B</span>. 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 <span class="ampm">A</span> will be impressed on a<span class='pagenum'><a name="Page_236" id="Page_236">[Pg 236]</a></span> sensitized photographic plate 1,000
+times more quickly.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image110.jpg" width="500" height="146" alt="Fig. 107." title="" />
+<span class="caption"><span class="smcap">Fig. 107.</span></span>
+</div>
+
+
+<p class="section">THE IMAGE CAST BY A CONVEX LENS.</p>
+
+<p>Fig. 108 shows diagrammatically how a convex lens forms an image. From <span class="ampm">A</span>
+and <span class="ampm">B</span>, 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 A<sup>1</sup> and B<sup>1</sup>. In reality a countless number
+of rays would be transmitted from every point of the object and
+collected to form the image.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image111.jpg" width="500" height="177" alt="Fig. 108." title="" />
+<span class="caption"><span class="smcap">Fig. 108.</span>&mdash;Showing how an image is cast by a convex lens.</span>
+</div>
+
+
+<p class="section">FOCUS.</p>
+
+<p>We must now take special notice of that word heard so often in
+photographic talk&mdash;"focus." What<span class='pagenum'><a name="Page_237" id="Page_237">[Pg 237]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image112.jpg" width="500" height="404" alt="Fig. 109." title="" />
+<span class="caption"><span class="smcap">Fig. 109.</span></span>
+</div>
+
+<p>We must here digress a moment to draw attention to the three simple
+diagrams of Fig. 109. The object, <span class="ampm">O</span>, 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 <span class="ampm">F</span>. If the distance
+between <span class="ampm">F</span> and the centre of the lens is six inches, we say<span class='pagenum'><a name="Page_238" id="Page_238">[Pg 238]</a></span> 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 <i>principal</i> focus,
+and is denoted by the symbol <i>f</i>. 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,
+<span class="ampm">F<sup>1</sup></span>, further from the lens than <span class="ampm">F</span>. The nearer we approach <span class="ampm">O</span> to the
+lens, the further away on the other side is the focal point, until a
+distance equal to that of <span class="ampm">F</span> 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 <span class="ampm">O</span> be brought nearer than the
+focal distance, the rays would <i>diverge</i> after passing through the lens.</p>
+
+
+<p class="section">RELATIVE POSITIONS OF OBJECT AND IMAGE.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image113.jpg" width="500" height="155" alt="Fig. 110." title="" />
+<span class="caption"><span class="smcap">Fig. 110.</span>&mdash;Showing how the position of the image alters
+relatively to the position of the object.</span>
+</div>
+
+<p>From what has been said above we deduce two main conclusions&mdash;(1.) The
+nearer an object is brought to the lens, the further away from the<span class='pagenum'><a name="Page_239" id="Page_239">[Pg 239]</a></span> 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 <span class="ampm">A B</span>, or <span class="ampm">A B<sup>1</sup></span>, denotes the principal
+focal length of the lens, and <span class="ampm">A C</span>, or <span class="ampm">A C<sup>1</sup></span>, denotes twice the focal
+length. We will take the positions in order:&mdash;</p>
+
+<p><i>Position I.</i> Object further away than 2<i>f</i>. Inverted image <i>smaller</i>
+than object, at distance somewhat exceeding <i>f</i>.</p>
+
+<p><i>Position II.</i> Object at distance = 2<i>f</i>. Inverted image at distance =
+2<i>f</i>, and of size equal to that of object.</p>
+
+<p><i>Position III</i> Object nearer than 2<i>f</i>. Inverted image further away than
+2<i>f</i>; <i>larger</i> than the object.</p>
+
+<p><span class='pagenum'><a name="Page_240" id="Page_240">[Pg 240]</a></span></p><p><i>Position IV.</i> Object at distance = <i>f</i>. As rays are parallel after
+passing the lens <i>no</i> image is cast.</p>
+
+<p><i>Position V.</i> Object at distance less than <i>f</i>. No real image&mdash;that is,
+one that can be caught on a focussing screen&mdash;is now given by the lens,
+but a magnified, erect, <i>virtual</i> image exists on the same side of the
+lens as the object.</p>
+
+<p>We shall refer to <i>virtual</i> 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&mdash;namely, that to copy a diagram,
+etc., full size, both it and the plate must be exactly 2<i>f</i> from the
+optical centre of the lens. And it follows from this that the further he
+can rack his camera out beyond 2<i>f</i> the greater will be the possible
+enlargement of the original.</p>
+
+
+<p class="section">CORRECTION OF LENSES FOR COLOUR.</p>
+
+<p>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<span class='pagenum'><a name="Page_241" id="Page_241">[Pg 241]</a></span> 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 <span class="ampm">R</span>, in which the red rays meet, is much further from the lens
+than is <span class="ampm">V</span>, 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image114.jpg" width="500" height="210" alt="Fig. 111." title="" />
+<span class="caption"><span class="smcap">Fig. 111.</span></span>
+</div>
+
+<p><span class='pagenum'><a name="Page_242" id="Page_242">[Pg 242]</a></span></p>
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image115.jpg" width="500" height="226" alt="Fig. 112." title="" />
+<span class="caption"><span class="smcap">Fig. 112.</span></span>
+</div>
+
+<p>This defect can fortunately be corrected by the method shown in Fig.
+112. A <i>compound</i> lens is needed, made up of a <i>crown</i> glass convex
+element, <span class="ampm">B</span>, and a concave element, <span class="ampm">A</span>, of <i>flint</i> 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&mdash;a meniscus lens in fact, since <span class="ampm">A</span> is not so concave as
+<span class="ampm">B</span> 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&mdash;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<span class='pagenum'><a name="Page_243" id="Page_243">[Pg 243]</a></span> inward refraction
+by <span class="ampm">B</span>, and both sets of rays come to a focus in the same plane. Such a
+lens is called <i>achromatic</i>, 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.</p>
+
+
+<p class="section">SPHERICAL ABERRATION.</p>
+
+<p>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 <i>spherical aberration</i>. You will be able to
+understand the reason from Figs. 113 and 114. Two rays, <span class="ampm">A</span>, are parallel
+to the axis and enter the lens near the centre (Fig. 113). These meet in
+one plane. Two other rays, <span class="ampm">B</span>, 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 <span class="ampm">A</span>. If this happened in a camera
+the results would be very bad. Either <span class="ampm">A</span> or <span class="ampm">B</span> would be out of focus.<span class='pagenum'><a name="Page_244" id="Page_244">[Pg 244]</a></span> 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 <span class="ampm">B</span> of Fig. 113 are stopped by this plate, which is
+therefore called a <i>stop</i>. 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 <span class="ampm">A</span>.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image116.jpg" width="500" height="285" alt="Fig. 113." title="" />
+<span class="caption"><span class="smcap">Fig. 113.</span></span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image117.jpg" width="500" height="280" alt="Fig. 114." title="" />
+<span class="caption"><span class="smcap">Fig. 114.</span></span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_245" id="Page_245">[Pg 245]</a></span></p><p class="section">DISTORTION OF IMAGE.</p>
+
+<div class="figright" style="width: 300px;">
+<img src="images/image118.jpg" width="300" height="341" alt="Fig. 115." title="" />
+<span class="caption"><span class="smcap">Fig. 115.</span>&mdash;Section of a rectilinear lens.</span>
+</div>
+
+<p>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 <i>barrel</i>
+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 <i>pin-cushion</i> distortion. For a long time opticians
+were unable to find a remedy. Then Mr. George S. Cundell suggested that
+<i>two</i> meniscus lenses should be used in combination, one on either side
+of the stop, as in Fig 115.<span class='pagenum'><a name="Page_246" id="Page_246">[Pg 246]</a></span> Each produces distortion, but it is
+counteracted by the opposite distortion of the other, and a square is
+represented as a square. Lenses of this kind are called <i>rectilinear</i>,
+or straight-line producing.</p>
+
+<p>We have now reviewed the three chief defects of a lens&mdash;chromatic
+aberration, spherical aberration, and distortion&mdash;and have seen how they
+may be remedied. So we will now pass on to the most perfect of cameras,</p>
+
+
+<p class="section">THE HUMAN EYE.</p>
+
+<p>The eye (Fig. 116) is nearly spherical in form, and is surrounded
+outside, except in front, by a hard, horny coat called the
+<i>sclerotica</i> (<span class="ampm">S</span>). In front is the <i>cornea</i> (<span class="ampm">A</span>), which bulges outwards,
+and acts as a transparent window to admit light to the lens of the eye
+(<span class="ampm">C</span>). Inside the sclerotica, and next to it, comes the <i>choroid</i> coat;
+and inside that again is the <i>retina</i>, 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 <i>vitreous
+humour</i>; and the cavity between the lens and the cornea is full of
+water.</p>
+
+<p><span class='pagenum'><a name="Page_247" id="Page_247">[Pg 247]</a></span></p><p>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?</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image119.jpg" width="500" height="467" alt="Fig. 116." title="" />
+<span class="caption"><span class="smcap">Fig. 116.</span>&mdash;Section of the human eye.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_248" id="Page_248">[Pg 248]</a></span> 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 <i>in
+effect</i> the box has been racked out sufficiently.</p>
+
+<p>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 (<span class="ampm">L L</span>), 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.</p>
+
+<p>This wonderful lens is achromatic, and free from spherical aberration
+and distortion of image. Nor<span class='pagenum'><a name="Page_249" id="Page_249">[Pg 249]</a></span> must we forget that it is aided by an
+automatic "stop," the <i>iris</i>, the central hole of which is named the
+<i>pupil</i>. 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.</p>
+
+
+<p class="section">THE USE OF SPECTACLES.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 117a and 117b">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image120.jpg" width="300" height="201" alt="Fig. 117a." title="" />
+<span class="caption"><span class="smcap">Fig. 117</span><i>a</i>.</span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image121.jpg" width="300" height="201" alt="Fig. 117b." title="" />
+<span class="caption"><span class="smcap">Fig. 117</span><i>b</i>.</span>
+</div></td></tr>
+</table></div>
+
+<div class='center'><br />
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 118a and 118b">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image122.jpg" width="300" height="201" alt="Fig. 118a." title="" />
+<span class="caption"><span class="smcap">Fig. 118</span><i>a</i>.</span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image123.jpg" width="300" height="201" alt="Fig. 118b." title="" />
+<span class="caption"><span class="smcap">Fig. 118</span><i>b</i>.</span>
+</div></td></tr>
+</table></div>
+
+<p>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 117<i>a</i>, is too convex&mdash;has its
+minimum focus too short&mdash;and the rays meet and<span class='pagenum'><a name="Page_250" id="Page_250">[Pg 250]</a></span> cross before they reach
+the retina, causing general confusion of outline. This defect is simply
+remedied by placing in front of the eye (Fig. 117<i>b</i>) a <i>concave</i> 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 <i>myopia</i>. Long-sight, or <i>hypermetropia</i>, signifies that
+the eyeball is too short or the lens too flat. Fig. 118<i>a</i> represents
+the normal condition of a long-sighted eye. When<span class='pagenum'><a name="Page_251" id="Page_251">[Pg 251]</a></span> 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 <i>convex</i> spectacles
+for reading the newspaper. As seen in Fig. 118<i>b</i>, the spectacle lens
+concentrates the rays before they enter the eye, and so does part of the
+eye's work for it.</p>
+
+<p>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<span class='pagenum'><a name="Page_252" id="Page_252">[Pg 252]</a></span> 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."</p>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_253" id="Page_253">[Pg 253]</a></span></p>
+<h3><a name="Chapter_XIII" id="Chapter_XIII"></a>Chapter XIII.</h3>
+
+<h4>THE MICROSCOPE, THE TELESCOPE, AND THE MAGIC-LANTERN.</h4>
+
+<div class="blockquot"><p class="hang">The simple microscope&mdash;Use of the simple microscope in the
+telescope&mdash;The terrestrial telescope&mdash;The Galilean telescope&mdash;The
+prismatic telescope&mdash;The reflecting telescope&mdash;The parabolic
+mirror&mdash;The compound microscope&mdash;The magic-lantern&mdash;The
+bioscope&mdash;The plane mirror. </p></div>
+
+
+<p class="noin"><span class="dcap">I</span><span class="caps">n</span> Fig. 119 is represented an eye looking at a vase, three inches high,
+situated at <span class="ampm">A</span>, a foot away. If we were to place another vase, <span class="ampm">B</span>, six
+inches high, at a distance of two feet; or <span class="ampm">C</span>, nine inches high, at three
+feet; or <span class="ampm">D</span>, a foot high, at four feet, the image on the retina would in
+every case be of the same size as that cast by <span class="ampm">A</span>. We can therefore lay
+down the<span class='pagenum'><a name="Page_254" id="Page_254">[Pg 254]</a></span> rule that <i>the apparent size of an object depends on the angle
+that it subtends at the eye</i>.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image124.jpg" width="500" height="131" alt="Fig. 119." title="" />
+<span class="caption"><span class="smcap">Fig. 119.</span></span>
+</div>
+
+<p>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."</p>
+
+
+<p class="section">THE SIMPLE MICROSCOPE.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_120" id="Fig_120"></a>
+<img src="images/image125.jpg" width="500" height="306" alt="Fig. 120." title="" />
+<span class="caption"><span class="smcap">Fig. 120.</span></span>
+</div>
+
+<p>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, <span class="ampm">F</span> (<a href="#Fig_120">see Fig. 120</a>), from<span class='pagenum'><a name="Page_255" id="Page_255">[Pg 255]</a></span>
+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 <i>impression</i> 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.<a name="FNanchor_22_22" id="FNanchor_22_22"></a><a href="#Footnote_22_22" class="fnanchor">[22]</a> 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&mdash;that is, one-fifth of the least distance of distinct vision&mdash;we
+shall get an image on the retina five times larger in diameter than
+would be possible without the lens.</p>
+
+<p>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, <span class="ampm">F</span>. We
+have already seen (Fig. 109) that rays coming from a point in the
+principal focal plane emerge as a<span class='pagenum'><a name="Page_256" id="Page_256">[Pg 256]</a></span> 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 <i>diverging</i> rays (Fig. 121), the eye lens thickening the necessary
+amount, and we therefore put our magnifying glass a bit nearer than <span class="ampm">F</span> to
+get full advantage of proximity. If we had the object <i>outside</i> 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 <i>flatten</i> more than is required for focussing parallel rays.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image126.jpg" width="500" height="268" alt="Fig. 121." title="" />
+<span class="caption"><span class="smcap">Fig. 121.</span></span>
+</div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image127.jpg" width="500" height="186" alt="Fig. 122." title="" />
+<span class="caption"><span class="smcap">Fig. 122.</span></span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_257" id="Page_257">[Pg 257]</a></span></p><p class="section">USE OF THE SIMPLE MICROSCOPE IN THE TELESCOPE.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image128.jpg" width="500" height="139" alt="Fig. 123." title="" />
+<span class="caption"><span class="smcap">Fig. 123.</span></span>
+</div>
+
+<p>Let us now turn to Fig. 123. At <span class="ampm">A</span> is a distant object, say, a hundred
+yards away. <span class="ampm">B</span> 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 <span class="ampm">C</span>. If the eye were placed at
+<span class="ampm">C</span>, it would distinguish nothing. But if withdrawn to <span class="ampm">D</span>, the least
+distance of distinct vision,<a name="FNanchor_23_23" id="FNanchor_23_23"></a><a href="#Footnote_23_23" class="fnanchor">[23]</a> behind <span class="ampm">C</span>, the image is seen clearly.
+That the image really is at <span class="ampm">C</span> is proved by letting down the focussing
+screen, which at once catches it. Now, as the focus of the lens is twice
+<i>d</i>, 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:&mdash;</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Focal Length">
+<tr>
+<td align='center'>Magnification =</td>
+<td align='center'><span class="u">focal length of lens</span><br /><i>d</i></td>
+</tr>
+</table></div>
+
+<p><span class='pagenum'><a name="Page_258" id="Page_258">[Pg 258]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image129.jpg" width="500" height="177" alt="Fig. 124." title="" />
+<span class="caption"><span class="smcap">Fig. 124.</span></span>
+</div>
+
+<p>In Fig. 124 we have interposed between the eye and the object a small
+magnifying glass of 2&frac12;-inch focus, so that the eye can now clearly
+see the image when one-quarter <i>d</i> away from it. <span class="ampm">B</span> already magnifies the
+image twice; the eye-piece again magnifies it four times; so that the
+total magnification is 2 &times; 4 = 8 times. This result is arrived at
+quickly by dividing the focus of <span class="ampm">B</span> (which corresponds to the
+object-glass of a telescope) by the focus of the eye-piece, thus:&mdash;</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Eight Equation">
+<tr>
+<td align='center'><span class="u">20</span><br />2&frac12;</td>
+<td align='center'> = 8</td>
+</tr>
+</table></div>
+
+<p>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.</p>
+
+
+<p><span class='pagenum'><a name="Page_259" id="Page_259">[Pg 259]</a></span></p><p class="section">THE TERRESTRIAL TELESCOPE.</p>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image130.jpg" width="500" height="148" alt="Fig. 125." title="" />
+<span class="caption"><span class="smcap">Fig. 125.</span></span>
+</div>
+
+
+<p class="section">THE GALILEAN TELESCOPE.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image131.jpg" width="500" height="110" alt="Fig. 126." title="" />
+<span class="caption"><span class="smcap">Fig. 126.</span></span>
+</div>
+
+<p>A third form of telescope is that invented by the great Italian
+astronomer, Galileo,<a name="FNanchor_24_24" id="FNanchor_24_24"></a><a href="#Footnote_24_24" class="fnanchor">[24]</a> in 1609. Its principle is shown in Fig. 126.
+The rays transmitted<span class='pagenum'><a name="Page_260" id="Page_260">[Pg 260]</a></span> by the object-glass are caught, <i>before</i> 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.</p>
+
+
+<p class="section">THE PRISMATIC TELESCOPE.</p>
+
+<p>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 <span class="ampm">A</span> 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.</p>
+
+<div class="figleft" style="width: 300px;">
+<img src="images/image132.jpg" width="300" height="435" alt="Fig. 127." title="" />
+<span class="caption"><span class="smcap">Fig. 127.</span></span>
+</div>
+
+
+<p class="section">THE REFLECTING TELESCOPE.</p>
+
+<p>We must not omit reference to the <i>reflecting</i> telescope,<span class='pagenum'><a name="Page_261" id="Page_261">[Pg 261]</a></span> 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 reflected direct into an eye-piece pointing through the side of the
+tube towards the mirror.</p>
+
+
+<p class="section">THE PARABOLIC MIRROR.</p>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/image133.jpg" width="400" height="650" alt="Fig. 128." title="" />
+<span class="caption"><span class="smcap">Fig. 128.</span>&mdash;A parabolic reflector.</span>
+</div>
+
+
+<p class="section">THE COMPOUND MICROSCOPE.</p>
+
+<p>We have already observed (Fig. 110) that the<span class='pagenum'><a name="Page_262" id="Page_262">[Pg 262]</a></span> 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 <i>sharp</i> 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
+<i>conjugate foci</i>&mdash;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<span class='pagenum'><a name="Page_263" id="Page_263">[Pg 263]</a></span> 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 <i>short</i> focus.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_129" id="Fig_129"></a>
+<img src="images/image134.jpg" width="500" height="205" alt="Fig. 129." title="" />
+<span class="caption"><span class="smcap">Fig. 129.</span>&mdash;Diagram to explain the compound microscope.</span>
+</div>
+
+<p>Now, a compound microscope is practically a telescope with the object at
+the <i>long</i> focus, very close to a short-focus lens. A greatly enlarged
+image is thrown (<a href="#Fig_129">see Fig. 129</a>) at the conjugate focus, and this is
+caught and still further magnified by the eye-piece. We may add that the
+object-glass, or <i>objective</i>, of a microscope is usually compounded of
+several lenses, as is also the eye-piece.</p>
+
+
+<p class="section">THE MAGIC-LANTERN.</p>
+
+<p>The most essential features of a magic-lantern are:&mdash;(1) The <i>source of
+light</i>; (2) the <i>condenser</i> for concentrating the light rays on to the
+slide;<span class='pagenum'><a name="Page_264" id="Page_264">[Pg 264]</a></span> (3) the <i>lens</i> for projecting a magnified image on to a screen.</p>
+
+<p>Fig. 130 shows these diagrammatically. The <i>illuminant</i> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image135.jpg" width="500" height="201" alt="Fig. 130." title="" />
+<span class="caption"><span class="smcap">Fig. 130.</span>&mdash;Sketch of the elements of a magic-lantern.</span>
+</div>
+
+<p>The <i>condenser</i> is set somewhat further from the<span class='pagenum'><a name="Page_265" id="Page_265">[Pg 265]</a></span> illuminant than the
+principal focal length of the lenses, so that the rays falling on them
+are bent inwards, or to the slide.</p>
+
+<p>The <i>objective</i>, 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 (<a href="#Page_239">see p.
+239</a>), 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.</p>
+
+<p>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&ndash;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.</p>
+
+
+<p><span class='pagenum'><a name="Page_266" id="Page_266">[Pg 266]</a></span></p><p class="section">THE BIOSCOPE.</p>
+
+<p>"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.</p>
+
+<p>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 <i>motion</i>, if the position of some of the
+objects, or parts of the objects, varies slightly in each succeeding
+picture.<a name="FNanchor_25_25" id="FNanchor_25_25"></a><a href="#Footnote_25_25" class="fnanchor">[25]</a></p>
+
+
+<p><span class='pagenum'><a name="Page_267" id="Page_267">[Pg 267]</a></span></p><p class="section">THE PLANE MIRROR.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image136.jpg" width="500" height="498" alt="Fig. 131." title="" />
+<span class="caption"><span class="smcap">Fig. 131.</span></span>
+</div>
+
+<p>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, <span class="ampm">A</span> <i>b</i>, <span class="ampm">A</span> <i>c</i>, from a point <span class="ampm">A</span> strike the mirror <span class="ampm">M</span> at the
+points <i>b</i> and <i>c</i>. Lines <i>b</i> <span class="ampm">N</span>, <i>c</i> <span class="ampm">O</span>, drawn from these points
+perpendicular to the mirror are called<span class='pagenum'><a name="Page_268" id="Page_268">[Pg 268]</a></span> their <i>normals</i>. The angles <span class="ampm">A</span>
+<i>b</i> <span class="ampm">N</span>, <span class="ampm"> A</span> <i>c</i> <span class="ampm">O</span> are the <i>angles of incidence</i> of rays <span class="ampm">A</span> <i>b</i>, <span class="ampm">A</span> <i>c</i>. The
+paths which the rays take after reflection must make angles with <i>b</i> <span class="ampm">N</span>
+and <i>c</i> <span class="ampm">O</span> respectively equal to <span class="ampm">A</span> <i>b</i> <span class="ampm">N</span>, <span class="ampm">A</span> <i>c</i> <span class="ampm">O</span>. These are the <i>angles
+of reflection</i>. If the eye is so situated that the rays enter it as in
+our illustration, an image of the point <span class="ampm">A</span> is seen at the point <span class="ampm">A<sup>1</sup></span>, in
+which the lines <span class="ampm">D</span> <i>b</i>, <span class="ampm">E</span> <i>c</i> meet when produced backwards.</p>
+
+<div class="figcenter" style="width: 321px;">
+<img src="images/image137.jpg" width="321" height="236" alt="Fig. 132." title="" />
+<span class="caption"><span class="smcap">Fig. 132.</span></span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_269" id="Page_269">[Pg 269]</a></span> 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.</p>
+
+
+<div class="footnote"><p><a name="Footnote_22_22" id="Footnote_22_22"></a><a href="#FNanchor_22_22"><span class="label">[22]</span></a> Glazebrook, "Light," p. 157.</p></div>
+
+<div class="footnote"><p><a name="Footnote_23_23" id="Footnote_23_23"></a><a href="#FNanchor_23_23"><span class="label">[23]</span></a> Glazebrook, "Light," p. 157.</p></div>
+
+<div class="footnote"><p><a name="Footnote_24_24" id="Footnote_24_24"></a><a href="#FNanchor_24_24"><span class="label">[24]</span></a> Galileo was severely censured and imprisoned for daring to
+maintain that the earth moved round the sun, and revolved on its axis.</p></div>
+
+<div class="footnote"><p><a name="Footnote_25_25" id="Footnote_25_25"></a><a href="#FNanchor_25_25"><span class="label">[25]</span></a> For a full account of Animated Pictures the reader might
+advantageously consult "The Romance of Modern Invention," pp. 166 foll.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_270" id="Page_270">[Pg 270]</a></span></p>
+<h3><a name="Chapter_XIV" id="Chapter_XIV"></a>Chapter XIV.</h3>
+
+<h4>SOUND AND MUSICAL INSTRUMENTS.</h4>
+
+<div class="blockquot"><p class="hang">Nature of sound&mdash;The ear&mdash;Musical instruments&mdash;The vibration of
+strings&mdash;The sounding-board and the frame of a piano&mdash;The
+strings&mdash;The striking mechanism&mdash;The quality of a note. </p></div>
+
+
+<p class="noin"><span class="dcap">S</span><span class="caps">ound</span> 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.</p>
+
+<p>Sound resembles light and heat, however, thus far, that it can be
+concentrated by means of suitable lenses and curved surfaces. An <i>echo</i>
+is a proof of its <i>reflection</i> from a surface.</p>
+
+<p>Before dealing with the various appliances used<span class='pagenum'><a name="Page_271" id="Page_271">[Pg 271]</a></span> for producing
+sound-waves of a definite character, let us examine that wonderful
+natural apparatus</p>
+
+
+<p class="section">THE EAR,</p>
+
+<p class="noin">through which we receive those sensations which we call sound.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image138.jpg" width="500" height="251" alt="Fig. 133." title="" />
+<span class="caption"><span class="smcap">Fig. 133.</span>&mdash;Diagrammatic sketch of the parts of the ear.</span>
+</div>
+
+<p>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 <i>outer ear</i>, 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 <i>ossicles</i>, or small bones. The last of these presses against
+an opening in the <i>inner ear</i>, a cavity surrounded by the bones of the
+head. Inside the inner ear is a watery fluid,<span class='pagenum'><a name="Page_272" id="Page_272">[Pg 272]</a></span> <span class="ampm">P</span>, called <i>perilymph</i>
+("surrounding water"), immersed in which is a membranic envelope, <span class="ampm">M</span>,
+containing <i>endolymph</i> ("inside water"), also full of fluid. Into this
+fluid project <span class="ampm">E E E</span>, the terminations of the <i>auditory nerve</i>, leading
+to the brain.</p>
+
+<p>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 <span class="ampm">O</span>, 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.</p>
+
+<p>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 <span class="ampm">M</span>, <span class="ampm">I</span>, and <span class="ampm">S</span> are respectively the <i>malleus</i> (hammer),
+<i>incus</i> (anvil), and <i>stapes</i> (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 <span class="ampm">O</span> of Fig. 133, which is scientifically known as the <i>fenestra
+ovalis</i>, or oval window. As liquids are practically incompressible,
+nature has made<span class='pagenum'><a name="Page_273" id="Page_273">[Pg 273]</a></span> 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).</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image139.jpg" width="500" height="318" alt="Fig. 134." title="" />
+<span class="caption"><span class="smcap">Fig. 134.</span>&mdash;Diagrammatic section of the ear, showing the
+various parts.</span>
+</div>
+
+<p>The inner ear consists of two main parts, the <i>cochlea</i>&mdash;so called from
+its resemblance in shape to a snail's shell&mdash;and the <i>semicircular
+canals</i>. Each portion has its perilymph and endolymph, and contains a
+number of the nerve-ends, which are, however,<span class='pagenum'><a name="Page_274" id="Page_274">[Pg 274]</a></span> 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 <i>intensity</i> or loudness of sounds and the
+direction from which they come, while the latter enables us to determine
+the <i>pitch</i> of a note. In the cochlea are about 2,800 tiny nerve-ends,
+called the <i>rods of Corti</i>. 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 <span class="above">1</span>&#8260;<span class="below">64</span>th 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.</p>
+
+<p>A person who has a "good ear" for music is presumably one whose Corti
+rods are very perfect.<span class='pagenum'><a name="Page_275" id="Page_275">[Pg 275]</a></span> 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.</p>
+
+<p>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.</p>
+
+<p>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."<a name="FNanchor_26_26" id="FNanchor_26_26"></a><a href="#Footnote_26_26" class="fnanchor">[26]</a> 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<span class='pagenum'><a name="Page_276" id="Page_276">[Pg 276]</a></span> 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.</p>
+
+<p>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.</p>
+
+<p>The <i>Eustachian tube</i> (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.</p>
+
+<p>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.</p>
+
+
+<p><span class='pagenum'><a name="Page_277" id="Page_277">[Pg 277]</a></span></p><p class="section">MUSICAL INSTRUMENTS.</p>
+
+<p>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.</p>
+
+<p>We will begin our excursion into the region of musical instruments with
+an examination of that very familiar piece of furniture,</p>
+
+
+<p class="section">THE PIANOFORTE,</p>
+
+<p class="noin">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&mdash;if properly handled. The two forms of piano now generally
+used are the <i>upright</i>,<span class='pagenum'><a name="Page_278" id="Page_278">[Pg 278]</a></span> with vertical sound-board and wires, and the
+<i>grand</i>, with horizontal sound-board.<a name="FNanchor_27_27" id="FNanchor_27_27"></a><a href="#Footnote_27_27" class="fnanchor">[27]</a></p>
+
+
+<p class="section">THE VIBRATION OF STRINGS.</p>
+
+<p>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
+<i>pitch</i> of the note depends on several conditions:&mdash;(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:&mdash;(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 <i>square root</i> of the
+<i>tension</i>: 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<span class='pagenum'><a name="Page_279" id="Page_279">[Pg 279]</a></span> to the <i>length</i> 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 <i>density</i> 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.</p>
+
+<p>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.</p>
+
+<p>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<span class='pagenum'><a name="Page_280" id="Page_280">[Pg 280]</a></span> 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.</p>
+
+
+<p class="section">THE SOUNDING-BOARD AND FRAME OF A PIANO.</p>
+
+<p>A piano has its strings strained across a <i>frame</i> 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 <i>tons</i>.</p>
+
+<p>To the back of the frame is attached the <i>sounding-board</i>, 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.</p>
+
+
+<p class="section">THE STRINGS.</p>
+
+<p>These are made of extremely strong steel wire of<span class='pagenum'><a name="Page_281" id="Page_281">[Pg 281]</a></span> 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&mdash;weight, tension, and length&mdash;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.</p>
+
+
+<p class="section">THE STRIKING MECHANISM.</p>
+
+<p>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:&mdash;(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<span class='pagenum'><a name="Page_282" id="Page_282">[Pg 282]</a></span> succession. The <i>hammer</i> 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<span class='pagenum'><a name="Page_283" id="Page_283">[Pg 283]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image140.jpg" width="500" height="235" alt="Fig. 135." title="" />
+<span class="caption"><span class="smcap">Fig. 135.</span>&mdash;The striking mechanism of a "grand" piano.</span>
+</div>
+
+<p>The <i>action carriage</i> 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 <span class="ampm">B</span> pressing upon a knob, <span class="ampm">N</span>, called the <i>notch</i>, attached to the
+under side of the shank. When the jack has risen to a certain point, its
+arm, <span class="ampm">B<sup>1</sup></span>, catches against the button <span class="ampm">C</span> 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
+<i>repetition lever</i> <span class="ampm">R</span>, which lifts it to allow of perfect repetition.</p>
+
+<p>The <i>check</i> 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 <i>damper</i> by a vertical
+wire,<span class='pagenum'><a name="Page_284" id="Page_284">[Pg 284]</a></span> 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, <span class="ampm">L</span>,
+passing along under all the <i>damper lifters</i>, is raised by depressing
+the loud pedal. The <i>soft pedal</i> 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.</p>
+
+<p>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,<span class='pagenum'><a name="Page_285" id="Page_285">[Pg 285]</a></span> strength, and certainty of action throughout the
+whole keyboard.</p>
+
+
+<p class="section">THE QUALITY OF A NOTE.</p>
+
+<p>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 <i>timbre</i>, as musicians
+call it, is influenced by the presence of <i>overtones</i>, or <i>harmonics</i>,
+in combination with the <i>fundamental</i>, 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 <i>nodes</i>, 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;<a name="FNanchor_28_28" id="FNanchor_28_28"></a><a href="#Footnote_28_28" class="fnanchor">[28]</a> and as four parts, and give
+the third overtone, the double octave.</p>
+
+<p>Now, if a string be struck at a point corresponding<span class='pagenum'><a name="Page_286" id="Page_286">[Pg 286]</a></span> 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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+
+<div class="footnote"><p><a name="Footnote_26_26" id="Footnote_26_26"></a><a href="#FNanchor_26_26"><span class="label">[26]</span></a> Tyndall, "On Sound," p. 75.</p></div>
+
+<div class="footnote"><p><a name="Footnote_27_27" id="Footnote_27_27"></a><a href="#FNanchor_27_27"><span class="label">[27]</span></a> A Broadwood "grand" is made up of 10,700 separate pieces,
+and in its manufacture forty separate trades are concerned.</p></div>
+
+<div class="footnote"><p><a name="Footnote_28_28" id="Footnote_28_28"></a><a href="#FNanchor_28_28"><span class="label">[28]</span></a> Twelve notes higher up the scale.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_287" id="Page_287">[Pg 287]</a></span></p>
+<h3><a name="Chapter_XV" id="Chapter_XV"></a>Chapter XV.</h3>
+
+<h4>WIND INSTRUMENTS.</h4>
+
+<div class="blockquot"><p class="hang">Longitudinal vibration&mdash;Columns of air&mdash;Resonance of columns of
+air&mdash;Length and tone&mdash;The open pipe&mdash;The overtones of an open
+pipe&mdash;Where overtones are used&mdash;The arrangement of the pipes and
+pedals&mdash;Separate sound-boards&mdash;Varieties of stops&mdash;Tuning pipes and
+reeds&mdash;The bellows&mdash;Electric and pneumatic actions&mdash;The largest
+organ in the world&mdash;Human reeds. </p></div>
+
+
+<p class="section">LONGITUDINAL VIBRATION.</p>
+
+<p class="noin"><span class="dcap">I</span><span class="caps">n</span> stringed instruments we are concerned only with the transverse
+vibrations of a string&mdash;that is, its movements in a direction at right
+angles to the axis of the string. A string can also vibrate
+longitudinally&mdash;that is, in the direction of its axis&mdash;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.</p>
+
+<p>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.<span class='pagenum'><a name="Page_288" id="Page_288">[Pg 288]</a></span> 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 <i>timbre</i>, or quality, owing to
+the difference in the harmonics.</p>
+
+
+<p class="section">COLUMNS OF AIR.</p>
+
+<p>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.</p>
+
+
+<p class="section">RESONANCE OF COLUMNS OF AIR.</p>
+
+<p>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<span class='pagenum'><a name="Page_289" id="Page_289">[Pg 289]</a></span> 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.</p>
+
+<div class="figright" style="width: 400px;">
+<img src="images/image141.jpg" width="400" height="605" alt="Fig 136." title="" />
+<span class="caption"><span class="smcap">Fig 136.</span>&mdash;Showing how the harmonics of a "stopped" pipe
+are formed.</span>
+</div>
+
+<p>In Fig. 136, <i>1</i> 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 <i>compressed</i>.
+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<span class='pagenum'><a name="Page_290" id="Page_290">[Pg 290]</a></span> <i>rarefaction</i> 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.</p>
+
+
+<p class="section">LENGTH AND TONE.</p>
+
+<p>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 <i>fundamental</i>
+tones of the pipes. With them, as in the case of strings, are associated
+the <i>overtones</i>, or harmonics,<span class='pagenum'><a name="Page_291" id="Page_291">[Pg 291]</a></span> which can be brought into prominence by
+increasing the pressure of the blast at the top of the pipe. Blow very
+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.</p>
+
+<p>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, <i>1</i>, there is
+only one point of rest for an impulse&mdash;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 <i>ventral segment</i>. Remember this term. Therefore the pipe
+represents a semi-ventral segment when the fundamental note is sounding.</p>
+
+<p>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<span class='pagenum'><a name="Page_292" id="Page_292">[Pg 292]</a></span> to find another node, while an impulse going up would have
+to travel to the top and back again&mdash;that is, go twice as far. So the
+node forms itself <i>one-third</i> of the distance down the pipe. From <span class="ampm">B</span> to <span class="ampm">A</span>
+(Fig. 136, <i>2</i>) and back is now equal to from <span class="ampm">B</span> to <span class="ampm">C</span>. When the second
+overtone is blown (Fig. 136, <i>3</i>) a third node forms. The pipe is now
+divided into <i>five</i> semi-ventral segments. And with each succeeding
+overtone another node and ventral segment are added.</p>
+
+<p>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.<a name="FNanchor_29_29" id="FNanchor_29_29"></a><a href="#Footnote_29_29" class="fnanchor">[29]</a> If the
+fundamental tone gives 100 vibrations per second, the first overtone in
+a closed pipe must give 300, and the second 500 vibrations.</p>
+
+
+<p class="section">THE OPEN PIPE.</p>
+
+<p>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<span class='pagenum'><a name="Page_293" id="Page_293">[Pg 293]</a></span> explained by
+Fig. 137, <i>1</i>. 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 <span class="ampm">B</span>, the natural point to obtain equilibrium. A pulse
+will pass from <span class="ampm">A</span> or <span class="ampm">A<sup>1</sup></span> to <span class="ampm">B</span> and back again in half the time required
+to pass from <span class="ampm">A</span> to <span class="ampm">B</span> and back in Fig. 136, <i>1</i>; therefore the note is an
+octave higher.</p>
+
+<div class="figleft" style="width: 400px;">
+<img src="images/image142.jpg" width="400" height="602" alt="Fig. 137." title="" />
+<span class="caption"><span class="smcap">Fig. 137.</span>&mdash;Showing how harmonics of an open pipe are
+formed, <span class="ampm">B</span>, <span class="ampm">B<sup>1</sup></span>, and <span class="ampm">C</span> are "nodes." The arrows indicate the distance
+travelled by a sound impulse from a node to a node.</span>
+</div>
+
+
+<p class="section">THE OVERTONES OF AN OPEN PIPE.</p>
+
+<p>The first overtone results when nodes form as in Fig. 137, <i>2</i>, 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, <i>3</i>. The rate has now trebled. So that, while
+the<span class='pagenum'><a name="Page_294" id="Page_294">[Pg 294]</a></span> 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.</p>
+
+
+<p class="section">WHERE OVERTONES ARE USED.</p>
+
+<p>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.</p>
+
+
+<p class="section">THE ORGAN.</p>
+
+<p>From the theory of acoustics<a name="FNanchor_30_30" id="FNanchor_30_30"></a><a href="#Footnote_30_30" class="fnanchor">[30]</a> 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<span class='pagenum'><a name="Page_295" id="Page_295">[Pg 295]</a></span> lip <span class="ampm">A</span>, and causes a fluttering, the
+proper pulse of which is converted by the air-column above into a
+musical sound.</p>
+
+<div class="figright" style="width: 150px;">
+<img src="images/image143.jpg" width="150" height="621" alt="Fig. 138." title="" />
+<span class="caption"><span class="smcap">Fig. 138.</span>&mdash;Section of an ordinary wooden "flue" pipe.</span>
+</div>
+
+<p>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.</p>
+
+
+<p class="section">THE ARRANGEMENT OF THE PIPES.</p>
+
+<p>We will now study briefly the mechanism of a very simple single-keyboard
+organ, with five ranks of pipes, or stops.</p>
+
+<p><span class='pagenum'><a name="Page_296" id="Page_296">[Pg 296]</a></span></p>
+<div class="figleft" style="width: 300px;">
+<img src="images/image144.jpg" width="300" height="305" alt="Fig. 139." title="" />
+<span class="caption"><span class="smcap">Fig. 139.</span>&mdash;The table of a sound-board.</span>
+</div>
+
+<p>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
+<i>sound-board</i>, which is built up in several layers. At the top is the
+<i>upper board</i>; below it come the <i>sliders</i>, one for each stop; and
+underneath that the <i>table</i>. 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
+pipes. The grooves under the big pipes are wider than those<span class='pagenum'><a name="Page_297" id="Page_297">[Pg 297]</a></span> under the
+small pipes, as they have to pass more 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.</p>
+
+<div class="figright" style="width: 300px;">
+<img src="images/image145.jpg" width="300" height="299" alt="Fig. 140." title="" />
+<span class="caption"><span class="smcap">Fig. 140.</span></span>
+</div>
+
+<p>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 <i>pallets</i> which
+admit air from the <i>wind-chest</i> to the grooves; and Fig. 142 gives us an
+end section of table, sliders, and<span class='pagenum'><a name="Page_298" id="Page_298">[Pg 298]</a></span> wind-chest, together with the rods,
+etc., connecting the key to its pallet. When the key is depressed, the
+<i>sticker</i> (a slight wooden rod) is pushed up. This rocks a <i>backfall</i>,
+or pivoted lever, to which is attached the <i>pulldown</i>, 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.</p>
+
+<div class="figleft" style="width: 300px;">
+<img src="images/image146.jpg" width="300" height="234" alt="Fig. 141." title="" />
+<span class="caption"><span class="smcap">Fig. 141.</span></span>
+</div>
+
+
+<p class="section">PEDALS.</p>
+
+<p>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<span class='pagenum'><a name="Page_299" id="Page_299">[Pg 299]</a></span> <i>couplers</i> one or more of the keyboard stops may
+be linked to the pedals.</p>
+
+
+<p class="section">SEPARATE SOUND-BOARDS.</p>
+
+<p>The keyboard of a very large organ has as many as five <i>manuals</i>, or
+rows of keys. Each manual operates what is practically a separate organ
+mounted on its own sound-board.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image147.jpg" width="500" height="469" alt="Fig. 142." title="" />
+<span class="caption"><span class="smcap">Fig. 142.</span></span>
+</div>
+
+<p><span class='pagenum'><a name="Page_300" id="Page_300">[Pg 300]</a></span></p>
+<div class="figcenter" style="width: 600px;"><a name="Fig_143" id="Fig_143"></a><br />
+<img src="images/image148.jpg" width="600" height="970" alt="Fig. 143." title="" />
+<span class="caption"><span class="smcap">Fig. 143.</span>&mdash;General section of a two-manual organ.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_301" id="Page_301">[Pg 301]</a></span></p><p>The manuals are arranged in steps, each slightly overhanging that
+below. Taken in order from the top, they are:&mdash;(1.) <i>Echo organ</i>, of
+stops of small scale and very soft tone, enclosed in a "swell-box." (2.)
+<i>Solo organ</i>, of stops imitating orchestral instruments. The wonderful
+"vox humana" stop also belongs to this manual. (3.) <i>Swell organ</i>,
+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.) <i>Great organ</i>, including pipes of powerful tone. (5.) <i>Choir
+organ</i>, of soft, mellow stops, often enclosed in a swell-box. We may add
+to these the <i>pedal organ</i>, which can be coupled to any but the echo
+manual.</p>
+
+
+<p class="section">VARIETIES OF STOPS.</p>
+
+<p>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.</p>
+
+<p>The two main classes into which organ pipes may be divided are:&mdash;(1.)
+<i>Flue</i> pipes, in which the wind is directed against a lip, as in Fig.
+138. (2.) <i>Reed</i> pipes&mdash;that is, pipes used in combination with a
+simple<span class='pagenum'><a name="Page_302" id="Page_302">[Pg 302]</a></span> device for admitting air into the bottom of the pipe in a series
+of gusts. Fig. 144 shows a <i>striking</i> 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 <i>free</i> 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.</p>
+
+
+<p class="section">TUNING PIPES AND REEDS.</p>
+
+<div class="figcenter" style="width: 200px;"><a name="Fig_144" id="Fig_144"></a>
+<img src="images/image149.jpg" width="150" height="682" alt="Fig. 144." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 144.</span>&mdash;A reed pipe.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_303" id="Page_303">[Pg 303]</a></span> 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 (<a href="#Fig_144">see Fig. 144</a>) 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.</p>
+
+
+<p class="section">BELLOWS.</p>
+
+<p>Different stops require different wind-pressures, ranging from <span class="above">1</span>&#8260;<span class="below">10</span> 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 (<a href="#Fig_143">see Fig. 143</a>) 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.</p>
+
+<p><span class='pagenum'><a name="Page_304" id="Page_304">[Pg 304]</a></span></p>
+<div class="figcenter" style="width: 483px;">
+<img src="images/image150.jpg" width="483" height="335" alt="Fig. 145." title="" />
+<span class="caption"><span class="smcap">Fig. 145.</span>&mdash;The keyboard and part of the pneumatic
+mechanism of the Hereford Cathedral organ. <span class="ampm">C</span>, composition pedals for
+pushing out groups of stops; <span class="ampm">P</span> (at bottom), pedals; <span class="ampm">P P</span> (at top), pipes
+carrying compressed air; <span class="ampm">M</span>, manuals (4); <span class="ampm">S S</span>, stops.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_305" id="Page_305">[Pg 305]</a></span></p><p class="section">ELECTRIC AND PNEUMATIC ACTIONS.</p>
+
+<p>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
+<i>console</i>, 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&mdash;marvel of it all&mdash;the player, using
+the swell pedal in his ordinary manner, would obtain<span class='pagenum'><a name="Page_306" id="Page_306">[Pg 306]</a></span> crescendo and
+diminuendo with a more perfect effect than by the old way."<a name="FNanchor_31_31" id="FNanchor_31_31"></a><a href="#Footnote_31_31" class="fnanchor">[31]</a></p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+
+<p class="section">HUMAN REEDS.</p>
+
+<p>The most wonderful of all musical reeds is found in the human throat, in
+the anatomical part called the <i>larynx</i>, situated at the top of the
+<i>trachea</i>, or windpipe.</p>
+
+<p>Slip a piece of rubber tubing over the end of a pipe, allowing an inch
+or so to project. Take the<span class='pagenum'><a name="Page_307" id="Page_307">[Pg 307]</a></span> 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.</p>
+
+<p>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
+<i>arytenoid muscles</i> 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
+<span class="above">1</span>&#8260;<span class="below">17000</span>th of an inch!</p>
+
+<p>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.</p>
+
+<p>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,<span class='pagenum'><a name="Page_308" id="Page_308">[Pg 308]</a></span> and so we get the
+different vocal sounds. Helmholtz has shown that the fundamental tone of
+any note is represented by the sound <i>oo</i>. If the mouth is adjusted to
+bring out the octave of the fundamental, <i>o</i> results. <i>a</i> is produced by
+accentuating the second harmonic, the twelfth; <i>ee</i> by developing the
+second and fourth harmonics; while for <i>ah</i> the fifth and seventh must
+be prominent.</p>
+
+<p>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&mdash;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<span class='pagenum'><a name="Page_309" id="Page_309">[Pg 309]</a></span> 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.</p>
+
+<p>We should add that the cornet tube is an "open" pipe. So is that of the
+flute. The clarionet is a "stopped" pipe.</p>
+
+
+<div class="footnote"><p><a name="Footnote_29_29" id="Footnote_29_29"></a><a href="#FNanchor_29_29"><span class="label">[29]</span></a> It is obvious that in Fig. 136, <i>2</i>, a pulse will pass
+from <span class="ampm">A</span> to <span class="ampm">B</span> and back in one-third the time required for it to pass from
+<span class="ampm">A</span> to <span class="ampm">B</span> and back in Fig. 136, <i>1</i>.</p></div>
+
+<div class="footnote"><p><a name="Footnote_30_30" id="Footnote_30_30"></a><a href="#FNanchor_30_30"><span class="label">[30]</span></a> The science of hearing; from the Greek verb,
+&#7936;&#954;&#959;&#8059;&#949;&#953;&#957;, "to hear."</p></div>
+
+<div class="footnote"><p><a name="Footnote_31_31" id="Footnote_31_31"></a><a href="#FNanchor_31_31"><span class="label">[31]</span></a> "Organs and Tuning," p. 245.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_310" id="Page_310">[Pg 310]</a></span></p>
+<h3><a name="Chapter_XVI" id="Chapter_XVI"></a>Chapter XVI.</h3>
+
+<h4>TALKING-MACHINES.</h4>
+
+<div class="blockquot"><p class="hang">The phonograph&mdash;The recorder&mdash;The reproducer&mdash;The gramophone&mdash;The
+making of records&mdash;Cylinder records&mdash;Gramophone records. </p></div>
+
+
+<p class="noin"><span class="dcap">I</span><span class="caps">n</span> the Patent Office Museum at South Kensington is a curious little
+piece of machinery&mdash;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<span class='pagenum'><a name="Page_311" id="Page_311">[Pg 311]</a></span> the
+beautiful machines which record and reproduce human speech and musical
+sounds with startling accuracy.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image151.jpg" width="500" height="257" alt="Fig. 146." title="" />
+<span class="caption"><span class="smcap">Fig. 146.</span>&mdash;The "governor" of a phonograph.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_312" id="Page_312">[Pg 312]</a></span> used appears in
+Fig. 146. The last pinion of the clockwork train is mounted on a shaft
+carrying two triangular plates, <span class="ampm">A</span> and <span class="ampm">C</span>, to which are attached three
+short lengths of flat steel spring with a heavy ball attached to the
+centre of each. <span class="ampm">A</span> is fixed; <span class="ampm">C</span> moves up the shaft as the balls fly out,
+and pulls with it the disc <span class="ampm">D</span>, which rubs against the pad <span class="ampm">P</span> (on the end
+of a spring) and sets up sufficient friction to slow the clockwork. The
+limit rate is regulated by screw <span class="ampm">S</span>.</p>
+
+
+<p class="section">THE PHONOGRAPH.</p>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image152.jpg" width="500" height="509" alt="Fig. 147." title="" />
+<span class="caption"><span class="smcap">Fig. 147.</span>&mdash;Section of an Edison Bell phonograph
+recorder.</span>
+</div>
+
+<p>The <i>recorder</i> (Fig. 147) is a little circular box about one and a half
+inches in diameter.<a name="FNanchor_32_32" id="FNanchor_32_32"></a><a href="#Footnote_32_32" class="fnanchor">[32]</a> From the top a tube leads to the horn. The
+bottom is a circular plate, <span class="ampm">C C</span>, hinged at one side. This plate supports
+a glass disc, <span class="ampm">D</span>, about <span class="above">1</span>&#8260;<span class="below">150</span>th of an inch thick, to which is attached
+the cutting stylus&mdash;a tiny sapphire rod with a cup-shaped end having
+very<span class='pagenum'><a name="Page_313" id="Page_313">[Pg 313]</a></span> 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 <span class="ampm">N</span> on to the centre of the glass disc through the
+hole in <span class="ampm">C C</span>. You will notice that <span class="ampm">N</span> has a ball end, and <span class="ampm">C C</span> a socket to
+fit <span class="ampm">N</span> exactly, so that, though <span class="ampm">C C</span> and <span class="ampm">N</span> move up and down very rapidly,
+they still make perfect contact. The disc is vibrated by the<span class='pagenum'><a name="Page_314" id="Page_314">[Pg 314]</a></span>
+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 <span class="ampm">S</span> is that of <span class="ampm">N</span>, <span class="ampm">C C</span>, and the glass diaphragm.</p>
+
+<div class="figleft" style="width: 250px;">
+<img src="images/image153.jpg" width="250" height="267" alt="Fig. 148." title="" />
+<span class="caption"><span class="smcap">Fig. 148.</span>&mdash;Perspective view of a phonograph recorder.</span>
+</div>
+
+<p>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 <span class="above">1</span>&#8260;<span class="below">1000</span>th of an inch in depth.<a name="FNanchor_33_33" id="FNanchor_33_33"></a><a href="#Footnote_33_33" class="fnanchor">[33]</a> Seen under
+a microscope, the surface of the record is a succession of hills and
+valleys, some much larger than others (Fig. 151, <i>a</i>). A 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.</p>
+
+<p><span class='pagenum'><a name="Page_315" id="Page_315">[Pg 315]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image154.jpg" width="500" height="534" alt="Fig. 149." title="" />
+<span class="caption"><span class="smcap">Fig. 149.</span>&mdash;Section of the reproducer of an Edison Bell
+phonograph.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_316" id="Page_316">[Pg 316]</a></span></p>
+<div class="figright" style="width: 400px;"><br />
+<img src="images/image155.jpg" width="300" height="477" alt="Fig. 150." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 150.</span>&mdash;Perspective view of a phonograph reproducer.</span>
+</div>
+
+<p>The <i>reproducer</i> (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, <span class="ampm">R R</span>, by a screw collar, <span class="ampm">C</span>. To the
+centre is attached a little eye, from which hangs a link, <span class="ampm">L</span>. Pivoted at
+<span class="ampm">P</span> from one edge of the box is a <i>floating weight</i>, 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<span class='pagenum'><a name="Page_317" id="Page_317">[Pg 317]</a></span> 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 note.</p>
+
+
+<p class="section">THE GRAMOPHONE.</p>
+
+<p>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. 151<i>b</i>). 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.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 151a and 151b">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 75px;">
+<img src="images/image156.jpg" width="75" height="609" alt="Fig. 151a." title="" />
+<span class="caption"><span class="smcap">Fig. 151</span><i>a</i>.</span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 75px;">
+<img src="images/image157.jpg" width="75" height="609" alt="Fig. 151b." title="" />
+<span class="caption"><span class="smcap">Fig. 151</span><i>b</i>.</span>
+</div></td></tr>
+</table></div>
+
+<div class="figcenter" style="width: 500px;"><br />
+<img src="images/image158.jpg" width="500" height="584" alt="Fig. 151c." title="" />
+<span class="caption"><span class="smcap">Fig. 151</span><i>c</i>.&mdash;Section of a gramophone reproducer.</span>
+</div>
+
+<p>In Fig. 151<i>c</i> the construction of the gramophone reproducer is shown in
+section. <span class="ampm">A</span> is the cover which screws on to the bottom <span class="ampm">B</span>, and confines
+the diaphragm<span class='pagenum'><a name="Page_318" id="Page_318">[Pg 318]</a></span> <span class="ampm">D</span> between itself and a rubber ring. The portion <span class="ampm">B</span> 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 <span class="ampm">C</span> to make
+an air-tight joint. The needle-carrier <span class="ampm">N</span> 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, <span class="ampm">S</span>, working in a socket projecting from the<span class='pagenum'><a name="Page_319" id="Page_319">[Pg 319]</a></span> 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.</p>
+
+<p>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.</p>
+
+
+<p class="section">THE MAKING OF RECORDS.</p>
+
+<p>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.</p>
+
+
+<p class="section">CYLINDER, OR PHONOGRAPH RECORDS.</p>
+
+<p>First of all, a wax record is made in the ordinary way on a recording
+machine. After being tested and<span class='pagenum'><a name="Page_320" id="Page_320">[Pg 320]</a></span> 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."</p>
+
+<p><span class='pagenum'><a name="Page_321" id="Page_321">[Pg 321]</a></span></p><p>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.</p>
+
+<p>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.</p>
+
+<div class="footnote"><p><a name="Footnote_32_32" id="Footnote_32_32"></a><a href="#FNanchor_32_32"><span class="label">[32]</span></a> The Edison Bell phonograph is here referred to.</p></div>
+
+<div class="footnote"><p><a name="Footnote_33_33" id="Footnote_33_33"></a><a href="#FNanchor_33_33"><span class="label">[33]</span></a> 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!</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_322" id="Page_322">[Pg 322]</a></span></p>
+<h3><a name="Chapter_XVII" id="Chapter_XVII"></a>Chapter XVII.</h3>
+
+<h4>WHY THE WIND BLOWS.</h4>
+
+<div class="blockquot"><p class="hang">Why the wind blows&mdash;Land and sea breezes&mdash;Light air and
+moisture&mdash;The barometer&mdash;The column barometer&mdash;The wheel
+barometer&mdash;A very simple barometer&mdash;The aneroid
+barometer&mdash;Barometers and weather&mdash;The diving-bell&mdash;The
+diving-dress&mdash;Air-pumps&mdash;Pneumatic tyres&mdash;The air-gun&mdash;The
+self-closing door-stop&mdash;The action of wind on oblique surfaces&mdash;The
+balloon&mdash;The flying-machine. </p></div>
+
+
+<p class="noin"><span class="dcap">W</span><span class="caps">hen</span> 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."</p>
+
+<p><span class='pagenum'><a name="Page_323" id="Page_323">[Pg 323]</a></span></p><p>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, <i>tends to rise</i>. 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.</p>
+
+<p>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, <span class="ampm">A</span>, 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 <span class="ampm">B</span> and squeeze the rest of the warm air out.
+We may<span class='pagenum'><a name="Page_324" id="Page_324">[Pg 324]</a></span> 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 (<a href="#Page_17">see p. 17</a>).</p>
+
+
+<p class="section">LAND AND SEA BREEZES.</p>
+
+<p>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.</p>
+
+
+<p class="section">LIGHT AIR AND MOISTURE.</p>
+
+<p>Light, warm air absorbs moisture. As it cools, the moisture in it
+condenses. Breathe on a plate, and<span class='pagenum'><a name="Page_325" id="Page_325">[Pg 325]</a></span> 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.</p>
+
+
+<p class="section">THE BAROMETER.</p>
+
+<p>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.</p>
+
+<p>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&mdash;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<span class='pagenum'><a name="Page_326" id="Page_326">[Pg 326]</a></span> 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 <i>above</i> 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.</p>
+
+<div class="figcenter" style="width: 120px;">
+<img src="images/image159.jpg" width="120" height="498" alt="Fig. 152." title="" />
+<span class="caption"><span class="smcap">Fig. 152.</span>&mdash;A Fortin barometer.</span>
+</div>
+
+
+<p class="section">FORTIN'S COLUMN BAROMETER</p>
+
+<p class="noin">is a simple Torricellian tube, <span class="ampm">T</span>, 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 <span class="ampm">S</span>, pressing on the
+washleather, is adjusted until the mercury in<span class='pagenum'><a name="Page_327" id="Page_327">[Pg 327]</a></span> the tank rises to the tip
+of the little ivory point <span class="ampm">P</span>. The reading is the figure of the scale on
+the face of the case opposite which the surface of the column stands.</p>
+
+<div class="figcenter" style="width: 200px;">
+<img src="images/image160.jpg" width="200" height="570" alt="Fig. 153." title="" />
+<span class="caption"><span class="smcap">Fig. 153.</span></span>
+</div>
+
+
+<p class="section">THE WHEEL BAROMETER</p>
+
+<p class="noin">also employs the mercury column (Fig. 153). The lower end of the tube is
+turned up and expanded to form a tank, <span class="ampm">C</span>. The pointer <span class="ampm">P</span>, 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 <span class="ampm">C</span> 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<span class='pagenum'><a name="Page_328" id="Page_328">[Pg 328]</a></span> to end
+except for a contraction at the bend. The reading of a siphon is the
+distance between the two surfaces of the mercury.</p>
+
+
+<p class="section">A VERY SIMPLE BAROMETER</p>
+
+<p class="noin">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.</p>
+
+<div class="figcenter" style="width: 120px;">
+<img src="images/image161.jpg" width="120" height="430" alt="Fig. 154." title="" />
+<span class="caption"><span class="smcap">Fig. 154.</span></span>
+</div>
+
+
+<p class="section">THE ANEROID BAROMETER.</p>
+
+<p>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.</p>
+
+<p>Fig. 155 shows an aneroid in detail. The most<span class='pagenum'><a name="Page_329" id="Page_329">[Pg 329]</a></span> noticeable feature is the
+vacuum chamber, <span class="ampm">V C</span>, 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, <span class="ampm">P</span>, with a
+transverse hole through it to accommodate the pin <span class="ampm">K E</span>, which has a
+triangular section, and stands on one edge.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image162.jpg" width="500" height="454" alt="Fig. 155." title="" />
+<span class="caption"><span class="smcap">Fig. 155.</span>&mdash;An aneroid barometer.</span>
+</div>
+
+<p>Returning to Fig. 155, we see that <span class="ampm">P</span> projects<span class='pagenum'><a name="Page_330" id="Page_330">[Pg 330]</a></span> through <span class="ampm">S</span>, a powerful
+spring of sheet-steel. To this is attached a long arm, <span class="ampm">C</span>, the free end
+of which moves a link rotating, through the pin <span class="ampm">E</span>, a spindle mounted in
+a frame, <span class="ampm">D</span>. The spindle moves arm <span class="ampm">F</span>. This pulls on a very minute chain
+wound round the pointer spindle <span class="ampm">B</span>, in opposition to a hairspring, <span class="ampm">H S</span>. <span class="ampm">B</span>
+is mounted on arm <span class="ampm">H</span>, which is quite independent of the rest of the
+aneroid.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 156 and 157">
+<tr class='tr6'><td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image163.jpg" width="300" height="179" alt="Fig. 156." title="" />
+<span class="caption"><span class="smcap">Fig. 156.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image164.jpg" width="300" height="179" alt="Fig. 157." title="" />
+<span class="caption"><span class="smcap">Fig. 157.</span></span>
+</div></td></tr>
+<tr class='tr6'><td align='center' colspan='2'><span class="caption">The vacuum chamber of an aneroid
+barometer extended and compressed.</span></td></tr>
+</table></div>
+
+<p>The vacuum chamber is exhausted during manufacture and sealed. It would
+naturally assume the shape of Fig. 157, but the spring <span class="ampm">S</span>, 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 <span class="ampm">C</span>, <span class="ampm">E</span>, <span class="ampm">F</span>, to the pointer.</p>
+
+<p>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<span class='pagenum'><a name="Page_331" id="Page_331">[Pg 331]</a></span>
+therefore a valuable help to mountaineers for determining their altitude
+above sea-level.</p>
+
+
+<p class="section">BAROMETERS AND WEATHER.</p>
+
+<p>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 <i>weight</i> recorder. How is weather connected
+with atmospheric weight?</p>
+
+<p>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.</p>
+
+<p>A sinking of the barometer heralds the approach of heated air&mdash;that is,
+moist air&mdash;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<span class='pagenum'><a name="Page_332" id="Page_332">[Pg 332]</a></span> or the north, the winds blowing thence would be
+the rainy winds, while south-westerly winds might bring hot and dry
+weather.</p>
+
+
+<p class="section">THE DIVING-BELL.</p>
+
+<p>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&mdash;that is, there
+would be a 30-lb. <i>absolute</i> 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.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/image165.jpg" width="400" height="703" alt="Fig. 158." title="" />
+<span class="caption"><span class="smcap">Fig. 158.</span>&mdash;A diving bell.</span>
+</div>
+
+<p>The diving-bell is used to enable people to work under water without
+having recourse to the diving-dress.<span class='pagenum'><a name="Page_333" id="Page_333">[Pg 333]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/image166.jpg" width="300" height="382" alt="Fig. 159." title="" />
+<span class="caption"><span class="smcap">Fig. 159.</span></span>
+</div>
+
+<p>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.</p>
+
+<p>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<span class='pagenum'><a name="Page_334" id="Page_334">[Pg 334]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/image167.jpg" width="300" height="613" alt="Fig. 160." title="" />
+<span class="caption"><span class="smcap">Fig. 160.</span></span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_335" id="Page_335">[Pg 335]</a></span> 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."<a name="FNanchor_34_34" id="FNanchor_34_34"></a><a href="#Footnote_34_34" class="fnanchor">[34]</a></p>
+
+<p>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</p>
+
+
+<p class="section">DIVING-DRESS,</p>
+
+<p class="noin">which consists of two main parts, the helmet and the dress proper. The
+helmet (Fig. 161) is made of copper. A breastplate, <span class="ampm">B</span>, 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 <span class="ampm">P<sup>1</sup></span>, screwed down tightly on it by
+the wing-nuts <span class="ampm">N N</span>, the bolts of which pass through the breastplate. Air
+enters the helmet<span class='pagenum'><a name="Page_336" id="Page_336">[Pg 336]</a></span> 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 <span class="ampm">O V</span> can be adjusted by the diver to maintain any
+pressure. At the sides of the headpiece are two hooks, <span class="ampm">H</span>, 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 <span class="ampm">K K</span>. 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, <span class="ampm">R W</span>, 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image168.jpg" width="500" height="590" alt="Fig. 161." title="" />
+<span class="caption"><span class="smcap">Fig. 161.</span>&mdash;A diver's helmet.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_337" id="Page_337">[Pg 337]</a></span></p><p>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.</p>
+
+<p>The pressure on a diver's body increases in the<span class='pagenum'><a name="Page_338" id="Page_338">[Pg 338]</a></span> ratio of 4&#8531; 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 <i>Cape Horn</i> sunk off the South American coast, made
+seven descents of 201 feet, one of which lasted for forty-two minutes.</p>
+
+<div class="figcenter" style="width: 190px;">
+<img src="images/image169.jpg" width="190" height="313" alt="Fig. 162." title="" />
+<span class="caption"><span class="smcap">Fig. 162.</span>&mdash;Diver's electric lamp.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_339" id="Page_339">[Pg 339]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 340px;">
+<img src="images/image170.jpg" width="340" height="469" alt="Fig. 163." title="" />
+<span class="caption"><span class="smcap">Fig. 163.</span>&mdash;Divers at work below water with pneumatic
+tools.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_340" id="Page_340">[Pg 340]</a></span></p><p class="section">AIR-PUMPS.</p>
+
+<div class='center'><br />
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 164 and 165">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 150px;">
+<img src="images/image171.jpg" width="150" height="478" alt="Fig. 164." title="" />
+<span class="caption"><span class="smcap">Fig. 164.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 150px;">
+<img src="images/image172.jpg" width="150" height="497" alt="Fig. 165." title="" />
+<span class="caption"><span class="smcap">Fig. 165.</span></span>
+</div></td></tr>
+</table></div>
+
+<p>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<span class='pagenum'><a name="Page_341" id="Page_341">[Pg 341]</a></span> to the barrel, as the cups under these
+circumstances cannot act as valves.</p>
+
+
+<p class="section">PNEUMATIC TYRES.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 166 and 167">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image173.jpg" width="300" height="316" alt="Fig. 166." title="" />
+<span class="caption"><span class="smcap">Fig. 166.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image174.jpg" width="300" height="349" alt="Fig. 167." title="" />
+<span class="caption"><span class="smcap">Fig. 167.</span></span>
+</div></td></tr>
+</table></div>
+
+<p>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<span class='pagenum'><a name="Page_342" id="Page_342">[Pg 342]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image175.jpg" width="500" height="191" alt="Fig. 168." title="" />
+<span class="caption"><span class="smcap">Fig. 168.</span>&mdash;Section of the mechanism of an air-gun.</span>
+</div>
+
+
+<p class="section">THE AIR-GUN.</p>
+
+<p>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<span class='pagenum'><a name="Page_343" id="Page_343">[Pg 343]</a></span> firing. In the stock
+of the gun is the <i>cylinder</i>, in which an accurately fitting and hollow
+<i>piston</i> 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 <i>catch</i> is pressed down so that its
+hooked end disengages from the stock, and the barrel is bent downwards
+on pivot <span class="ampm">P</span>. This slides the lower end of the <i>compressing lever</i> towards
+the butt, and a projection on the guide <span class="ampm">B</span>, working in a groove, takes
+the piston with it. When the spring has been fully compressed, the
+triangular tip of the rocking cam <span class="ampm">R</span> 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.</p>
+
+<p>There are several other good types of air-gun, all of which employ the
+principles described above.</p>
+
+
+<p><span class='pagenum'><a name="Page_344" id="Page_344">[Pg 344]</a></span></p><p class="section">THE SELF-CLOSING DOOR-STOP</p>
+
+<p class="noin">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.</p>
+
+
+<p class="section">THE ACTION OF WIND ON OBLIQUE SURFACES.</p>
+
+<p>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<span class='pagenum'><a name="Page_345" id="Page_345">[Pg 345]</a></span> 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 <i>two</i> forces, coming from each side
+of the original line. These are called the <i>component</i> forces.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image176.jpg" width="500" height="383" alt="Fig. 169." title="" />
+<span class="caption"><span class="smcap">Fig. 169.</span></span>
+</div>
+
+<p>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 <span class="ampm">A</span>.
+The oblique surface of the kite resolves its force into the two
+components indicated by the dotted arrows <span class="ampm">B</span> and <span class="ampm">C</span>. Of these <span class="ampm">C</span> only has
+lifting power to overcome the<span class='pagenum'><a name="Page_346" id="Page_346">[Pg 346]</a></span> force of gravity. The kite assumes a
+position in which force <span class="ampm">C</span> and gravity counterbalance one another.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image177.jpg" width="500" height="382" alt="Fig. 170." title="" />
+<span class="caption"><span class="smcap">Fig. 170.</span></span>
+</div>
+
+<p>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 <span class="ampm">A</span> is
+resolved into forces <span class="ampm">B</span> and <span class="ampm">C</span>, of which <span class="ampm">C</span> 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 <span class="ampm">B</span> on the
+boat, masts, etc., overcomes the<span class='pagenum'><a name="Page_347" id="Page_347">[Pg 347]</a></span> force <span class="ampm">C</span>. The capability of a boat for
+sailing up wind depends on her "lines" and the amount of surface she
+offers to the wind.</p>
+
+
+<p class="section">THE BALLOON</p>
+
+<p class="noin">is a pear-shaped bag&mdash;usually made of silk&mdash;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;<span class='pagenum'><a name="Page_348" id="Page_348">[Pg 348]</a></span> and this process
+can be repeated until the ballast is exhausted. The greatest height ever
+attained by aeronauts is the 7&frac14; 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image178.jpg" width="500" height="221" alt="Fig. 171." title="" />
+<span class="caption"><span class="smcap">Fig. 171.</span></span>
+</div>
+
+<p>The <i>flying-machine</i>, 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, <span class="ampm">E</span>,
+extremely<span class='pagenum'><a name="Page_349" id="Page_349">[Pg 349]</a></span> powerful for its weight. This drove large propellers, <span class="ampm">S S</span>.
+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.</p>
+
+<p>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.</p>
+
+<p>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."</p>
+
+<div class="footnote"><p><a name="Footnote_34_34" id="Footnote_34_34"></a><a href="#FNanchor_34_34"><span class="label">[34]</span></a> The "Romance of Modern Mechanism," p. 243</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_350" id="Page_350">[Pg 350]</a></span></p>
+<h3><a name="Chapter_XVIII" id="Chapter_XVIII"></a>Chapter XVIII.</h3>
+
+<h4>HYDRAULIC MACHINERY.</h4>
+
+<div class="blockquot"><p class="hang">The siphon&mdash;The bucket pump&mdash;The force-pump&mdash;The most marvellous
+pump&mdash;The blood channels&mdash;The course of the blood&mdash;The hydraulic
+press&mdash;Household water-supply fittings&mdash;The ball-cock&mdash;The
+water-meter&mdash;Water-supply systems&mdash;The household filter&mdash;Gas
+traps&mdash;Water engines&mdash;The cream separator&mdash;The "hydro." </p></div>
+
+
+<p class="noin"><span class="dcap">I</span><span class="caps">n</span> 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<span class='pagenum'><a name="Page_351" id="Page_351">[Pg 351]</a></span> a
+further raising of the piston would not raise the water any farther. At
+sea-level, therefore, the <i>lifting</i> 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&mdash;in
+fact, be proportional to the pressure.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 172 and 173">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 150px;">
+<img src="images/image179.jpg" width="150" height="580" alt="Fig. 172." title="" />
+<span class="caption"><span class="smcap">Fig. 172.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 200px;">
+<img src="images/image180.jpg" width="200" height="464" alt="Fig. 173." title="" />
+<span class="caption"><span class="smcap">Fig. 173.</span></span>
+</div></td></tr>
+</table></div>
+
+<p class="section">THE SIPHON</p>
+
+<p class="noin">is an interesting application of the principle of<span class='pagenum'><a name="Page_352" id="Page_352">[Pg 352]</a></span> 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, <span class="ampm">A B C D</span>, is in the
+first instance filled by suction. The weight of the water between <span class="ampm">A</span> and
+<span class="ampm">B</span> counter-balances that between <span class="ampm">B</span> and <span class="ampm">C</span>. But the column <span class="ampm">C D</span> hangs, as it
+were, to the heels of <span class="ampm">B C</span>, and draws it down. Or, to put it otherwise,
+the column <span class="ampm">B D</span>, being heavier than the column <span class="ampm">B A</span>, draws it over the
+topmost point of the siphon. Any parting between the columns, provided
+that <span class="ampm">B A</span> does not exceed 34 feet, is impossible, as the pressure of the
+atmosphere on the mouth of <span class="ampm">B A</span> is sufficient to prevent the formation of
+a vacuum.</p>
+
+
+<p class="section">THE BUCKET PUMP.</p>
+
+<p>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 (<i>a</i>) 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 (<i>b</i>) shows the first<span class='pagenum'><a name="Page_353" id="Page_353">[Pg 353]</a></span> 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 (<i>c</i>) the water above the piston is raised
+until it overflows through the spout, while a fresh supply is being
+sucked in below.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image181.jpg" width="500" height="509" alt="Fig. 174." title="" />
+<span class="caption"><span class="smcap">Fig. 174.</span></span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_354" id="Page_354">[Pg 354]</a></span></p><p class="section">THE FORCE-PUMP.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 175 and 176">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 350px;">
+<img src="images/image182.jpg" width="300" height="580" alt="Fig. 175." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 175.</span> Force-pump; suction stroke.</span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 350px;">
+<img src="images/image183.jpg" width="300" height="532" alt="Fig. 176." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 176.</span> Force-pump; delivery stroke.</span>
+</div></td></tr>
+</table></div>
+
+<p>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<span class='pagenum'><a name="Page_355" id="Page_355">[Pg 355]</a></span> 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.</p>
+
+<p>A <i>double-action</i> 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.</p>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/image184.jpg" width="300" height="554" alt="Fig. 177." title="" />
+<span class="caption"><span class="smcap">Fig. 177.</span></span>
+</div>
+
+
+<p class="section">THE MOST MARVELLOUS PUMP</p>
+
+<p class="noin">known is the <i>heart</i>. We give in Fig. 178 a diagrammatic<span class='pagenum'><a name="Page_356" id="Page_356">[Pg 356]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image185.jpg" width="500" height="749" alt="Fig. 178." title="" />
+<span class="caption"><span class="smcap">Fig. 178.</span>&mdash;A diagrammatic representation of the
+circulatory system of the blood.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_357" id="Page_357">[Pg 357]</a></span></p><p>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 <i>auricles</i>, and the right and
+left <i>ventricles</i>. 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 <i>mitral</i> valve,
+that between the right auricle and ventricle the <i>tricuspid</i> 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.)</p>
+
+<p>The action of the heart is this:&mdash;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<span class='pagenum'><a name="Page_358" id="Page_358">[Pg 358]</a></span> 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.</p>
+
+
+<p class="section">THE BLOOD CHANNELS</p>
+
+<p class="noin">are of two kinds&mdash;(1) The <i>arteries</i>, which lead the blood into the
+circulatory system; (2) the <i>veins</i>, 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 <i>capillaries</i> (Latin,
+<i>capillus</i>, a hair), are minute tubes having an average diameter of
+<span class="above">1</span>&#8260;<span class="below">3000</span>th 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.</p>
+
+<p>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<span class='pagenum'><a name="Page_359" id="Page_359">[Pg 359]</a></span> 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.</p>
+
+<p>Arterial blood is <i>red</i>, and comes out from a cut in gulps, on account
+of the contraction of the elastic walls. If you cut a vein, <i>blue</i> 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.</p>
+
+<p>The <i>lungs</i> 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<span class='pagenum'><a name="Page_360" id="Page_360">[Pg 360]</a></span> 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.</p>
+
+
+<p class="section">THE COURSE OF THE BLOOD.</p>
+
+<p>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.</p>
+
+<p>We may briefly summarize the course of the circulation of the blood
+thus:&mdash;It is expelled from the left ventricle into the <i>aorta</i> 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<span class='pagenum'><a name="Page_361" id="Page_361">[Pg 361]</a></span> <i>pulmonary</i> (lung)
+<i>arteries</i>; enters the lungs, and is purified. It returns to the left
+auricle through the <i>pulmonary veins</i>; enters the left auricle, passes
+to left ventricle, and so on.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+
+<p class="section">THE HYDRAULIC PRESS.</p>
+
+<p>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<span class='pagenum'><a name="Page_362" id="Page_362">[Pg 362]</a></span> a simple illustration is given in Fig. 179. Two
+cylinders, <span class="ampm">A</span> and <span class="ampm">B</span>, 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 <span class="ampm">B</span> is
+placed a load of 10 lbs. To prevent <span class="ampm">A</span> rising above the level of <span class="ampm">B</span>, it
+must be loaded proportionately. The area of piston <span class="ampm">A</span> is four times that
+of <span class="ampm">B</span>, 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 <span class="ampm">B</span>.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image186.jpg" width="500" height="422" alt="Fig. 179." title="" />
+<span class="caption"><span class="smcap">Fig. 179.</span></span>
+</div>
+
+<p><span class='pagenum'><a name="Page_363" id="Page_363">[Pg 363]</a></span></p>
+<div class="figright" style="width: 350px;"><br />
+<img src="images/image187.jpg" width="350" height="376" alt="Fig. 180." title="" />
+<span class="caption"><span class="smcap">Fig. 180.</span>&mdash;The cylinder and ram of a hydraulic press.</span>
+</div>
+
+<p>The hydraulic press is an application of this law. Cylinder <span class="ampm">B</span> 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
+<span class="ampm">A</span> we have a stout cylinder with a solid plunger, <span class="ampm">P</span> (Fig. 180), carrying
+the <i>table</i> 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,<a name="FNanchor_35_35" id="FNanchor_35_35"></a><a href="#Footnote_35_35" class="fnanchor">[35]</a><span class='pagenum'><a name="Page_364" id="Page_364">[Pg 364]</a></span> 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 <span class="bigletter">U</span>-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.</p>
+
+<p>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.</p>
+
+
+<p class="section">HOUSEHOLD WATER-SUPPLY FITTINGS.</p>
+
+<p>Among these, the most used is the tap, 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<span class='pagenum'><a name="Page_365" id="Page_365">[Pg 365]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image188.jpg" width="500" height="413" alt="Fig. 181." title="" />
+<span class="caption"><span class="smcap">Fig. 181.</span>&mdash;A screw-down water cock.</span>
+</div>
+
+<p>Its place has been taken by the screw-down cock. A very common and
+effective pattern is shown in Fig. 181. The valve <span class="ampm">V</span>, with a facing of
+rubber, leather, or some other sufficiently elastic substance,<span class='pagenum'><a name="Page_366" id="Page_366">[Pg 366]</a></span> is
+attached to a pin, <span class="ampm">C</span>, which projects upwards into the spindle <span class="ampm">A</span> of the
+tap. This spindle has a screw thread on it engaging with a collar, <span class="ampm">B</span>.
+When the spindle is turned it rises or falls, allowing the valve to
+leave its seating, <span class="ampm">V S</span>, or forcing it down on to it. A packing <span class="ampm">P</span> in the
+neck of <span class="ampm">B</span> 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.</p>
+
+
+<p class="section">THE BALL-COCK</p>
+
+<p class="noin">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, <span class="ampm">L</span>, 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<span class='pagenum'><a name="Page_367" id="Page_367">[Pg 367]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image189.jpg" width="500" height="276" alt="Fig. 182." title="" />
+<span class="caption"><span class="smcap">Fig. 182.</span>&mdash;An automatic ball-valve.</span>
+</div>
+
+
+<p class="section">THE WATER-METER.</p>
+
+<div class="figcenter" style="width: 245px;">
+<img src="images/image190.jpg" width="245" height="294" alt="Fig. 183." title="" />
+<span class="caption"><span class="smcap">Fig. 183.</span></span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_368" id="Page_368">[Pg 368]</a></span> 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<span class='pagenum'><a name="Page_369" id="Page_369">[Pg 369]</a></span> 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 (<a href="#Fig_184">see Fig. 184</a>). 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.</p>
+
+<p>In order to follow the working of the meter, we must keep an eye on
+Figs. 183 and 184 simultaneously. Water is entering from <span class="ampm">A</span>, the supply
+pipe. It flows through the cock downwards through channel <span class="ampm">D</span> into the
+lower half of the cylinder. The piston rises, driving out the water
+above it through <span class="ampm">C</span> to the delivery pipe <span class="ampm">B</span>. 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<span class='pagenum'><a name="Page_370" id="Page_370">[Pg 370]</a></span> 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 <span class="ampm">A</span> down <span class="ampm">C</span> into the top of the
+cylinder, forcing the piston down, while the water admitted below during
+the last stroke is forced up the passage <span class="ampm">D</span>, and out by the outlet <span class="ampm">B</span>.
+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<span class='pagenum'><a name="Page_371" id="Page_371">[Pg 371]</a></span> its former position, ready to
+begin another upward stroke.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_184" id="Fig_184"></a>
+<img src="images/image191.jpg" width="500" height="484" alt="Fig. 184." title="" />
+<span class="caption"><span class="smcap">Fig. 184.</span></span>
+</div>
+
+<p>The <i>index mechanism</i> 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.</p>
+
+
+<p class="section">WATER-SUPPLY SYSTEMS.</p>
+
+<p>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 <i>settling tanks</i>, where the suspended
+matter sinks to the bottom. The water is then drawn off<span class='pagenum'><a name="Page_372" id="Page_372">[Pg 372]</a></span> into
+<i>filtration beds</i>, 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,<a name="FNanchor_36_36" id="FNanchor_36_36"></a><a href="#Footnote_36_36" class="fnanchor">[36]</a> 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.</p>
+
+<p>It is sometimes necessary to send the water through a succession of
+beds, arranged in terraces, before it is sufficiently pure for drinking
+purposes.</p>
+
+
+<p class="section">THE HOUSEHOLD FILTER.</p>
+
+<p>When there is any doubt as to the wholesomeness<span class='pagenum'><a name="Page_373" id="Page_373">[Pg 373]</a></span> 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. <span class="ampm">R</span> is the reservoir for the filtered
+water; <span class="ampm">A</span> the filter case proper; <span class="ampm">D</span> a conical perforated frame; <span class="ampm">B</span> a
+jacket of asbestos cloth secured top and bottom by asbestos cords to <span class="ampm">D</span>;
+<span class="ampm">C</span> powdered carbon, between which and the asbestos is a layer of special
+chemical filtering medium. A perforated cap, <span class="ampm">E</span>, 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.</p>
+
+<div class="figcenter" style="width: 350px;">
+<img src="images/image192.jpg" width="350" height="696" alt="Fig. 185." title="" />
+<span class="caption"><span class="smcap">Fig. 185.</span></span>
+</div>
+
+<p>The most useful form of household filter is one which can be attached to
+a tap connected with<span class='pagenum'><a name="Page_374" id="Page_374">[Pg 374]</a></span> 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.</p>
+
+<p>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.</p>
+
+
+<p class="section">GAS TRAPS.</p>
+
+<p>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<span class='pagenum'><a name="Page_375" id="Page_375">[Pg 375]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/image193.jpg" width="300" height="389" alt="Fig. 186." title="" />
+<span class="caption"><span class="smcap">Fig. 186.</span>&mdash;A trap for foul air.</span>
+</div>
+
+
+<p class="section">WATER-ENGINES.</p>
+
+<p>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<span class='pagenum'><a name="Page_376" id="Page_376">[Pg 376]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image194.jpg" width="500" height="594" alt="Fig. 187." title="" />
+<span class="caption"><span class="smcap">Fig. 187.</span>&mdash;A Pelton wheel which develops 5,000
+horse-power. Observe the shape of the double buckets.</span>
+</div>
+
+<p>The <i>turbine</i>, 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<span class='pagenum'><a name="Page_377" id="Page_377">[Pg 377]</a></span> wheel takes the first place. By the
+courtesy of the manufacturers we are able to give some interesting
+details and illustrations of this device.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_188" id="Fig_188"></a>
+<img src="images/image195.jpg" width="500" height="372" alt="Fig. 188." title="" />
+<span class="caption"><span class="smcap">Fig. 188.</span>&mdash;Pelton wheel mounted, with nozzle in
+position.</span>
+</div>
+
+<p>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 (<a href="#Fig_188">see Fig. 188</a>). The water strikes the
+bucket on the partition between<span class='pagenum'><a name="Page_378" id="Page_378">[Pg 378]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image196.jpg" width="500" height="342" alt="Fig. 189." title="" />
+<span class="caption"><span class="smcap">Fig. 189.</span>&mdash;Speed regulator for Pelton wheel.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_379" id="Page_379">[Pg 379]</a></span> in its highest position the needle tip is withdrawn;
+as the nozzle sinks the needle protrudes, gradually decreasing the
+discharge area of the nozzle.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_380" id="Page_380">[Pg 380]</a></span></p>
+<div class="figcenter" style="width: 400px;">
+<img src="images/image197.jpg" width="325" height="453" alt="Fig. 190." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 190.</span>&mdash;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.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_381" id="Page_381">[Pg 381]</a></span></p><p>The full-page illustration on <a href="#Page_380">p. 380</a> 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 72&frac12; 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.</p>
+
+<p>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.</p>
+
+
+<p class="section">THE CREAM SEPARATOR.</p>
+
+<p>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<span class='pagenum'><a name="Page_382" id="Page_382">[Pg 382]</a></span> 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.</p>
+
+<p><span class='pagenum'><a name="Page_383" id="Page_383">[Pg 383]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image198.jpg" width="500" height="837" alt="Fig. 191." title="" />
+<span class="caption"><span class="smcap">Fig. 191.</span>&mdash;Section of a Cream Separator.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_384" id="Page_384">[Pg 384]</a></span></p><p>How does it work? asks the reader. Centrifugal force<a name="FNanchor_37_37" id="FNanchor_37_37"></a><a href="#Footnote_37_37" class="fnanchor">[37]</a> 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, <span class="ampm">D</span>, 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 <span class="ampm">R</span> (supported on a stout arm) through tap <span class="ampm">A</span>
+into a little distributer on the top of the separator, and from it drops
+into the central tube <span class="ampm">C</span> of the bowl. Falling to the bottom, it is flung
+outwards by centrifugal force, finds an escape upwards through the holes
+<i>a a</i>, and climbs up the perforated grid <i>e</i>, 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&mdash;that is, the watery&mdash;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.</p>
+
+<p>As more milk enters the drum it forces upwards what is already there.
+The cap of the drum has an inner jacket, <span class="ampm">F</span>, which at the bottom <i>all but
+touches</i> the side of the drum. The distance between them is the merest
+slit; but the cream is deflected up outside <span class="ampm">F</span> into space <span class="ampm">E</span>, and escapes
+through a hole one-sixteenth of an inch in diameter perforating the
+plate <span class="ampm">G</span>. The cream is flung into space <span class="ampm">K</span> and<span class='pagenum'><a name="Page_385" id="Page_385">[Pg 385]</a></span> trickles out of spout <span class="ampm">B</span>,
+while the water flies into space <span class="ampm">H</span> and trickles away through spout <span class="ampm">A</span>.</p>
+
+
+<p class="section">THE "HYDRO.,"</p>
+
+<p class="noin">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.</p>
+
+<div class="footnote"><p><a name="Footnote_35_35" id="Footnote_35_35"></a><a href="#FNanchor_35_35"><span class="label">[35]</span></a> Inventor of the lathe slide-rest.</p></div>
+
+<div class="footnote"><p><a name="Footnote_36_36" id="Footnote_36_36"></a><a href="#FNanchor_36_36"><span class="label">[36]</span></a> Living germs; some varieties the cause of disease.</p></div>
+
+<div class="footnote"><p><a name="Footnote_37_37" id="Footnote_37_37"></a><a href="#FNanchor_37_37"><span class="label">[37]</span></a> 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.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_386" id="Page_386">[Pg 386]</a></span></p>
+<h3><a name="Chapter_XIX" id="Chapter_XIX"></a>Chapter XIX.</h3>
+
+<h4>HEATING AND LIGHTING.</h4>
+
+<div class="blockquot"><p class="hang">The hot-water supply&mdash;The tank system&mdash;The cylinder system&mdash;How a
+lamp works&mdash;Gas and gasworks&mdash;Automatic stoking&mdash;A gas
+governor&mdash;The gas meter&mdash;Incandescent gas lighting. </p></div>
+
+
+<p class="section">HOT-WATER SUPPLY.</p>
+
+<p class="noin"><span class="dcap">A</span> <span class="caps">well-equipped</span> 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.</p>
+
+<p>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&mdash;(1) The <i>tank</i> system; (2) the <i>cylinder</i> system.</p>
+
+
+<p><span class='pagenum'><a name="Page_387" id="Page_387">[Pg 387]</a></span></p><p class="section">THE TANK SYSTEM</p>
+
+<p class="noin">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 <i>flow
+pipe</i> into the hot-water tank <span class="ampm">A</span>, displacing the somewhat colder water
+there, which descends through the <i>return pipe</i> to the bottom of the
+boiler.</p>
+
+<p>Water is drawn off from the flow pipe. This pipe projects some distance
+through the bottom of <span class="ampm">A</span>, 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 <span class="ampm">A</span> through the siphon pipe <span class="ampm">S</span>. The
+bend in this pipe prevents the ascent of hot water, which cannot sink
+through water colder than itself. From the top of <span class="ampm">A</span> an <i>expansion pipe</i>
+is led up and turned over the cold-water tank to discharge any steam
+which may be generated in the boiler.</p>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_388" id="Page_388">[Pg 388]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image199.jpg" width="500" height="797" alt="Fig. 192." title="" />
+<span class="caption"><span class="smcap">Fig. 192.</span>&mdash;The "tank" system of hot-water supply.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_389" id="Page_389">[Pg 389]</a></span></p><p>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 <span class="ampm">A</span> may be entirely emptied,
+circulation cease, and the water in the boiler and pipes boil away
+rapidly.</p>
+
+
+<p class="section">THE CYLINDER SYSTEM</p>
+
+<p class="noin">(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.</p>
+
+<p><span class='pagenum'><a name="Page_390" id="Page_390">[Pg 390]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image200.jpg" width="500" height="781" alt="Fig. 193." title="" />
+<span class="caption"><span class="smcap">Fig. 193.</span>&mdash;The "cylinder" system of hot-water supply.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_391" id="Page_391">[Pg 391]</a></span></p><p>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,"<a name="FNanchor_38_38" id="FNanchor_38_38"></a><a href="#Footnote_38_38" class="fnanchor">[38]</a> 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.</p>
+
+<p>Another cause of disaster is the <i>furring up</i> 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.</p>
+
+
+<p><span class='pagenum'><a name="Page_392" id="Page_392">[Pg 392]</a></span></p><p class="section">HOW A LAMP WORKS.</p>
+
+<p>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 <i>capillary attraction</i>. 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&mdash;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.</p>
+
+<p>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<span class='pagenum'><a name="Page_393" id="Page_393">[Pg 393]</a></span> 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.</p>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_394" id="Page_394">[Pg 394]</a></span></p>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image201.jpg" width="300" height="549" alt="Fig. 194." title="" />
+<span class="caption"><span class="smcap">Fig. 194.</span>&mdash;Showing how a blower-plate draws up the
+fire.</span>
+</div>
+
+
+<p class="section">GAS AND GASWORKS.</p>
+
+<p>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:&mdash;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<span class='pagenum'><a name="Page_395" id="Page_395">[Pg 395]</a></span> 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 <i>coke</i>. 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.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/image202.jpg" width="600" height="343" alt="Fig. 195." title="" />
+<span class="caption"><span class="smcap">Fig. 195.</span>&mdash;Sketch of the apparatus used in the
+manufacture of coal gas.</span>
+</div>
+
+<p>Our home-made gas yields a smoky and unsatisfactory flame, owing to the
+presence of certain impurities&mdash;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 <i>retorts</i>, which correspond to our
+canister. These are usually long fire-brick tubes of <span class="ampm">D</span>-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<span class='pagenum'><a name="Page_396" id="Page_396">[Pg 396]</a></span> door for filling the
+retort through, immediately behind which rises an iron exit pipe, <span class="ampm">A</span>, 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 <i>hydraulic main</i>, a tubular vessel half full
+of water running the whole<span class='pagenum'><a name="Page_397" id="Page_397">[Pg 397]</a></span> length of the retorts. The end of pipe <span class="ampm">A</span>
+dips below the surface of the water, which condenses most of the tar and
+steam. The partly-purified gas now passes through pipe <span class="ampm">B</span> to the
+<i>condensers</i>, a series of inverted <span class="bigletter">U</span>-pipes standing on an iron chest
+with vertical cross divisions between the mouths of each <span class="bigletter">U</span>. 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.</p>
+
+<p>The next stage is the passage of the <i>scrubber</i>, 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 <i>lime purifier</i>, 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 <i>gasometer</i>, 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,<span class='pagenum'><a name="Page_398" id="Page_398">[Pg 398]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/image203.jpg" width="400" height="393" alt="Fig. 196." title="" />
+<span class="caption"><span class="smcap">Fig. 196.</span>&mdash;The largest gasholder in the world: South
+Metropolitan Gas Co., Greenwich Gas Works. Capacity, 12,158,600 cubic
+feet.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_399" id="Page_399">[Pg 399]</a></span></p><p>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 &amp; 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.</p>
+
+<p><span class='pagenum'><a name="Page_400" id="Page_400">[Pg 400]</a></span></p>
+<div class="figcenter" style="width: 482px;">
+<img src="images/image204.jpg" width="482" height="333" alt="Fig. 197." title="" />
+<span class="caption"><span class="smcap">Fig. 197.</span>&mdash;Drawing retorts. (<i>Photo by F. Marsh.</i>)</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_401" id="Page_401">[Pg 401]</a></span></p><p class="section">AUTOMATIC STOKING.</p>
+
+<p>The labour of feeding the retorts with coal and removing the coke is
+exceedingly severe. In the illustration on <a href="#Page_400">p. 400</a> (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 <span class="ampm">P</span> from hoppers in the story above, which have
+openings, <span class="ampm">H H</span>, controlled by shutters. The coal as it falls is caught by
+a rubber belt working round part of the<span class='pagenum'><a name="Page_402" id="Page_402">[Pg 402]</a></span> circumference of the large
+wheel <span class="ampm">W</span> 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.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/image205.jpg" width="400" height="431" alt="Fig. 198." title="" />
+<span class="caption"><span class="smcap">Fig. 198.</span>&mdash;De Brouwer automatic retort charger.</span>
+</div>
+
+
+<p class="section">A GAS GOVERNOR.</p>
+
+<p>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<span class='pagenum'><a name="Page_403" id="Page_403">[Pg 403]</a></span> 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 <span class="above">1</span>&#8260;<span class="below">50</span> lb. to the square inch.
+With less it gives a smoky, flickering light, and with more the
+combustion is also imperfect.</p>
+
+<div class="figleft" style="width: 400px;">
+<img src="images/image206.jpg" width="400" height="362" alt="Fig. 199." title="" />
+<span class="caption"><span class="smcap">Fig. 199.</span></span>
+</div>
+
+<p>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, <span class="ampm">D</span>, which has top and bottom apertures
+closed by the valves <span class="ampm">V V</span>. Attached to the valve shaft is a large
+inverted<span class='pagenum'><a name="Page_404" id="Page_404">[Pg 404]</a></span> 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 <span class="ampm">W</span>,
+with which the valve spindle is loaded at the top. As soon as this
+pressure is exceeded, the gas in <span class="ampm">C C</span> lifts the metal cup, and <span class="ampm">V V</span> are
+pressed against their seats, so cutting off the supply. Gas cannot
+escape from <span class="ampm">C C</span>, as it has not sufficient pressure to force its way
+through the mercury under the lip of the cup. <span class='pagenum'><a name="Page_405" id="Page_405">[Pg 405]</a></span>Immediately the pressure
+in <span class="ampm">C C</span> 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&mdash;that is, they pass just
+as much gas as the burners in use can consume at the pressure arranged
+for.</p>
+
+<p>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.</p>
+
+
+<p class="section">THE GAS-METER</p>
+
+<p class="noin">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, <span class="ampm">B</span>, <span class="ampm">C</span>, and <span class="ampm">D</span>. Gas enters at <span class="ampm">A</span>, and passes to the
+valve chamber <span class="ampm">B</span>. The slide-valves of this allow it to pass into <span class="ampm">C</span> and <span class="ampm">D</span>,
+and also into the two circular leather bellows <span class="ampm">E, F</span>, which are attached
+to the central division <span class="ampm">G</span>, but are quite independent of one another.</p>
+
+<p><span class='pagenum'><a name="Page_406" id="Page_406">[Pg 406]</a></span></p>
+<div class="figright" style="width: 300px;">
+<img src="images/image207.jpg" width="300" height="448" alt="Fig. 200." title="" />
+<span class="caption"><span class="smcap">Fig. 200.</span>&mdash;Sketch of the bellows and chambers of a "dry"
+gas meter.</span>
+</div>
+
+<p>We will suppose that in the illustration the valves are admitting gas to
+chamber <span class="ampm">C</span> and bellows <span class="ampm">F</span>. The pressure in <span class="ampm">C</span> presses the circular head of
+<span class="ampm">E</span> towards the division <span class="ampm">G</span>, expelling the contents of the bellows through
+an outlet pipe (not shown) to the burners in operation within the house.
+Simultaneously the inflation of <span class="ampm">F</span> forces the gas in chamber <span class="ampm">D</span> also
+through the outlet. The head-plates of the bellows are attached to rods
+and levers (not shown) working the slide-valves in <span class="ampm">B</span>. As soon as <span class="ampm">E</span> is
+fully in, and <span class="ampm">F</span> fully expanded, the valves begin to open and put the
+inlet pipe in communication with <span class="ampm">D</span> and <span class="ampm">E</span>, and allow the contents of <span class="ampm">F</span>
+and <span class="ampm">C</span> 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 a certain volume of gas has been
+ejected either from them or from the chambers in which they move: this
+is registered by the counter.<span class='pagenum'><a name="Page_407" id="Page_407">[Pg 407]</a></span> 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&mdash;the central, or exhaust, leading to the outlet,
+the outer ones from the inlet. The bellows are fed through channels in
+the division <span class="ampm">G</span>.</p>
+
+
+<p class="section">INCANDESCENT GAS LIGHTING.</p>
+
+<p>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 <i>incandescent mantle</i> for
+gas-burners opened a prosperous era in the history of gas lighting.</p>
+
+<p>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<span class='pagenum'><a name="Page_408" id="Page_408">[Pg 408]</a></span> 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.</p>
+
+<p>Dr. Auer von Welsbach found that the substances most suitable for
+incandescent mantles were the oxides of certain rare metals, <i>thorium</i>,
+and <i>cerium</i>. 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<span class='pagenum'><a name="Page_409" id="Page_409">[Pg 409]</a></span>
+applied to the bottom, and the collodion burned off, leaving nothing but
+the heat-resisting oxides.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<div class="footnote"><p><a name="Footnote_38_38" id="Footnote_38_38"></a><a href="#FNanchor_38_38"><span class="label">[38]</span></a> If, of course, there is no safety-valve in proper working
+order included in the installation.</p></div>
+
+
+
+<hr /><p><span class='pagenum'><a name="Page_410" id="Page_410">[Pg 410]</a></span></p>
+<h3><a name="Chapter_XX" id="Chapter_XX"></a>Chapter XX.</h3>
+
+<h4>VARIOUS MECHANISMS.</h4>
+
+<div class="blockquot"><p class="hang"><span class="smcap">Clocks and Watches</span>:&mdash;A short history of timepieces&mdash;The
+construction of timepieces&mdash;The driving power&mdash;The
+escapement&mdash;Compensating pendulums&mdash;The spring balance&mdash;The
+cylinder escapement&mdash;The lever escapement&mdash;Compensated
+balance-wheels&mdash;Keyless winding mechanism for watches&mdash;The hour
+hand train. <span class="smcap">Locks</span>:&mdash;The Chubb lock&mdash;The Yale lock. <span class="smcap">The Cycle</span>:&mdash;The
+gearing of a cycle&mdash;The free wheel&mdash;The change-speed gear.
+<span class="smcap">Agricultural Machines</span>:&mdash;The threshing-machine&mdash;Mowing-machines.
+<span class="smcap">Some Natural Phenomena</span>:&mdash;Why sun-heat varies in intensity&mdash;The
+tides&mdash;Why high tide varies daily. </p></div>
+
+<p class="center"><big>CLOCKS AND WATCHES.</big></p>
+
+
+<p class="section">A SHORT HISTORY OF TIMEPIECES.</p>
+
+<p class="noin"><span class="dcap">T</span><span class="caps">he</span> 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.</p>
+
+<p>The clepsydra, or water-clock, also of great antiquity,<span class='pagenum'><a name="Page_411" id="Page_411">[Pg 411]</a></span> 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&mdash;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 <span class="ampm">A.D.</span> Charlemagne received from the King of
+Persia a water-clock which struck the hours. It is thus described in
+Gifford's "History of France":&mdash;"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."</p>
+
+<p>Sand-glasses were introduced about 330 <span class="ampm">A.D.<span class='pagenum'><a name="Page_412" id="Page_412">[Pg 412]</a></span></span> Except for special
+purposes, such as timing sermons and boiling eggs, they have not been of
+any practical value.</p>
+
+<p>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 <i>weight-driven
+clock</i> is attributed, like a good many other things, to Archimedes, the
+famous Sicilian mathematician of the third century <span class="ampm">B.C.</span>; but no record
+exists of any actual clock composed of wheels operated by a weight prior
+to 1120 <span class="ampm">A.D.</span> So we may take that year as opening the era of the clock as
+we know it.</p>
+
+<p>About 1500 Peter Hele of Nuremberg invented the <i>mainspring</i> as a
+substitute for the weight, and the <i>watch</i> appeared soon afterwards
+(1525 <span class="ampm">A.D.</span>). The pendulum was first adopted for controlling the motion
+of the wheels by Christian Huygens, a distinguished Dutch mechanician,
+in 1659.</p>
+
+<p>To Thomas Tompion, "the father of English watchmaking," is ascribed the
+honour of first fitting a <i>hairspring</i> to the escapement of a watch, in
+or about the year 1660. He also introduced the <i>cylinder escapement</i> now
+so commonly used in<span class='pagenum'><a name="Page_413" id="Page_413">[Pg 413]</a></span> 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.</p>
+
+
+<p class="section">THE CONSTRUCTION OF TIMEPIECES.</p>
+
+<p>A clock or watch contains three main elements:&mdash;(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&mdash;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.</p>
+
+
+<p class="section">THE DRIVING POWER.</p>
+
+<p><i>Weights</i> 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. <i>Springs</i> occupy much less
+room than weights, and are indispensable for portable timepieces. The
+employment of them<span class='pagenum'><a name="Page_414" id="Page_414">[Pg 414]</a></span> 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 <i>fusee</i>, 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, <span class="ampm">B</span>, and click, and therefore the spring in its
+effort to uncoil causes the barrel to rotate.</p>
+
+<div class="figleft" style="width: 300px;">
+<img src="images/image208.jpg" width="300" height="460" alt="Fig. 201." title="" />
+<span class="caption"><span class="smcap">Fig. 201.</span></span>
+</div>
+
+<p>A string of catgut (or a very fine chain) is<span class='pagenum'><a name="Page_415" id="Page_415">[Pg 415]</a></span> 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 <span class="ampm">A</span> 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.</p>
+
+<p>The fusee is still used for marine 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
+<i>going-barrel</i> 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<span class='pagenum'><a name="Page_416" id="Page_416">[Pg 416]</a></span> 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.</p>
+
+
+<p class="section">THE ESCAPEMENT.</p>
+
+<div class="figright" style="width: 250px;">
+<img src="images/image209.jpg" width="250" height="326" alt="Fig. 202." title="" />
+<span class="caption"><span class="smcap">Fig. 202.</span></span>
+</div>
+
+<p>The spring or weight transmits its power through a train of cogs to the
+<i>escapement</i>, or device for regulating the rate at which the wheels are
+to revolve. In clocks a <i>pendulum</i> 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<span class='pagenum'><a name="Page_417" id="Page_417">[Pg 417]</a></span> which depends a split arm embracing the rod and the pendulum. We
+must be careful to note that the pendulum <i>controls</i> motion only; it
+does not cause movement.</p>
+
+<p>The escape-wheel revolves in a clockwise direction. The two pallets <i>a</i>
+and <i>b</i> are so designed that only one can rest on the teeth at one time.
+In the sketch the sloping end of <i>b</i> has just been forced upwards by the
+pressure of a tooth. This swings the pallet and the pendulum. The
+momentum of the latter causes <i>a</i> to descend, and at the instant when
+<i>b</i> clears its tooth <i>a</i> catches and holds another. The left-hand side
+of <i>a</i>, called the <i>locking-face</i>, is part of a circle, so that the
+escape-wheel is held motionless as long as it touches <i>a</i>: hence the
+term, "dead beat"&mdash;that is, brought to a dead stop. As the pendulum
+swings back, to the left, under the influence of gravity, <i>a</i> is raised
+and frees the tooth. The wheel jerks round, and another tooth is caught
+by the locking-face of <i>b</i>. Again the pendulum swings to the right, and
+the sloping end of <i>b</i> is pushed up once more, giving the pendulum fresh
+impetus. This process repeats itself as long as the driving power
+lasts&mdash;for weeks, months, or years, as the case may be, and the
+mechanism continues to be in good working order.</p>
+
+
+<p><span class='pagenum'><a name="Page_418" id="Page_418">[Pg 418]</a></span></p><p class="section">COMPENSATING PENDULUMS.</p>
+
+<p>Metal expands when heated; therefore a steel pendulum which is of the
+exact length to govern a clock correctly at a temperature of 60&deg; would
+become too long at 80&deg;, and slow the clock, and too short at 40&deg;, 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 <i>self-compensating</i> pendulum on the principle illustrated by
+Fig. 203. He used steel for the rod, and formed the <i>bob</i>, 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 <i>upwards</i> 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<span class='pagenum'><a name="Page_419" id="Page_419">[Pg 419]</a></span> still used in observatories and other places where
+timekeepers of extreme precision are required. The milled nut <span class="ampm">S</span> in Fig.
+203 is fitted at the end of the pendulum rod to permit the exact
+adjustment of the pendulum's length.</p>
+
+<p>For watches, chronometers, and small clocks</p>
+
+
+<p class="section">THE SPRING BALANCE</p>
+
+<p class="noin">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.</p>
+
+<div class="figcenter" style="width: 150px;">
+<img src="images/image210.jpg" width="150" height="612" alt="Fig. 203." title="" />
+<span class="caption"><span class="smcap">Fig. 203.</span></span>
+</div>
+
+<p>The <i>hairspring</i> 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 <i>balance-wheel</i>, 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<span class='pagenum'><a name="Page_420" id="Page_420">[Pg 420]</a></span> 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&mdash;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.</p>
+
+<p>Motion is transmitted to the balance by one of two methods. Either (1)
+directly, by a cylinder escapement; or (2) indirectly, through a lever.</p>
+
+<div class="figleft" style="width: 250px;">
+<img src="images/image211.jpg" width="250" height="258" alt="Fig. 204." title="" />
+<span class="caption"><span class="smcap">Fig. 204.</span>&mdash;"Cylinder" watch escapement.</span>
+</div>
+
+
+<p class="section">THE CYLINDER ESCAPEMENT</p>
+
+<p class="noin">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<span class='pagenum'><a name="Page_421" id="Page_421">[Pg 421]</a></span> would appear like
+<i>a</i> in our illustration. A tooth is just beginning to shove its point
+under the nearer edge of the opening. As it is forced forwards, <i>b</i> is
+revolved in a clockwise direction, winding up the hairspring. When the
+tooth has passed the nearer edge it flies 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.</p>
+
+
+<p class="section">THE LEVER ESCAPEMENT</p>
+
+<p class="noin">is somewhat more complicated. The escape-wheel teeth are locked and
+unlocked by the pallets <span class="ampm">P P<sup>1</sup></span> 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, <span class="ampm">R</span>, which
+carries a small pin, <span class="ampm">I</span>. Two pins, <span class="ampm">B B</span>, projecting from the plate of the
+watch prevent the lever moving too far. We must further notice the
+little pin <span class="ampm">C</span> on the lever, and a notch in the edge of the roller.</p>
+
+<p><span class='pagenum'><a name="Page_422" id="Page_422">[Pg 422]</a></span></p>
+<div class="figcenter" style="width: 500px;">
+<img src="images/image212.jpg" width="500" height="412" alt="Fig. 205." title="" />
+<span class="caption"><span class="smcap">Fig. 205.</span>&mdash;"Lever" watch escapement.</span>
+</div>
+
+<p>In the illustration a tooth has just passed under the "impulse face" <i>b</i>
+of <span class="ampm">P<sup>1</sup></span>. The lever has been moved upwards at the right end; and its
+forked end has given an impulse to <span class="ampm">R</span>, and through it to the
+balance-wheel. The spring winds up. The pin <span class="ampm">C</span> prevents the lever
+dropping, because it no longer has the notch opposite to it, but presses
+on the circumference of <span class="ampm">R</span>. As the spring unwinds it strikes the lever at
+the moment when the notch and <span class="ampm">C</span> are opposite. The lever is knocked
+downwards, and the tooth, which had been arrested by the locking-face
+<i>a</i> of<span class='pagenum'><a name="Page_423" id="Page_423">[Pg 423]</a></span> pallet <span class="ampm">P</span>, now presses on the impulse face <i>b</i>, forcing the left
+end of the lever up. The impulse pin <span class="ampm">I</span> receives a blow, assisting the
+unwinding of the spring, and <span class="ampm">C</span> again locks the lever. The same thing is
+repeated in alternate directions over and over again.</p>
+
+
+<p class="section">COMPENSATING BALANCE-WHEELS.</p>
+
+<p>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<span class='pagenum'><a name="Page_424" id="Page_424">[Pg 424]</a></span> of the
+<i>compensating balance</i>, using the same principle&mdash;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).<a name="FNanchor_39_39" id="FNanchor_39_39"></a><a href="#Footnote_39_39" class="fnanchor">[39]</a>
+This ingenious contrivance keeps the leverage of the rim constant<span class='pagenum'><a name="Page_425" id="Page_425">[Pg 425]</a></span>
+within very fine limits. The screws <span class="ampm">S S</span> are inserted in the rim to
+balance it correctly, and very fine adjustment is made by means of the
+four tiny weights <span class="ampm">W W</span>. In ships' chronometers,<a name="FNanchor_40_40" id="FNanchor_40_40"></a><a href="#Footnote_40_40" class="fnanchor">[40]</a> the rim pieces are
+<i>sub</i>-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.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 206, 207, and 208">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 200px;">
+<img src="images/image213.jpg" width="200" height="202" alt="Fig. 206." title="" />
+<span class="caption"><span class="smcap">Fig. 206.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 200px;">
+<img src="images/image214.jpg" width="200" height="202" alt=" Fig. 207." title="" />
+<span class="caption"><span class="smcap">Fig. 207.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 200px;">
+<img src="images/image215.jpg" width="200" height="202" alt=" Fig. 208." title="" />
+<span class="caption"><span class="smcap">Fig. 208.</span></span>
+</div></td></tr>
+<tr class='tr2'><td align='center' colspan='3'>
+<span class="caption">A "compensating" watch balance, at normal, super-normal, and sub-normal temperatures.</span>
+</td></tr>
+</table></div>
+
+<p class="section">KEYLESS WINDING MECHANISM FOR WATCHES.</p>
+
+<p>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.</p>
+
+<p>There are two forms of "going-barrel" keyless mechanism&mdash;(1) The rocking
+bar; (2) the shifting sleeve. The <i>rocking bar</i> device is shown in Figs.
+209, 210. The milled head <span class="ampm">M</span> turns a cog, <span class="ampm">G</span>, which is always in gear with
+a cog, <span class="ampm">F</span>. This cog gears<span class='pagenum'><a name="Page_426" id="Page_426">[Pg 426]</a></span> with two others, <span class="ampm">A</span> and <span class="ampm">B</span>, mounted at each end
+of the rocker <span class="ampm">R</span>, which moves on pivot <span class="ampm">S</span>. A spring, <span class="ampm">S P</span>, attached to the
+watch plate presses against a small stud on the rocking bar, and keeps <span class="ampm">A</span>
+normally in gear with <span class="ampm">C</span>, mounted on the arbor of the mainspring.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image216.jpg" width="500" height="513" alt="Fig. 209." title="" />
+<span class="caption"><span class="smcap">Fig. 209.</span>&mdash;The winding mechanism of a keyless watch.</span>
+</div>
+
+<p>To wind the watch, <span class="ampm">M</span> is turned so as to give <span class="ampm">F</span> an anti-clockwise motion.
+The teeth of <span class="ampm">F</span> now press<span class='pagenum'><a name="Page_427" id="Page_427">[Pg 427]</a></span> <span class="ampm">A</span> downwards and keep it in gear with <span class="ampm">C</span> while
+the winding is done. A spring click (marked solid black) prevents the
+spring uncoiling (Fig. 209). If <span class="ampm">F</span> is turned in a clockwise direction it
+lifts <span class="ampm">A</span> and prevents it biting the teeth of <span class="ampm">C</span>, and no strain is thrown
+on <span class="ampm">C</span>.</p>
+
+<p>To set the hands, the little push-piece <span class="ampm">P</span> is pressed inwards by the
+thumb (Fig. 210) so as to depress the right-hand end of <span class="ampm">R</span> and bring <span class="ampm">B</span>
+into gear with <span class="ampm">D</span>, which in turn moves <span class="ampm">E</span>, mounted on the end of the
+minute-hand shaft. The hands can now be moved in either direction by
+turning <span class="ampm">M</span>. On releasing the push-piece the winding-wheels engage again.</p>
+
+<p>The <i>shifting sleeve</i> mechanism has a bevel pinion in the place of <span class="ampm">G</span>
+(Fig. 209) gearing with the mainspring cog. The shaft of the knob <span class="ampm">M</span> 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 <span class="ampm">G</span> (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,<span class='pagenum'><a name="Page_428" id="Page_428">[Pg 428]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image217.jpg" width="500" height="508" alt="Fig. 210." title="" />
+<span class="caption"><span class="smcap">Fig. 210.</span>&mdash;The hand-setting mechanism in action.</span>
+</div>
+
+<p>In one form of this mechanism the push-piece is<span class='pagenum'><a name="Page_429" id="Page_429">[Pg 429]</a></span> dispensed with, and the
+minute-wheel pinion is engaged by pulling the knob upwards.</p>
+
+
+<p class="section">THE HOUR-HAND TRAIN.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image218.jpg" width="500" height="261" alt="Fig. 211." title="" />
+<span class="caption"><span class="smcap">Fig. 211.</span>&mdash;The hour-hand train of a clock.</span>
+</div>
+
+<p>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 <span class="ampm">A</span> (solid
+black) can be moved round inside the cog <span class="ampm">B</span>, driven by the mainspring
+drum. It carries a cog, <span class="ampm">C</span>. This gears with a cog, <span class="ampm">D</span>, having three times
+as many teeth. The cog <span class="ampm">E</span>, united to <span class="ampm">D</span>, drives cog <span class="ampm">F</span>, having four times
+as many teeth as <span class="ampm">E</span>. To <span class="ampm">F</span> is attached the collar <span class="ampm">G</span> of the<span class='pagenum'><a name="Page_430" id="Page_430">[Pg 430]</a></span> hour-hand. <span class="ampm">F</span>
+and <span class="ampm">G</span> revolve outside the minute-hand shaft. On turning <span class="ampm">A</span>, <span class="ampm">C</span> turns <span class="ampm">D</span> and
+<span class="ampm">E</span>, <span class="ampm">E</span> turns <span class="ampm">F</span> and the hour-hand, which revolves &#8531; of &frac14; = <span class="above">1</span>&#8260;<span class="below">12</span> as fast
+as <span class="ampm">A</span>.<a name="FNanchor_41_41" id="FNanchor_41_41"></a><a href="#Footnote_41_41" class="fnanchor">[41]</a></p>
+
+<hr style='width: 45%;' />
+
+<p class="center"><big>LOCKS.</big></p>
+
+<p class="noin"><span class="smcap">On</span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image219.jpg" width="500" height="306" alt="Fig. 212." title="" />
+<span class="caption"><span class="smcap">Fig. 212.</span></span>
+</div>
+
+<p>The simplest form of lock, such as is found in<span class='pagenum'><a name="Page_431" id="Page_431">[Pg 431]</a></span> 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</p>
+
+<p class="section">TUMBLER LOCK.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image220.jpg" width="500" height="347" alt="Fig. 213." title="" />
+<span class="caption"><span class="smcap">Fig. 213.</span></span>
+</div>
+
+<p>The bolt now can move only in a horizontal direction. It has an opening
+cut in it with two<span class='pagenum'><a name="Page_432" id="Page_432">[Pg 432]</a></span> notches (Figs. 213, 214). Behind the bolt lies the
+<i>tumbler</i> <span class="ampm">T</span> (indicated by the dotted line), pivoted at the angle on a
+pin. From the face of the tumbler a stud, <span class="ampm">S</span>, projects through the hole
+in the bolt. This stud is forced into one or other of the notches by the
+spring, <span class="ampm">S<sup>1</sup></span>, which presses on the tail of the tumbler.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image221.jpg" width="500" height="324" alt="Fig. 214." title="" />
+<span class="caption"><span class="smcap">Fig. 214.</span></span>
+</div>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_433" id="Page_433">[Pg 433]</a></span></p><p>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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image222.jpg" width="500" height="126" alt="Fig. 215." title="" />
+<span class="caption"><span class="smcap">Fig. 215.</span>&mdash;The bolt of a Barron lock.</span>
+</div>
+
+<p class="section">THE CHUBB LOCK</p>
+
+<p class="noin">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<span class='pagenum'><a name="Page_434" id="Page_434">[Pg 434]</a></span> 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.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image223.jpg" width="500" height="312" alt="Fig. 216." title="" />
+<span class="caption"><span class="smcap">Fig. 216.</span>&mdash;Tumbler of Chubb lock.</span>
+</div>
+
+<p>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.</p>
+
+<div class="figcenter" style="width: 250px;">
+<img src="images/image224.jpg" width="250" height="452" alt="Fig. 217." title="" />
+<span class="caption"><span class="smcap">Fig. 217.</span>&mdash;A Chubb key.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_435" id="Page_435">[Pg 435]</a></span></p><p>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 <span class="ampm">D</span> on the extreme right (lifted by step 2 of the key) has a stud,
+<span class="ampm">D S</span>, projecting from it over the other tumblers. This is called the
+<i>detector tumbler</i>. 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 <span class="ampm">D S</span>, 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.</p>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/image225.jpg" width="300" height="238" alt="Fig. 218." title="" />
+<span class="caption"><span class="smcap">Fig. 218.</span>&mdash;A Chubb key raising all the tumblers to the
+correct height.</span>
+</div>
+
+<p>Each tumbler step of a large Chubb key can be<span class='pagenum'><a name="Page_436" id="Page_436">[Pg 436]</a></span> 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!</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image226.jpg" width="500" height="352" alt="Fig. 219." title="" />
+<span class="caption"><span class="smcap">Fig. 219.</span>&mdash;Section of a Yale lock.</span>
+</div>
+
+
+<p class="section">THE YALE LOCK,</p>
+
+<p class="noin">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<span class='pagenum'><a name="Page_437" id="Page_437">[Pg 437]</a></span> the key. The keyhole is a narrow
+twisted slot in the face of a cylinder, <span class="ampm">G</span> (Fig. 219), which revolves
+inside a larger fixed cylinder, <span class="ampm">F</span>. As the key is pushed in, the notches
+in its upper edge raise up the pins <span class="ampm">A<sup>1</sup>, B<sup>1</sup>, C<sup>1</sup>, D<sup>1</sup>, E<sup>1</sup></span>,
+until their tops exactly reach the surface of <span class="ampm">G</span>, which can now be
+revolved by the key in Fig. 220, and work the bolt through the medium of
+the arm <span class="ampm">H</span>. (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 <span class="ampm">F</span> (<a href="#Fig_221">see Fig. 221</a>), or some of the pins in <span class="ampm">F</span> will sink into <span class="ampm">G</span>. It
+is then impossible to turn the key.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image227.jpg" width="500" height="290" alt="Fig. 220." title="" />
+<span class="caption"><span class="smcap">Fig. 220.</span>&mdash;Yale key turning.</span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_438" id="Page_438">[Pg 438]</a></span> account can be given. We might, however, notice the <i>time</i>
+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 <i>automatic combination</i> lock.
+This may have twenty or more keys, any one of which can lock it; but the
+same one must be used to <i>un</i>lock 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.</p>
+
+<div class="figcenter" style="width: 500px;"><a name="Fig_221" id="Fig_221"></a>
+<img src="images/image228.jpg" width="500" height="290" alt="Fig. 221." title="" />
+<span class="caption"><span class="smcap">Fig. 221.</span>&mdash;The wrong key inserted. The pins do not allow
+the lock to be turned.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_439" id="Page_439">[Pg 439]</a></span></p><hr style='width: 45%;' />
+
+<p class="center"><big>THE CYCLE.</big></p>
+
+<p class="noin"><span class="smcap">There</span> are a few features of this useful and in some ways wonderful
+contrivance which should be noticed. First,</p>
+
+
+<p class="section">THE GEARING OF A CYCLE.</p>
+
+<p>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.</p>
+
+<p>The safety cycle is always "geared up"&mdash;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:&mdash;The teeth are 75
+and 30 in number respectively; the ratio of speed therefore = <span class="above">75</span>&#8260;<span class="below">30</span> =
+<span class="above">5</span>&#8260;<span class="below">2</span> = 2&frac12;. One turn of the pedal turns the rear wheel 2&frac12; 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 2&frac12; &times; 28 = 70 inches.</p>
+
+<p><span class='pagenum'><a name="Page_440" id="Page_440">[Pg 440]</a></span></p><p>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&mdash;</p>
+
+<p class="center">
+<span class="above">75</span>&#8260;<span class="below">25</span> &times; 28 inches = 84 inches.<br />
+</p>
+
+<p class="noin">A handy formula to remember is, gearing = T/<i>t</i> &times; D, where T = teeth on
+large chain-wheel; <i>t</i> = teeth on small chain-wheel; and D = diameter of
+driving-wheel in inches.</p>
+
+<p>Two of the most important improvements recently added to the cycle
+are&mdash;(1) The free wheel; (2) the change-speed gear.</p>
+
+
+<p class="section">THE FREE WHEEL</p>
+
+<p>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<span class='pagenum'><a name="Page_441" id="Page_441">[Pg 441]</a></span> (Fig. 222), which is extremely simple. The
+<i>ratchet-wheel</i> <span class="ampm">R</span> 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 <i>pawls</i>, 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.</p>
+
+<div class="figleft" style="width: 300px;">
+<img src="images/image229.jpg" width="300" height="288" alt="Fig. 222." title="" />
+<span class="caption"><span class="smcap">Fig. 222.</span></span>
+</div>
+
+<p>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<span class='pagenum'><a name="Page_442" id="Page_442">[Pg 442]</a></span> 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.</p>
+
+
+<p class="section">THE CHANGE-SPEED GEAR.</p>
+
+<p>A gain in speed means a loss in power, and <i>vice vers&acirc;</i>. 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.</p>
+
+<p>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&mdash;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<span class='pagenum'><a name="Page_443" id="Page_443">[Pg 443]</a></span> speed as, or faster than the
+small chain-wheel,<a name="FNanchor_42_42" id="FNanchor_42_42"></a><a href="#Footnote_42_42" class="fnanchor">[42]</a> according to the wish of the rider.</p>
+
+<p>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 <i>force</i> 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.</p>
+
+<p><span class='pagenum'><a name="Page_444" id="Page_444">[Pg 444]</a></span></p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 223, 224, and 225">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 200px;">
+<img src="images/image230.jpg" width="200" height="225" alt="Fig. 223." title="" />
+<span class="caption"><span class="smcap">Fig. 223.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 200px;">
+<img src="images/image231.jpg" width="200" height="225" alt="Fig. 224." title="" />
+<span class="caption"><span class="smcap">Fig. 224.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 200px;">
+<img src="images/image232.jpg" width="200" height="225" alt="Fig. 225." title="" />
+<span class="caption"><span class="smcap">Fig. 225.</span></span>
+</div></td></tr>
+</table></div>
+
+<p>Now see how this principle is applied to the change-speed gear. The
+lower rule is replaced by a cog-wheel, <span class="ampm">C</span> (Fig. 223); the cylinder by a
+cog, <span class="ampm">B</span>, running round it; and the upper rule by a ring, <span class="ampm">A</span>, with internal
+teeth. We may suppose that <span class="ampm">A</span> is the chain-ring, <span class="ampm">B</span> a cog mounted on a pin
+projecting from the hub, and <span class="ampm">C</span> a cog attached to the fixed axle. It is
+evident that <span class="ampm">B</span> will not move so fast round <span class="ampm">C</span> as <span class="ampm">A</span> does. The amount by
+which <span class="ampm">A</span> will get ahead of <span class="ampm">B</span> can be calculated easily. We begin with the
+wheels in the position shown in Fig. 223. A point, <span class="ampm">I</span>, on <span class="ampm">A</span> is exactly
+over the topmost point of <span class="ampm">C</span>. For the sake of convenience we will first
+assume that instead of <span class="ampm">B</span> running round <span class="ampm">C</span>, <span class="ampm">B</span> is revolved on its axis for
+one complete revolution in a clockwise direction, and that <span class="ampm">A</span> and <span class="ampm">C</span> move
+as in Fig. 224. If <span class="ampm">B</span> has 10 teeth, <span class="ampm">C</span> 30, and <span class="ampm">A</span> 40, <span class="ampm">A</span> will have been
+moved <span class="above">10</span>&#8260;<span class="below">40</span> = &frac14; of a revolution<span class='pagenum'><a name="Page_445" id="Page_445">[Pg 445]</a></span> in a clockwise direction, and <span class="ampm">C</span> <span class="above">10</span>&#8260;<span class="below">30</span>
+= &#8531; of a revolution in an anti-clockwise direction.</p>
+
+<p>Now, coming back to what actually does happen, we shall be able to
+understand how far <span class="ampm">A</span> rotates round <span class="ampm">C</span> relatively to the motion of <span class="ampm">B</span>, when
+<span class="ampm">C</span> is fixed and <b>B</b> rolls (Fig. 225). <span class="ampm">B</span> advances &#8531; of distance round <span class="ampm">C</span>; <span class="ampm">A</span>
+advances &#8531; + &frac14; = <span class="above">7</span>&#8260;<span class="below">12</span> of distance round <span class="ampm">B</span>. The fractions, if reduced
+to a common denominator, are as 4:7, and this is equivalent to 40
+(number of teeth on <span class="ampm">A</span>): 40 + 30 (teeth on <span class="ampm">A</span> + teeth on <span class="ampm">C</span>.)</p>
+
+<p>To leave the reader with a very clear idea we will summarize the matter
+thus:&mdash;If T = number of teeth on <span class="ampm">A</span>, <i>t</i> = number of teeth on <span class="ampm">C</span>, then
+movement of <span class="ampm">A</span>: movement of <span class="ampm">B</span>:: T + <i>t</i>: T.</p>
+
+<p>Here is a two-speed hub. Let us count the teeth. The chain-ring (= <span class="ampm">A</span>)
+has 64 internal teeth, and the central cog (= <span class="ampm">C</span>) on the axle has 16
+teeth. There are four cogs (= <span class="ampm">B</span>) equally spaced, running on pins
+projecting from the hub-shell between <span class="ampm">A</span> and <span class="ampm">C</span>. How much faster than <span class="ampm">B</span>
+does <span class="ampm">A</span> run round <span class="ampm">C</span>? Apply the formula:&mdash;Motion of <span class="ampm">A</span>: motion of <span class="ampm">B</span>:: 64 +
+16: 64. That is, while <span class="ampm">A</span> revolves once, <span class="ampm">B</span> and the hub and the
+driving-wheel will revolve only <span class="above">64</span>&#8260;<span class="below">80</span> = &#8536; of a turn. To use scientific
+language, <span class="ampm">B</span> revolves 20 per cent. slower than <span class="ampm">A</span>.</p>
+
+<p><span class='pagenum'><a name="Page_446" id="Page_446">[Pg 446]</a></span></p><p>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 <span class="ampm">A</span> and <span class="ampm">C</span> must revolve together. In
+one well-known gear this is effected by sliding <span class="ampm">C</span> along the spindle of
+the wheel till it disengages itself from the spindle, and one end locks
+with the plate which carries <span class="ampm">A</span>. Since <span class="ampm">B</span> 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 <i>solidly</i>&mdash;that is, while <span class="ampm">A</span> turns through a
+circle <span class="ampm">B</span> does the same.</p>
+
+<p>To get an <i>increase</i> of gearing, matters must be so arranged that the
+drive is transmitted from the chain-wheel to <span class="ampm">B</span>, and from <span class="ampm">A</span> to the hub.
+While <span class="ampm">B</span> describes a circle, <span class="ampm">A</span> and the driving-wheel turn through a
+circle and a part of a circle&mdash;that is, the driving-wheel revolves
+faster than the hub. Given the same number of teeth as before, the
+proportional rates will be <span class="ampm">A</span> = 80, <span class="ampm">B</span> = 64, so that the gear <i>rises</i> 25
+per cent.</p>
+
+<p>By means of proper mechanism the power is transmitted in a three-speed
+gear either (1) from chain-wheel to <span class="ampm">A</span>, <span class="ampm">A</span> to <span class="ampm">B</span>, <span class="ampm">B</span> to wheel = <i>low</i> gear;
+or (2) from chain-wheel to <span class="ampm">A</span> and <span class="ampm">C</span> simultaneously = solid,<span class='pagenum'><a name="Page_447" id="Page_447">[Pg 447]</a></span> normal, or
+<i>middle</i> gear; or (3) from chain-wheel to <span class="ampm">B</span>, <span class="ampm">B</span> to <span class="ampm">A</span>, <span class="ampm">A</span> to wheel = <i>high</i>
+gear. In two-speed gears either 1 or 3 is omitted.</p>
+
+<hr style='width: 45%;' />
+
+<p class="center"><big>AGRICULTURAL MACHINES.</big></p>
+
+
+<p class="section">THE THRESHING-MACHINE.</p>
+
+<p class="noin"><span class="smcap">Bread</span> 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 (<a href="#Page_384">p. 384</a>), the threshing-machine has done for
+agriculture. A page or two ought therefore to be spared for this useful
+invention.</p>
+
+<p><span class='pagenum'><a name="Page_448" id="Page_448">[Pg 448]</a></span></p>
+<div class="figcenter" style="width: 600px;">
+<img src="images/image233.jpg" width="600" height="282" alt="Fig. 226." title="" />
+<span class="caption"><span class="smcap">Fig. 226.</span>&mdash;Section of a threshing machine.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_449" id="Page_449">[Pg 449]</a></span></p><p>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 <i>drum</i> <span class="ampm">A</span> 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, <span class="ampm">B</span>, 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 <span class="ampm">C</span>, 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 <span class="ampm">D</span>, and works backwards to the <i>caving riddles</i>, or moving
+sieves, <span class="ampm">E</span>. The <i>main blower</i>, by means of a revolving fan, <span class="ampm">N</span>, sends air
+along the channel <span class="ampm">X</span> upwards through these riddles, blowing the short
+straws away to the left. The grain, husks, and dust fall through <span class="ampm">E</span> on to
+<span class="ampm">G</span>, over the end of which they fall on to the <i>chaff riddle</i>, <span class="ampm">H</span>. A second
+column of air from the blower drives the chaff away. The heavy grain,
+seeds, dust, etc., fall on to <span class="ampm">I</span>, <span class="ampm">J</span>, and <span class="ampm">K</span> in turn, and are shaken until
+only the grain remains to pass along <span class="ampm">L</span> to the elevator bottom, <span class="ampm">M</span>. An
+endless band with cups attached to it scoops up the grain, carries it
+aloft, and shoots it into hopper <span class="ampm">P</span>. It then goes through the shakers <span class="ampm">Q,
+R</span>, is dusted by the <i>back end blower</i>, <span class="ampm">S</span>, and slides down <span class="ampm">T</span> into the
+open end of the rotary screen-drum <span class="ampm">U</span>, 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.</p>
+
+
+<p><span class='pagenum'><a name="Page_450" id="Page_450">[Pg 450]</a></span></p><p class="section">MOWING-MACHINES.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/image234.jpg" width="500" height="345" alt="Fig. 227." title="" />
+<span class="caption"><span class="smcap">Fig. 227.</span></span>
+</div>
+
+<p>The ordinary <i>lawn&mdash;mower</i> 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<span class='pagenum'><a name="Page_451" id="Page_451">[Pg 451]</a></span> them as
+it passes along the edges. The same thing happens with grass, which is
+so soft that it is cut right through.</p>
+
+<p class="section">HAY-CUTTER.</p>
+
+<p>The <i>hay-cutter</i> 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.</p>
+
+<hr style='width: 45%;' />
+
+<p class="center"><big>SOME NATURAL PHENOMENA.</big></p>
+
+
+<p class="section">WHY SUN-HEAT VARIES IN INTENSITY.</p>
+
+<p class="noin"><span class="smcap">The</span> 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 <span class="ampm">A B</span> 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<span class='pagenum'><a name="Page_452" id="Page_452">[Pg 452]</a></span> 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.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 228 and 229">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image235.jpg" width="300" height="167" alt="Fig. 228." title="" />
+<span class="caption"><span class="smcap">Fig. 228.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image236.jpg" width="300" height="167" alt="Fig. 229." title="" />
+<span class="caption"><span class="smcap">Fig. 229.</span></span>
+</div></td></tr>
+</table></div>
+
+<p>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.</p>
+
+
+<p class="section">THE TIDES.</p>
+
+<p>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;<span class='pagenum'><a name="Page_453" id="Page_453">[Pg 453]</a></span> 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.)</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 230 and 231">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image237.jpg" width="300" height="430" alt="Fig. 230." title="" />
+<span class="caption"><span class="smcap">Fig. 230.</span></span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 300px;">
+<img src="images/image238.jpg" width="300" height="430" alt="Fig. 231." title="" />
+<span class="caption"><span class="smcap">Fig. 231.</span></span>
+</div></td></tr>
+</table></div>
+
+<p class="section">WHY HIGH TIDE VARIES DAILY.</p>
+
+<p>The moon travels round the earth once in twenty-eight days. In Fig. 231
+the point <i>a</i> is nearest the moon at, say, twelve noon. At the end of
+twenty-four<span class='pagenum'><a name="Page_454" id="Page_454">[Pg 454]</a></span> 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 <span class="above">1</span>&#8260;<span class="below">28</span>th of a revolution round the earth.<a name="FNanchor_43_43" id="FNanchor_43_43"></a><a href="#Footnote_43_43" class="fnanchor">[43]</a> Consequently high
+tide will not occur till <i>a</i> has reached position <i>b</i> 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.</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Figs 232 and 233">
+<tr class='tr2'><td align='center'>
+<div class="figcenter" style="width: 350px;">
+<img src="images/image239.jpg" width="300" height="460" alt="Fig. 232." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 232.</span>&mdash;Relative positions of sun, moon, and earth at
+"spring" tides.</span>
+</div></td>
+<td align='center'>
+<div class="figcenter" style="width: 350px;">
+<img src="images/image240.jpg" width="300" height="460" alt="Fig. 233." title="" /><br />
+<span class="caption"><span class="smcap">Fig. 233.</span>&mdash;Relative positions of sun, moon, and earth at
+"neap" tides.</span>
+</div></td></tr>
+</table></div>
+
+<p class="section">NEAP TIDES AND SPRING TIDES.</p>
+
+<p>The sun, as well as the moon, attracts the ocean,<span class='pagenum'><a name="Page_455" id="Page_455">[Pg 455]</a></span> 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 <i>spring</i> tides (Fig. 232). When sun
+and moon pull at right angles to one another&mdash;namely, at the first and
+third quarters&mdash;the excrescence caused by the moon is flattened (Fig.
+233), and we get the lowest, or <i>neap</i> tides.</p>
+
+<div class="footnote"><p><a name="Footnote_39_39" id="Footnote_39_39"></a><a href="#FNanchor_39_39"><span class="label">[39]</span></a> In both Figs. 207 and 208 the degree of expansion is very
+greatly exaggerated.</p></div>
+
+<div class="footnote"><p><a name="Footnote_40_40" id="Footnote_40_40"></a><a href="#FNanchor_40_40"><span class="label">[40]</span></a> 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.</p></div>
+
+<div class="footnote"><p><a name="Footnote_41_41" id="Footnote_41_41"></a><a href="#FNanchor_41_41"><span class="label">[41]</span></a> 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.</p></div>
+
+<div class="footnote"><p><a name="Footnote_42_42" id="Footnote_42_42"></a><a href="#FNanchor_42_42"><span class="label">[42]</span></a> We shall here notice only those gears which are included
+in the hub of the driving-wheel.</p></div>
+
+<div class="footnote"><p><a name="Footnote_43_43" id="Footnote_43_43"></a><a href="#FNanchor_43_43"><span class="label">[43]</span></a> The original position of the moon is indicated by the
+dotted circle.</p></div>
+
+
+
+<hr />
+<h2>INDEX.</h2>
+
+<p class="center"><span class="smcap">Note.</span>&mdash;Figures in italics signify that an illustration of<br /> the thing
+referred to appears on the page.</p>
+
+
+<p class="index">
+Aberration, spherical, of lens, <a href="#Page_243">243</a>.<br />
+<br />
+Acoustics, <a href="#Page_294">294</a>.<br />
+<br />
+Achromatic lens, <a href="#Page_243">243</a>.<br />
+<br />
+Action carriage of piano, <a href="#Page_283">283</a>.<br />
+<br />
+Advancing the spark, <a href="#Page_102">102</a>.<br />
+<br />
+Air-gun, <i><a href="#Page_342">342</a></i>.<br />
+<br />
+Air-pump for cycle tyres, <i><a href="#Page_340">340</a></i>;<br />
+<span style="margin-left: 1em;">for Westinghouse brake, <a href="#Page_199">199</a>.</span><br />
+<br />
+Alternating currents, <a href="#Page_164">164</a>;<br />
+<span style="margin-left: 1em;">dynamo, <a href="#Page_164">164</a>.</span><br />
+<br />
+Amperage, <a href="#Page_125">125</a>.<br />
+<br />
+Angle of advance, <a href="#Page_57">57</a>, <a href="#Page_58">58</a>;<br />
+<span style="margin-left: 1em;">incidence, <a href="#Page_268">268</a>;</span><br />
+<span style="margin-left: 1em;">reflection, <a href="#Page_268">268</a>.</span><br />
+<br />
+Aorta, <a href="#Page_360">360</a>.<br />
+<br />
+Arc lamp, <a href="#Page_182">182</a>.<br />
+<br />
+Archimedes, <a href="#Page_412">412</a>.<br />
+<br />
+Armature, <a href="#Page_162">162</a>.<br />
+<br />
+Arteries, <a href="#Page_358">358</a>.<br />
+<br />
+Arterial blood, <a href="#Page_359">359</a>.<br />
+<br />
+Atmospheric pressure, <a href="#Page_350">350</a>.<br />
+<br />
+Auditory nerve, <a href="#Page_272">272</a>.<br />
+<br />
+Automatic brakes, <a href="#Page_188">188</a>;<br />
+<span style="margin-left: 1em;">signalling, <a href="#Page_228">228</a>;</span><br />
+<span style="margin-left: 1em;">stoker, <a href="#Page_399">399</a>.</span><br />
+<br />
+<br />
+Backfall, <a href="#Page_298">298</a>.<br />
+<br />
+Balance-wheel, <a href="#Page_419">419</a>.<br />
+<br />
+Ball cock, <a href="#Page_366">366</a>, <i><a href="#Page_367">367</a></i>.<br />
+<br />
+Balloon, fire, <a href="#Page_323">323</a>;<br />
+<span style="margin-left: 1em;">gas, <a href="#Page_347">347</a>.</span><br />
+<br />
+Barometer, aneroid, <a href="#Page_328">328</a>, <i><a href="#Page_329">329</a></i>;<br />
+<span style="margin-left: 1em;">and weather, <a href="#Page_331">331</a>;</span><br />
+<span style="margin-left: 1em;">Fortin's, <i><a href="#Page_326">326</a></i>;</span><br />
+<span style="margin-left: 1em;">meaning of, <a href="#Page_325">325</a>;</span><br />
+<span style="margin-left: 1em;">simple, <i><a href="#Page_328">328</a></i>;</span><br />
+<span style="margin-left: 1em;">wheel, <i><a href="#Page_327">327</a></i>.</span><br />
+<br />
+Beau de Rochas, <a href="#Page_89">89</a>.<br />
+<br />
+Bell, diving, <i><a href="#Page_332">332</a></i>;<br />
+<span style="margin-left: 1em;">electric, <a href="#Page_119">119</a>, <i><a href="#Page_120">120</a></i>.</span><br />
+<br />
+Bellows of organ, <a href="#Page_303">303</a>.<br />
+<br />
+Bioscope, <a href="#Page_266">266</a>.<br />
+<br />
+Blades, turbine, <i><a href="#Page_81">81</a></i>, <a href="#Page_83">83</a>.<br />
+<br />
+Block system, <a href="#Page_201">201</a>, <a href="#Page_212">212</a>.<br />
+<br />
+Blood, arterial, <a href="#Page_359">359</a>;<br />
+<span style="margin-left: 1em;">circulation of, <i><a href="#Page_356">356</a></i>, <i><a href="#Page_357">357</a></i>, <a href="#Page_360">360</a>;</span><br />
+<span style="margin-left: 1em;">venous, <a href="#Page_359">359</a>.</span><br />
+<br />
+Blower-plate, <a href="#Page_393">393</a>, <i><a href="#Page_394">394</a></i>.<br />
+<br />
+Boat, sails of, <a href="#Page_346">346</a>.<br />
+<br />
+Boiler, Babcock and Wilcox, <i><a href="#Page_21">21</a></i>, <a href="#Page_22">22</a>;<br />
+<span style="margin-left: 1em;">explosions, <a href="#Page_34">34</a>, <a href="#Page_391">391</a>;</span><br />
+<span style="margin-left: 1em;">fire-tube, <a href="#Page_21">21</a>;</span><br />
+<span style="margin-left: 1em;">fittings, <a href="#Page_31">31</a>;</span><br />
+<span style="margin-left: 1em;">Lancashire, <a href="#Page_25">25</a>, <i><a href="#Page_26">26</a></i>;</span><br />
+<span style="margin-left: 1em;">locomotive, <i><a href="#Page_20">20</a></i>, <a href="#Page_23">23</a>;</span><br />
+<span style="margin-left: 1em;">multitubular, <a href="#Page_21">21</a>;</span><br />
+<span style="margin-left: 1em;">principle of, <a href="#Page_15">15</a>;</span><br />
+<span style="margin-left: 1em;">stored energy in, <a href="#Page_32">32</a>;</span><br />
+<span style="margin-left: 1em;">vertical, <i><a href="#Page_25">25</a></i>;</span><br />
+<span style="margin-left: 1em;">water supply to, <a href="#Page_39">39</a>;</span><br />
+<span style="margin-left: 1em;">water-tube, <a href="#Page_21">21</a>.</span><br />
+<br />
+Brakes, hydraulic, <a href="#Page_188">188</a>;<br />
+<span style="margin-left: 1em;">motor car, <a href="#Page_110">110</a>;</span><br />
+<span style="margin-left: 1em;">railway, <a href="#Page_187">187</a>;</span><br />
+<span style="margin-left: 1em;">vacuum, <a href="#Page_189">189</a>, <i><a href="#Page_190">190</a></i>, <i><a href="#Page_191">191</a></i>;</span><br />
+<span style="margin-left: 1em;">Westinghouse, <a href="#Page_194">194</a>, <i><a href="#Page_195">195</a></i>, <i><a href="#Page_197">197</a></i>.</span><br />
+<br />
+Bramah, <a href="#Page_363">363</a>, <a href="#Page_437">437</a>.<br />
+<br />
+Breezes, land and sea, <a href="#Page_324">324</a>.<br />
+<br />
+Brushes of dynamo, <a href="#Page_161">161</a>, <i><a href="#Page_172">172</a></i>.<br />
+<br />
+Bunsen burner, <a href="#Page_409">409</a>.<br />
+<br />
+Burning-glass, <a href="#Page_232">232</a>.<br />
+<br />
+<br />
+Camera, the, <a href="#Page_233">233</a>;<br />
+<span style="margin-left: 1em;">pinhole, <i><a href="#Page_234">234</a></i>, <i><a href="#Page_235">235</a></i>.</span><br />
+<br />
+Canals, semicircular, <a href="#Page_273">273</a>.<br />
+<br />
+Capillary attraction, <a href="#Page_392">392</a>;<br />
+<span style="margin-left: 1em;">veins, <a href="#Page_358">358</a>.</span><br />
+<br />
+Carbon dioxide, <a href="#Page_27">27</a>, <a href="#Page_359">359</a>;<br />
+<span style="margin-left: 1em;">monoxide, <a href="#Page_27">27</a>.</span><br />
+<br />
+Carburetter, <a href="#Page_98">98</a>, <i><a href="#Page_99">99</a></i>.<br />
+<br />
+Cardan shaft, <a href="#Page_93">93</a>.<br />
+<br />
+<i>Carmania</i>, the, <a href="#Page_83">83</a>.<br />
+<br />
+Centrifugal force, <a href="#Page_382">382</a>.<br />
+<br />
+Change-speed gear, <a href="#Page_105">105</a>, <a href="#Page_442">442</a>.<br />
+<br />
+Chassis of motor car, <a href="#Page_92">92</a>.<br />
+<br />
+Circulation of water in a boiler, <i><a href="#Page_17">17</a></i>, <i><a href="#Page_18">18</a></i>, <i><a href="#Page_19">19</a></i>;<br />
+<span style="margin-left: 1em;">of water in a motor car, <a href="#Page_95">95</a>, <i><a href="#Page_97">97</a></i>.</span><br />
+<br />
+Clarionet, <a href="#Page_308">308</a>.<br />
+<br />
+Clock, first weight-driven, <a href="#Page_412">412</a>;<br />
+<span style="margin-left: 1em;">water, <a href="#Page_410">410</a>.</span><br />
+<br />
+Clutch of motor car, <a href="#Page_105">105</a>.<br />
+<br />
+Coal, as fuel, <a href="#Page_15">15</a>;<br />
+<span style="margin-left: 1em;">gas, <a href="#Page_394">394</a>;</span><br />
+<span style="margin-left: 1em;">gas making, <a href="#Page_394">394</a>;</span><br />
+<span style="margin-left: 1em;">gas plant, <i><a href="#Page_396">396</a></i>;</span><br />
+<span style="margin-left: 1em;">gas, purification of, <a href="#Page_397">397</a>.</span><br />
+<br />
+Cochlea, <a href="#Page_273">273</a>.<br />
+<br />
+Coherer, <a href="#Page_140">140</a>.<br />
+<br />
+Coil, Ruhmkorff, <a href="#Page_121">121</a>.<br />
+<br />
+Coke, <a href="#Page_395">395</a>.<br />
+<br />
+Combinations in Chubb lock, <a href="#Page_436">436</a>;<br />
+<span style="margin-left: 1em;">Yale lock, <a href="#Page_436">436</a>.</span><br />
+<br />
+Combustion, <a href="#Page_26">26</a>, <a href="#Page_393">393</a>;<br />
+<span style="margin-left: 1em;">perfect, <a href="#Page_28">28</a>.</span><br />
+<br />
+Compensating gear, <a href="#Page_107">107</a>, <i><a href="#Page_108">108</a></i>.<br />
+<br />
+Compound engines, <a href="#Page_59">59</a>;<br />
+<span style="margin-left: 1em;">arrangement of, <a href="#Page_61">61</a>;</span><br />
+<span style="margin-left: 1em;">invention of, <a href="#Page_59">59</a>.</span><br />
+<br />
+Compound locomotives, <a href="#Page_62">62</a>.<br />
+<br />
+Compound microscope, <a href="#Page_261">261</a>.<br />
+<br />
+Condenser, marine, <a href="#Page_71">71</a>, <i><a href="#Page_72">72</a></i>;<br />
+<span style="margin-left: 1em;">of Ruhmkorff coil, <a href="#Page_123">123</a>.</span><br />
+<br />
+Conduit, <a href="#Page_176">176</a>.<br />
+<br />
+Convex lens, image cast by, <i><a href="#Page_236">236</a></i>.<br />
+<br />
+Conjugate foci, <a href="#Page_262">262</a>.<br />
+<br />
+Cornet, <a href="#Page_308">308</a>.<br />
+<br />
+Corti, rods of, <a href="#Page_274">274</a>.<br />
+<br />
+Coxwell, <a href="#Page_348">348</a>.<br />
+<br />
+Cream separator, <a href="#Page_381">381</a>, <i><a href="#Page_383">383</a></i>.<br />
+<br />
+Current, reversal of electric, <i><a href="#Page_130">130</a></i>, <a href="#Page_131">131</a>;<br />
+<span style="margin-left: 1em;">transformation of, <a href="#Page_124">124</a>.</span><br />
+<br />
+Cushioning of steam, <a href="#Page_55">55</a>.<br />
+<br />
+Cycle, gearing of, <a href="#Page_439">439</a>.<br />
+<br />
+Cylinder, hydraulic press, <i><a href="#Page_363">363</a></i>;<br />
+<span style="margin-left: 1em;">steam, <i><a href="#Page_49">49</a></i>.</span><br />
+<br />
+<br />
+Danes, <a href="#Page_382">382</a>.<br />
+<br />
+Dead point, <a href="#Page_47">47</a>.<br />
+<br />
+De Brouwer stoker, <a href="#Page_401">401</a>.<br />
+<br />
+Detector in Chubb lock, <a href="#Page_435">435</a>.<br />
+<br />
+Diving-bell, <i><a href="#Page_332">332</a></i>;<br />
+<span style="margin-left: 1em;">simple, <i><a href="#Page_333">333</a></i>, <i><a href="#Page_334">334</a></i>.</span><br />
+<br />
+Diving-dress, <a href="#Page_335">335</a>.<br />
+<br />
+Direction of current in dynamo circuit, <a href="#Page_163">163</a>.<br />
+<br />
+Diver's feats, <a href="#Page_338">338</a>;<br />
+<span style="margin-left: 1em;">helmet, <i><a href="#Page_336">336</a></i>;</span><br />
+<span style="margin-left: 1em;">lamp, <i><a href="#Page_338">338</a></i>.</span><br />
+<br />
+Donkey-engines, <a href="#Page_68">68</a>.<br />
+<br />
+Doorstop, self-closing, <a href="#Page_344">344</a>.<br />
+<br />
+Double-cylinder engines, <a href="#Page_47">47</a>.<br />
+<br />
+Draught, forced, <a href="#Page_28">28</a>, <i><a href="#Page_29">29</a></i>;<br />
+<span style="margin-left: 1em;">induced, <a href="#Page_29">29</a>.</span><br />
+<br />
+Drum and fusee, <i><a href="#Page_414">414</a></i>.<br />
+<br />
+Durability of motor-car engine, <a href="#Page_96">96</a>.<br />
+<br />
+<span class="bigletter">D</span>-valve, <a href="#Page_67">67</a>.<br />
+<br />
+Dynamo, alternating, <a href="#Page_164">164</a>, <a href="#Page_174">174</a>;<br />
+<span style="margin-left: 1em;">brushes, <i><a href="#Page_172">172</a></i>;</span><br />
+<span style="margin-left: 1em;">compound, <a href="#Page_174">174</a>;</span><br />
+<span style="margin-left: 1em;">continuous-current, <a href="#Page_165">165</a>;</span><br />
+<span style="margin-left: 1em;">multipolar, <a href="#Page_169">169</a>;</span><br />
+<span style="margin-left: 1em;">series wound, <i><a href="#Page_173">173</a></i>;</span><br />
+<span style="margin-left: 1em;">shunt wound, <i><a href="#Page_173">173</a></i>;</span><br />
+<span style="margin-left: 1em;">simple, <a href="#Page_161">161</a>, <i><a href="#Page_162">162</a></i>.</span><br />
+<br />
+<br />
+Ear, the, <i><a href="#Page_271">271</a></i>, <i><a href="#Page_273">273</a></i>;<br />
+<span style="margin-left: 1em;">a good, <a href="#Page_274">274</a>, <a href="#Page_307">307</a>;</span><br />
+<span style="margin-left: 1em;">sensitiveness of, <a href="#Page_275">275</a>.</span><br />
+<br />
+Eccentric, <i><a href="#Page_52">52</a></i>, <a href="#Page_53">53</a>;<br />
+<span style="margin-left: 1em;">setting of, <a href="#Page_53">53</a>.</span><br />
+<br />
+Edison, Thomas, <a href="#Page_310">310</a>.<br />
+<br />
+Edison-Bell phonograph, <a href="#Page_310">310</a>.<br />
+<br />
+Electricity, current, <a href="#Page_115">115</a>;<br />
+<span style="margin-left: 1em;">forms of, <a href="#Page_113">113</a>;</span><br />
+<span style="margin-left: 1em;">nature of, <a href="#Page_112">112</a>;</span><br />
+<span style="margin-left: 1em;">static, <a href="#Page_114">114</a>.</span><br />
+<br />
+Electric bell, <a href="#Page_119">119</a>, <i><a href="#Page_120">120</a></i>;<br />
+<span style="margin-left: 1em;">signalling, <a href="#Page_225">225</a>;</span><br />
+<span style="margin-left: 1em;">slot, <a href="#Page_226">226</a>.</span><br />
+<br />
+Electroplating, <a href="#Page_185">185</a>, <i><a href="#Page_186">186</a></i>.<br />
+<br />
+Electro-magnets, <a href="#Page_117">117</a>.<br />
+<br />
+Endolymph, <a href="#Page_272">272</a>.<br />
+<br />
+Engines, compound, <a href="#Page_59">59</a>;<br />
+<span style="margin-left: 1em;">donkey, <a href="#Page_68">68</a>;</span><br />
+<span style="margin-left: 1em;">double-cylinder, <a href="#Page_47">47</a>;</span><br />
+<span style="margin-left: 1em;">internal-combustion, <a href="#Page_87">87</a>, <a href="#Page_95">95</a>;</span><br />
+<span style="margin-left: 1em;">reciprocating, <a href="#Page_44">44</a>.</span><br />
+<br />
+Escapement of timepieces, <a href="#Page_416">416</a>;<br />
+<span style="margin-left: 1em;">cylinder, <i><a href="#Page_420">420</a></i>;</span><br />
+<span style="margin-left: 1em;">lever, <a href="#Page_421">421</a>, <i><a href="#Page_422">422</a></i>.</span><br />
+<br />
+Ether, <a href="#Page_270">270</a>.<br />
+<br />
+Eustachian tube, <a href="#Page_276">276</a>.<br />
+<br />
+Eye, human, <a href="#Page_246">246</a>, <i><a href="#Page_247">247</a></i>;<br />
+<span style="margin-left: 1em;">self-accommodation of, <a href="#Page_248">248</a>.</span><br />
+<br />
+Expansive working of steam, <a href="#Page_56">56</a>.<br />
+<br />
+<br />
+Faraday, Michael, <a href="#Page_159">159</a>.<br />
+<br />
+Field, magnetic, <a href="#Page_159">159</a>;<br />
+<span style="margin-left: 1em;">magnets, <a href="#Page_171">171</a>;</span><br />
+<span style="margin-left: 1em;">ring, <a href="#Page_174">174</a>.</span><br />
+<br />
+Filters, <a href="#Page_374">374</a>;<br />
+<span style="margin-left: 1em;">Maignen, <i><a href="#Page_373">373</a></i>;</span><br />
+<span style="margin-left: 1em;">Berkefeld, <a href="#Page_374">374</a>.</span><br />
+<br />
+Filtration beds, <a href="#Page_372">372</a>.<br />
+<br />
+Flute, <a href="#Page_308">308</a>.<br />
+<br />
+Flying-machines, <a href="#Page_348">348</a>.<br />
+<br />
+Fly-wheel, use of, <a href="#Page_48">48</a>.<br />
+<br />
+Focus, meaning of, <a href="#Page_237">237</a>;<br />
+<span style="margin-left: 1em;">principal, <a href="#Page_238">238</a>.</span><br />
+<br />
+Foci, conjugate, <a href="#Page_262">262</a>.<br />
+<br />
+Force, lines of, <a href="#Page_116">116</a>.<br />
+<br />
+Forces, component, <a href="#Page_345">345</a>.<br />
+<br />
+Free wheel, <i><a href="#Page_440">440</a></i>.<br />
+<br />
+Furring-up of pipes, <a href="#Page_391">391</a>.<br />
+<br />
+Fusee, drum and, <a href="#Page_414">414</a>.<br />
+<br />
+<br />
+Galileo, <a href="#Page_259">259</a>, <a href="#Page_325">325</a>, <a href="#Page_416">416</a>.<br />
+<br />
+Galilean telescope, <i><a href="#Page_259">259</a></i>.<br />
+<br />
+Gas, coal, <a href="#Page_394">394</a>;<br />
+<span style="margin-left: 1em;">governor, <a href="#Page_402">402</a>;</span><br />
+<span style="margin-left: 1em;">meter, <a href="#Page_405">405</a>;</span><br />
+<span style="margin-left: 1em;">traps, <a href="#Page_374">374</a>;</span><br />
+<span style="margin-left: 1em;">works, <a href="#Page_394">394</a>.</span><br />
+<br />
+Gasometer, <a href="#Page_397">397</a>;<br />
+<span style="margin-left: 1em;">largest, <i><a href="#Page_398">398</a></i>, <a href="#Page_399">399</a>.</span><br />
+<br />
+Gauge, steam, <a href="#Page_36">36</a>, <i><a href="#Page_38">38</a></i>;<br />
+<span style="margin-left: 1em;">water, <a href="#Page_35">35</a>, <i><a href="#Page_36">36</a></i>.</span><br />
+<br />
+Gear, compensating, <a href="#Page_107">107</a>, <i><a href="#Page_108">108</a></i>.<br />
+<br />
+Gear-box of motor car, <a href="#Page_105">105</a>.<br />
+<br />
+Gearing of cycle, <a href="#Page_439">439</a>.<br />
+<br />
+Glaisher, <a href="#Page_348">348</a>.<br />
+<br />
+Gland, <a href="#Page_50">50</a>, <a href="#Page_363">363</a>.<br />
+<br />
+Glass, flint and crown, <a href="#Page_242">242</a>.<br />
+<br />
+Going-barrel for watches, <a href="#Page_415">415</a>.<br />
+<br />
+Gooch reversing gear, <a href="#Page_65">65</a>.<br />
+<br />
+Governors, speed, <a href="#Page_67">67</a>;<br />
+<span style="margin-left: 1em;">of motor car, <a href="#Page_103">103</a>, <i><a href="#Page_104">104</a></i>.</span><br />
+<br />
+Graham, <a href="#Page_418">418</a>.<br />
+<br />
+Gramophone, <a href="#Page_317">317</a>;<br />
+<span style="margin-left: 1em;">records, <a href="#Page_319">319</a>, <a href="#Page_321">321</a>;</span><br />
+<span style="margin-left: 1em;">reproducer, <i><a href="#Page_318">318</a></i>.</span><br />
+<br />
+<br />
+Hairspring, <a href="#Page_412">412</a>.<br />
+<br />
+Hay-cutter, <a href="#Page_451">451</a>.<br />
+<br />
+Heart, the, <a href="#Page_355">355</a>;<br />
+<span style="margin-left: 1em;">disease, <a href="#Page_361">361</a>;</span><br />
+<span style="margin-left: 1em;">rate of pulsation of, <a href="#Page_361">361</a>;</span><br />
+<span style="margin-left: 1em;">size of, <a href="#Page_357">357</a>.</span><br />
+<br />
+Heat of sun, <a href="#Page_451">451</a>.<br />
+<br />
+Hele, Peter, <a href="#Page_412">412</a>.<br />
+<br />
+Helmet, diver's, <i><a href="#Page_336">336</a></i>.<br />
+<br />
+Helmholtz, <a href="#Page_274">274</a>, <a href="#Page_308">308</a>.<br />
+<br />
+Hero of Alexandria, <a href="#Page_74">74</a>.<br />
+<br />
+Herschel, <a href="#Page_261">261</a>.<br />
+<br />
+Hertz, Dr., <a href="#Page_138">138</a>.<br />
+<br />
+Hertzian waves, <a href="#Page_138">138</a>.<br />
+<br />
+Hot-water supply, <a href="#Page_386">386</a>.<br />
+<br />
+Hour-hand train in timepieces, <i><a href="#Page_429">429</a></i>.<br />
+<br />
+Household water supply, <a href="#Page_364">364</a>.<br />
+<br />
+Hughes type-printer, <a href="#Page_134">134</a>.<br />
+<br />
+Hydraulic press, <a href="#Page_361">361</a>, <i><a href="#Page_362">362</a></i>.<br />
+<br />
+Hydro, <a href="#Page_385">385</a>.<br />
+<br />
+<br />
+Ignition of charge in motor-car cylinder, <a href="#Page_100">100</a>, <i><a href="#Page_101">101</a></i>.<br />
+<br />
+Image and object, relative positions of, <a href="#Page_239">239</a>;<br />
+<span style="margin-left: 1em;">distortion of, <a href="#Page_245">245</a>.</span><br />
+<br />
+Incandescent gas mantle, <a href="#Page_407">407</a>;<br />
+<span style="margin-left: 1em;">electric lamp, <a href="#Page_179">179</a>.</span><br />
+<br />
+Incus, <a href="#Page_272">272</a>.<br />
+<br />
+Index mechanism of water-meter, <a href="#Page_37">37</a>.<br />
+<br />
+Indicator of electric bell, <a href="#Page_119">119</a>.<br />
+<br />
+Induction coil, <a href="#Page_121">121</a>;<br />
+<span style="margin-left: 1em;">uses of, <a href="#Page_125">125</a>.</span><br />
+<br />
+Injector, <a href="#Page_39">39</a>;<br />
+<span style="margin-left: 1em;">Giffard's, <i><a href="#Page_41">41</a></i>;</span><br />
+<span style="margin-left: 1em;">principle of, <a href="#Page_40">40</a>;</span><br />
+<span style="margin-left: 1em;">self-starting, <a href="#Page_42">42</a>.</span><br />
+<br />
+Interlocking of signals, <a href="#Page_204">204</a>, <a href="#Page_222">222</a>.<br />
+<br />
+Internal-combustion engine, <a href="#Page_87">87</a>.<br />
+<br />
+Iris of eye, <a href="#Page_249">249</a>;<br />
+<span style="margin-left: 1em;">stop, <a href="#Page_249">249</a>.</span><br />
+<br />
+<br />
+Kelvin, Lord, <a href="#Page_158">158</a>.<br />
+<br />
+Keyless winding mechanism, <a href="#Page_425">425</a>, <i><a href="#Page_426">426</a></i>, <a href="#Page_428">428</a>.<br />
+<br />
+Kite, <a href="#Page_345">345</a>.<br />
+<br />
+<br />
+Lamp, arc, <a href="#Page_182">182</a>;<br />
+<span style="margin-left: 1em;">how it works, <a href="#Page_392">392</a>;</span><br />
+<span style="margin-left: 1em;">incandescent, <a href="#Page_179">179</a>;</span><br />
+<span style="margin-left: 1em;">manufacture of incandescent lamps, <a href="#Page_180">180</a>.</span><br />
+<br />
+Lap of slide-valve, <i><a href="#Page_57">57</a></i>, <a href="#Page_59">59</a>.<br />
+<br />
+Larynx, <a href="#Page_306">306</a>.<br />
+<br />
+Laxey wheel, <i><a href="#Page_380">380</a></i>, <a href="#Page_381">381</a>.<br />
+<br />
+Leads, <a href="#Page_208">208</a>.<br />
+<br />
+Lenses, <a href="#Page_231">231</a>;<br />
+<span style="margin-left: 1em;">correction of for colour, <a href="#Page_240">240</a>, <i><a href="#Page_241">241</a></i>;</span><br />
+<span style="margin-left: 1em;">focus of, <a href="#Page_236">236</a>;</span><br />
+<span style="margin-left: 1em;">rectilinear, <i><a href="#Page_245">245</a></i>;</span><br />
+<span style="margin-left: 1em;">spherical aberration in, <a href="#Page_243">243</a>.</span><br />
+<br />
+Levers, signal, colours of, <a href="#Page_208">208</a>.<br />
+<br />
+Limit of error in cylinder, <a href="#Page_52">52</a>.<br />
+<br />
+Light, electric, <a href="#Page_179">179</a>;<br />
+<span style="margin-left: 1em;">nature of, <a href="#Page_230">230</a>;</span><br />
+<span style="margin-left: 1em;">propagation of, <a href="#Page_231">231</a>.</span><br />
+<br />
+Li Hung Chang, <a href="#Page_157">157</a>.<br />
+<br />
+Lindsay, James Bowman, <a href="#Page_145">145</a>.<br />
+<br />
+Lines of force, <a href="#Page_116">116</a>, <a href="#Page_162">162</a>.<br />
+<br />
+"Linking up," <a href="#Page_65">65</a>.<br />
+<br />
+Locks, <a href="#Page_430">430</a>;<br />
+<span style="margin-left: 1em;">Barron, <a href="#Page_433">433</a>;</span><br />
+<span style="margin-left: 1em;">Bramah, <a href="#Page_437">437</a>;</span><br />
+<span style="margin-left: 1em;">Chubb, <a href="#Page_433">433</a>, <a href="#Page_434">434</a>;</span><br />
+<span style="margin-left: 1em;">Hobbs, <a href="#Page_437">437</a>;</span><br />
+<span style="margin-left: 1em;">simplest, <i><a href="#Page_431">431</a></i>;</span><br />
+<span style="margin-left: 1em;">tumbler, <i><a href="#Page_432">432</a></i>;</span><br />
+<span style="margin-left: 1em;">Yale, <i><a href="#Page_436">436</a></i>.</span><br />
+<br />
+Locking gear for signals, <a href="#Page_205">205</a>.<br />
+<br />
+Locomotive, electric, <a href="#Page_178">178</a>;<br />
+<span style="margin-left: 1em;">advantages of, <a href="#Page_179">179</a>.</span><br />
+<br />
+Lungs, <a href="#Page_359">359</a>.<br />
+<br />
+<br />
+Magic-lantern, <a href="#Page_263">263</a>, <i><a href="#Page_264">264</a></i>.<br />
+<br />
+Magnet, <a href="#Page_115">115</a>;<br />
+<span style="margin-left: 1em;">permanent, <a href="#Page_115">115</a>, <a href="#Page_116">116</a>;</span><br />
+<span style="margin-left: 1em;">temporary, <a href="#Page_115">115</a>.</span><br />
+<br />
+Magnetism, <a href="#Page_115">115</a>.<br />
+<br />
+Magnetic needle, influence of current on, <a href="#Page_129">129</a>.<br />
+<br />
+Mainspring, invention of, <a href="#Page_412">412</a>.<br />
+<br />
+Malleus, <a href="#Page_272">272</a>.<br />
+<br />
+Marconi, <a href="#Page_140">140</a>, <a href="#Page_146">146</a>.<br />
+<br />
+Marine chronometers, <a href="#Page_415">415</a>;<br />
+<span style="margin-left: 1em;">delicacy of, <a href="#Page_425">425</a>.</span><br />
+<br />
+Marine speed governor, <a href="#Page_71">71</a>.<br />
+<br />
+Marine turbine, advantages of, <a href="#Page_84">84</a>.<br />
+<br />
+Maudslay, Henry, <a href="#Page_363">363</a>.<br />
+<br />
+Maxim, Sir Hiram, <a href="#Page_348">348</a>.<br />
+<br />
+Micrometer free wheel, <a href="#Page_441">441</a>.<br />
+<br />
+Micro-photography, <a href="#Page_265">265</a>.<br />
+<br />
+Microscope, <a href="#Page_254">254</a>;<br />
+<span style="margin-left: 1em;">compound, <a href="#Page_261">261</a>, <i><a href="#Page_263">263</a></i>;</span><br />
+<span style="margin-left: 1em;">in telescope, <a href="#Page_257">257</a>;</span><br />
+<span style="margin-left: 1em;">simple, <i><a href="#Page_254">254</a></i>.</span><br />
+<br />
+Mineral oil, <a href="#Page_392">392</a>.<br />
+<br />
+Mirror, parabolic, <a href="#Page_261">261</a>, <i><a href="#Page_262">262</a></i>;<br />
+<span style="margin-left: 1em;">plane, <i><a href="#Page_267">267</a></i>.</span><br />
+<br />
+Morse, <a href="#Page_132">132</a>, <a href="#Page_145">145</a>;<br />
+<span style="margin-left: 1em;">code, <a href="#Page_128">128</a>;</span><br />
+<span style="margin-left: 1em;">inker, <a href="#Page_142">142</a>;</span><br />
+<span style="margin-left: 1em;">sounder, <a href="#Page_132">132</a>.</span><br />
+<br />
+Motor car, the, <a href="#Page_92">92</a>;<br />
+<span style="margin-left: 1em;">electric, <a href="#Page_177">177</a>.</span><br />
+<br />
+Mouth, <a href="#Page_307">307</a>.<br />
+<br />
+Mowing-machines, <a href="#Page_450">450</a>.<br />
+<br />
+Musical sounds, <a href="#Page_277">277</a>.<br />
+<br />
+<br />
+Nerve, auditory, <a href="#Page_272">272</a>;<br />
+<span style="margin-left: 1em;">optic, <a href="#Page_246">246</a>.</span><br />
+<br />
+Nodes on a string, <a href="#Page_285">285</a>;<br />
+<span style="margin-left: 1em;">column of air, <a href="#Page_291">291</a>.</span><br />
+<br />
+Note, fundamental, <a href="#Page_285">285</a>;<br />
+<span style="margin-left: 1em;">quality of, <a href="#Page_285">285</a>.</span><br />
+<br />
+Niagara Falls, power station at, <a href="#Page_174">174</a>.<br />
+<br />
+<br />
+Organ, the, <a href="#Page_294">294</a>, <i><a href="#Page_300">300</a></i>;<br />
+<span style="margin-left: 1em;">bellows, <a href="#Page_303">303</a>;</span><br />
+<span style="margin-left: 1em;">console, <a href="#Page_305">305</a>;</span><br />
+<span style="margin-left: 1em;">echo, solo, swell, great, and choir, <a href="#Page_301">301</a>;</span><br />
+<span style="margin-left: 1em;">electric and pneumatic, <a href="#Page_305">305</a>;</span><br />
+<span style="margin-left: 1em;">largest in the world, <a href="#Page_306">306</a>;</span><br />
+<span style="margin-left: 1em;">pedals, <a href="#Page_298">298</a>;</span><br />
+<span style="margin-left: 1em;">pipes, <a href="#Page_295">295</a>;</span><br />
+<span style="margin-left: 1em;">pipes, arrangement of, <a href="#Page_295">295</a>;</span><br />
+<span style="margin-left: 1em;">sound-board, <i><a href="#Page_296">296</a></i>;</span><br />
+<span style="margin-left: 1em;">wind-chest, <a href="#Page_297">297</a>.</span><br />
+<br />
+Otto cycle, <a href="#Page_91">91</a>.<br />
+<br />
+Overtones, <a href="#Page_285">285</a>.<br />
+<br />
+<br />
+Pallets of organ, <a href="#Page_297">297</a>.<br />
+<br />
+Parallel arrangement of electric lamps, <a href="#Page_184">184</a>.<br />
+<br />
+Paris, siege of, <a href="#Page_265">265</a>.<br />
+<br />
+Pedals of organ, <a href="#Page_298">298</a>.<br />
+<br />
+Pelton wheel, <i><a href="#Page_377">377</a></i>.<br />
+<br />
+Pendulum, <a href="#Page_412">412</a>;<br />
+<span style="margin-left: 1em;">compensating, <a href="#Page_418">418</a>, <i><a href="#Page_419">419</a></i>.</span><br />
+<br />
+Perilymph, <a href="#Page_272">272</a>.<br />
+<br />
+Perry, Professor, <a href="#Page_16">16</a>.<br />
+<br />
+Petrol, <a href="#Page_98">98</a>.<br />
+<br />
+Phonograph, <a href="#Page_310">310</a>;<br />
+<span style="margin-left: 1em;">governor, <i><a href="#Page_311">311</a></i>;</span><br />
+<span style="margin-left: 1em;">recorder, <a href="#Page_312">312</a>, <i><a href="#Page_313">313</a></i>;</span><br />
+<span style="margin-left: 1em;">records, making of, <a href="#Page_319">319</a>;</span><br />
+<span style="margin-left: 1em;">reproducer, <a href="#Page_315">315</a>;</span><br />
+<span style="margin-left: 1em;">tracings on record of, <i><a href="#Page_317">317</a></i>.</span><br />
+<br />
+Pianoforte, <a href="#Page_277">277</a>;<br />
+<span style="margin-left: 1em;">sounding-board, <a href="#Page_280">280</a>;</span><br />
+<span style="margin-left: 1em;">striking mechanism, <a href="#Page_281">281</a>;</span><br />
+<span style="margin-left: 1em;">strings, <a href="#Page_281">281</a>.</span><br />
+<br />
+Piccolo, <a href="#Page_308">308</a>.<br />
+<br />
+Pipes, closed, <a href="#Page_289">289</a>;<br />
+<span style="margin-left: 1em;">flue, <a href="#Page_301">301</a>;</span><br />
+<span style="margin-left: 1em;">open, <a href="#Page_292">292</a>;</span><br />
+<span style="margin-left: 1em;">organ, <a href="#Page_295">295</a>;</span><br />
+<span style="margin-left: 1em;">reed, <a href="#Page_301">301</a>, <i><a href="#Page_302">302</a></i>;</span><br />
+<span style="margin-left: 1em;">tuning, <a href="#Page_302">302</a>.</span><br />
+<br />
+Piston valve, <a href="#Page_67">67</a>.<br />
+<br />
+Pneumatic tyres, <a href="#Page_341">341</a>.<br />
+<br />
+Poldhu, signalling station at, <a href="#Page_138">138</a>.<br />
+<br />
+Points, railway, <a href="#Page_208">208</a>, <i><a href="#Page_210">210</a></i>;<br />
+<span style="margin-left: 1em;">and signals in combination, <a href="#Page_211">211</a>.</span><br />
+<br />
+Poles of a magnet, <a href="#Page_115">115</a>.<br />
+<br />
+Popoff, Professor A., <a href="#Page_138">138</a>, <a href="#Page_145">145</a>.<br />
+<br />
+Power, transmission of, <a href="#Page_175">175</a>.<br />
+<br />
+Preece, Sir William, <a href="#Page_145">145</a>.<br />
+<br />
+Primary winding of induction coil, <a href="#Page_122">122</a>.<br />
+<br />
+Pump, air, <a href="#Page_340">340</a>;<br />
+<span style="margin-left: 1em;">bucket, <a href="#Page_352">352</a>, <i><a href="#Page_353">353</a></i>;</span><br />
+<span style="margin-left: 1em;">force, <a href="#Page_354">354</a>;</span><br />
+<span style="margin-left: 1em;">most marvellous, <a href="#Page_355">355</a>;</span><br />
+<span style="margin-left: 1em;">Westinghouse air, <a href="#Page_199">199</a>.</span><br />
+<br />
+<br />
+Railway brakes, <a href="#Page_187">187</a>;<br />
+<span style="margin-left: 1em;">signalling, <a href="#Page_200">200</a>.</span><br />
+<br />
+Rays, converging and diverging, <i><a href="#Page_256">256</a></i>;<br />
+<span style="margin-left: 1em;">heat, concentrated by lens, <i><a href="#Page_232">232</a></i>;</span><br />
+<span style="margin-left: 1em;">light, <a href="#Page_232">232</a>, <a href="#Page_235">235</a>, <a href="#Page_236">236</a>, <a href="#Page_237">237</a>.</span><br />
+<br />
+Records, master, <a href="#Page_319">319</a>, <a href="#Page_320">320</a>.<br />
+<br />
+Reciprocation, <a href="#Page_51">51</a>.<br />
+<br />
+Reed, human, <a href="#Page_306">306</a>;<br />
+<span style="margin-left: 1em;">pipes, <a href="#Page_301">301</a>, <i><a href="#Page_302">302</a></i>.</span><br />
+<br />
+Reflecting telescope, <a href="#Page_260">260</a>.<br />
+<br />
+Relays, telegraphic, <a href="#Page_133">133</a>, <a href="#Page_141">141</a>.<br />
+<br />
+Retina, <a href="#Page_247">247</a>.<br />
+<br />
+Retorts, <a href="#Page_395">395</a>.<br />
+<br />
+Reversing gear, <a href="#Page_62">62</a>;<br />
+<span style="margin-left: 1em;">Allan, <a href="#Page_65">65</a>;</span><br />
+<span style="margin-left: 1em;">Gooch, <a href="#Page_65">65</a>;</span><br />
+<span style="margin-left: 1em;">radial, <a href="#Page_66">66</a>.</span><br />
+<br />
+Rocking bar mechanism for watches, <a href="#Page_425">425</a>.<br />
+<br />
+Rods of Corti, <a href="#Page_274">274</a>.<br />
+<br />
+Ruhmkorff coil, <a href="#Page_121">121</a>, <i><a href="#Page_122">122</a></i>.<br />
+<br />
+<br />
+Safety-valve, <a href="#Page_32">32</a>, <i><a href="#Page_33">33</a></i>, <a href="#Page_391">391</a>.<br />
+<br />
+Sand-glasses, <a href="#Page_411">411</a>.<br />
+<br />
+Scissors, action of, <i><a href="#Page_450">450</a></i>.<br />
+<br />
+Secondary winding of induction coil, <a href="#Page_122">122</a>.<br />
+<br />
+Series arrangement of electric lamps, <a href="#Page_183">183</a>.<br />
+<br />
+Series winding of dynamo, <i><a href="#Page_173">173</a></i>.<br />
+<br />
+Shunt wound dynamo, <i><a href="#Page_173">173</a></i>.<br />
+<br />
+Sight, long and short, <a href="#Page_250">250</a>.<br />
+<br />
+Signalling, automatic, <a href="#Page_228">228</a>;<br />
+<span style="margin-left: 1em;">electric, <a href="#Page_225">225</a>;</span><br />
+<span style="margin-left: 1em;">pneumatic, <a href="#Page_225">225</a>;</span><br />
+<span style="margin-left: 1em;">power, <a href="#Page_225">225</a>.</span><br />
+<br />
+Signal levers, <i><a href="#Page_206">206</a></i>.<br />
+<br />
+Signals, interlocking of, <a href="#Page_204">204</a>;<br />
+<span style="margin-left: 1em;">position of, <a href="#Page_202">202</a>;</span><br />
+<span style="margin-left: 1em;">railway, <a href="#Page_200">200</a>;</span><br />
+<span style="margin-left: 1em;">single line, <a href="#Page_215">215</a>.</span><br />
+<br />
+Silencer on motor cars, <a href="#Page_109">109</a>.<br />
+<br />
+Siphon, <i><a href="#Page_351">351</a></i>.<br />
+<br />
+Slide-valve, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>, <a href="#Page_51">51</a>;<br />
+<span style="margin-left: 1em;">setting of, <a href="#Page_53">53</a>.</span><br />
+<br />
+Sliders, <a href="#Page_297">297</a>.<br />
+<br />
+Sound, nature of, <a href="#Page_270">270</a>;<br />
+<span style="margin-left: 1em;">board of organ, <a href="#Page_296">296</a>;</span><br />
+<span style="margin-left: 1em;">board of piano, <a href="#Page_280">280</a>.</span><br />
+<br />
+Spagnoletti disc instrument, <a href="#Page_212">212</a>.<br />
+<br />
+Sparking-plug, <i><a href="#Page_102">102</a></i>.<br />
+<br />
+Spectacles, use of, <a href="#Page_249">249</a>.<br />
+<br />
+Spectrum, colours of, <a href="#Page_230">230</a>.<br />
+<br />
+Speed governors, <a href="#Page_67">67</a>, <i><a href="#Page_68">68</a></i>, <i><a href="#Page_69">69</a></i>;<br />
+<span style="margin-left: 1em;">Hartwell, <a href="#Page_70">70</a>;</span><br />
+<span style="margin-left: 1em;">marine, <a href="#Page_71">71</a>.</span><br />
+<br />
+Speed of motor cars, <a href="#Page_110">110</a>.<br />
+<br />
+Spot, blind, in eye, <a href="#Page_251">251</a>;<br />
+<span style="margin-left: 1em;">yellow, in eye, <a href="#Page_251">251</a>.</span><br />
+<br />
+Spring balance for watches, <a href="#Page_419">419</a>;<br />
+<span style="margin-left: 1em;">compensating, <a href="#Page_423">423</a>, <i><a href="#Page_424">424</a></i>.</span><br />
+<br />
+Stapes, <a href="#Page_272">272</a>.<br />
+<br />
+Steam, what it is, <a href="#Page_13">13</a>;<br />
+<span style="margin-left: 1em;">energy of, <a href="#Page_14">14</a>;</span><br />
+<span style="margin-left: 1em;">engines, <a href="#Page_44">44</a>;</span><br />
+<span style="margin-left: 1em;">engines, reciprocating, <i><a href="#Page_45">45</a></i>;</span><br />
+<span style="margin-left: 1em;">expansive working of, <a href="#Page_59">59</a>, <a href="#Page_81">81</a>;</span><br />
+<span style="margin-left: 1em;">gauge, <a href="#Page_36">36</a>;</span><br />
+<span style="margin-left: 1em;">gauge, principle of, <a href="#Page_37">37</a>;</span><br />
+<span style="margin-left: 1em;">turbine, <a href="#Page_74">74</a>;</span><br />
+<span style="margin-left: 1em;">turbine, De Laval, <a href="#Page_76">76</a>, <i><a href="#Page_77">77</a></i>;</span><br />
+<span style="margin-left: 1em;">turbine, Hero's, <a href="#Page_74">74</a>;</span><br />
+<span style="margin-left: 1em;">turbine, Parsons, <a href="#Page_79">79</a>, <i><a href="#Page_80">80</a></i>;</span><br />
+<span style="margin-left: 1em;">volume of, as compared with water, <a href="#Page_15">15</a>.</span><br />
+<br />
+Stephenson, George, <a href="#Page_63">63</a>, <a href="#Page_375">375</a>.<br />
+<br />
+Stop, in lens, <a href="#Page_244">244</a>;<br />
+<span style="margin-left: 1em;">iris, <a href="#Page_249">249</a>;</span><br />
+<span style="margin-left: 1em;">use of, <a href="#Page_244">244</a>.</span><br />
+<br />
+Sun-dial of Ahaz, <a href="#Page_410">410</a>.<br />
+<br />
+Syntonic transmission of wireless messages, <a href="#Page_143">143</a>.<br />
+<br />
+<br />
+Talking-machines, <a href="#Page_310">310</a>.<br />
+<br />
+Tapper in wireless telegraphy receiver, <a href="#Page_141">141</a>.<br />
+<br />
+Tappet arm, <a href="#Page_205">205</a>.<br />
+<br />
+Telegraph, electric, <a href="#Page_127">127</a>;<br />
+<span style="margin-left: 1em;">insulator, <i><a href="#Page_133">133</a></i>;</span><br />
+<span style="margin-left: 1em;">needle, <i><a href="#Page_128">128</a></i>;</span><br />
+<span style="margin-left: 1em;">recording, <a href="#Page_133">133</a>;</span><br />
+<span style="margin-left: 1em;">sounder, <a href="#Page_132">132</a>.</span><br />
+<br />
+Telegraphy, high-speed, <a href="#Page_135">135</a>;<br />
+<span style="margin-left: 1em;">wireless, <a href="#Page_137">137</a>.</span><br />
+<br />
+Telephone, <a href="#Page_147">147</a>;<br />
+<span style="margin-left: 1em;">Bell, <i><a href="#Page_148">148</a></i>;</span><br />
+<span style="margin-left: 1em;">circuit, double-line, <a href="#Page_155">155</a>;</span><br />
+<span style="margin-left: 1em;">circuit, general arrangement, <i><a href="#Page_152">152</a></i>, <a href="#Page_153">153</a>;</span><br />
+<span style="margin-left: 1em;">exchange, <i><a href="#Page_154">154</a></i>, <a href="#Page_155">155</a>.</span><br />
+<br />
+Telephony, submarine, <a href="#Page_157">157</a>.<br />
+<br />
+Telescope, <a href="#Page_257">257</a>;<br />
+<span style="margin-left: 1em;">Galilean, <i><a href="#Page_259">259</a></i>;</span><br />
+<span style="margin-left: 1em;">prismatic, <i><a href="#Page_260">260</a></i>;</span><br />
+<span style="margin-left: 1em;">reflecting, <a href="#Page_260">260</a>;</span><br />
+<span style="margin-left: 1em;">terrestrial, <i><a href="#Page_259">259</a></i>.</span><br />
+<br />
+Threshing-machine, <a href="#Page_447">447</a>, <i><a href="#Page_448">448</a></i>.<br />
+<br />
+Thurston, Professor, <a href="#Page_31">31</a>.<br />
+<br />
+Tides, <a href="#Page_452">452</a>;<br />
+<span style="margin-left: 1em;">high, <a href="#Page_453">453</a>;</span><br />
+<span style="margin-left: 1em;">neap and spring, <a href="#Page_455">455</a>.</span><br />
+<br />
+Timbre, <a href="#Page_285">285</a>.<br />
+<br />
+Tompion, Thomas, <a href="#Page_412">412</a>.<br />
+<br />
+Torricelli, <a href="#Page_325">325</a>.<br />
+<br />
+Trachea, <a href="#Page_306">306</a>.<br />
+<br />
+Train staff signalling, <a href="#Page_216">216</a>;<br />
+<span style="margin-left: 1em;">single, <a href="#Page_216">216</a>;</span><br />
+<span style="margin-left: 1em;">and ticket, <a href="#Page_217">217</a>;</span><br />
+<span style="margin-left: 1em;">electric, <a href="#Page_218">218</a>.</span><br />
+<br />
+Transformation of current, <a href="#Page_124">124</a>, <a href="#Page_176">176</a>.<br />
+<br />
+Transmission of power, <a href="#Page_174">174</a>, <i><a href="#Page_175">175</a></i>.<br />
+<br />
+Transmitter, Edison telephone, <a href="#Page_150">150</a>;<br />
+<span style="margin-left: 1em;">granular carbon, <a href="#Page_150">150</a>, <i><a href="#Page_151">151</a></i>.</span><br />
+<br />
+Triple-valve, <a href="#Page_196">196</a>.<br />
+<br />
+Trolley arm, <a href="#Page_176">176</a>.<br />
+<br />
+Turbines, steam, <a href="#Page_74">74</a>.<br />
+<br />
+<i>Turbinia</i>, the, <a href="#Page_79">79</a>.<br />
+<br />
+Tympanum, <a href="#Page_137">137</a>, <a href="#Page_271">271</a>, <a href="#Page_272">272</a>.<br />
+<br />
+<br />
+Universal joint, <a href="#Page_93">93</a>.<br />
+<br />
+<br />
+Vacuum brake, <a href="#Page_189">189</a>, <i><a href="#Page_190">190</a></i>, <i><a href="#Page_191">191</a></i>.<br />
+<br />
+Vacuum chamber of aneroid barometer, <i><a href="#Page_330">330</a></i>.<br />
+<br />
+Valve, piston, <a href="#Page_67">67</a>;<br />
+<span style="margin-left: 1em;">safety, <a href="#Page_32">32</a>;</span><br />
+<span style="margin-left: 1em;">of internal-combustion engine, <a href="#Page_89">89</a>.</span><br />
+<br />
+Valves of the heart, <a href="#Page_357">357</a>.<br />
+<br />
+Veins, <a href="#Page_358">358</a>;<br />
+<span style="margin-left: 1em;">capillary, <a href="#Page_358">358</a>;</span><br />
+<span style="margin-left: 1em;">pulmonary, <a href="#Page_361">361</a>.</span><br />
+<br />
+Ventral segments, <a href="#Page_291">291</a>.<br />
+<br />
+Ventricles, <a href="#Page_357">357</a>.<br />
+<br />
+Vibration of columns of air, <a href="#Page_288">288</a>, <a href="#Page_289">289</a>;<br />
+<span style="margin-left: 1em;">of rods, <a href="#Page_287">287</a>;</span><br />
+<span style="margin-left: 1em;">of strings, <a href="#Page_278">278</a>;</span><br />
+<span style="margin-left: 1em;">of strings, conditions regulating, <a href="#Page_278">278</a>.</span><br />
+<br />
+<i>Viper</i>, the, <a href="#Page_86">86</a>.<br />
+<br />
+Virag, Pollak&mdash;high-speed telegraphy, <a href="#Page_136">136</a>.<br />
+<br />
+Vitreous humour, <a href="#Page_246">246</a>.<br />
+<br />
+Voltage, <a href="#Page_121">121</a>, <a href="#Page_161">161</a>.<br />
+<br />
+Vowel sounds, <a href="#Page_308">308</a>.<br />
+<br />
+<br />
+Wasborough, Matthew, <a href="#Page_51">51</a>.<br />
+<br />
+Watches, first, <a href="#Page_412">412</a>.<br />
+<br />
+Water cock, <i><a href="#Page_365">365</a></i>;<br />
+<span style="margin-left: 1em;">engines, <a href="#Page_375">375</a>;</span><br />
+<span style="margin-left: 1em;">gauge, <a href="#Page_35">35</a>, <i><a href="#Page_36">36</a></i>;</span><br />
+<span style="margin-left: 1em;">jacket, <a href="#Page_19">19</a>, <a href="#Page_95">95</a>;</span><br />
+<span style="margin-left: 1em;">meter, <i><a href="#Page_368">368</a></i>;</span><br />
+<span style="margin-left: 1em;">supply, <a href="#Page_371">371</a>;</span><br />
+<span style="margin-left: 1em;">turbines, <a href="#Page_174">174</a>, <a href="#Page_376">376</a>;</span><br />
+<span style="margin-left: 1em;">wheels, <a href="#Page_375">375</a>.</span><br />
+<br />
+Watt, James, <a href="#Page_51">51</a>, <a href="#Page_69">69</a>, <a href="#Page_375">375</a>.<br />
+<br />
+Welsbach incandescent mantle, <a href="#Page_407">407</a>.<br />
+<br />
+Westinghouse air-brake, <a href="#Page_194">194</a>, <i><a href="#Page_195">195</a></i>, <i><a href="#Page_197">197</a></i>;<br />
+<span style="margin-left: 1em;">George, <a href="#Page_194">194</a>.</span><br />
+<br />
+Wheatstone needle instrument, <a href="#Page_128">128</a>, <a href="#Page_131">131</a>;<br />
+<span style="margin-left: 1em;">automatic transmitter, <a href="#Page_135">135</a>.</span><br />
+<br />
+Wind, why it blows, <a href="#Page_323">323</a>;<br />
+<span style="margin-left: 1em;">action of on kites, <a href="#Page_345">345</a>;</span><br />
+<span style="margin-left: 1em;">on sails, <a href="#Page_346">346</a>.</span><br />
+<br />
+Windmills, <a href="#Page_375">375</a>.<br />
+<br />
+Window, oval, in ear, <a href="#Page_272">272</a>;<br />
+<span style="margin-left: 1em;">round, in ear, <a href="#Page_272">272</a>.</span><br />
+<br />
+Wireless telegraphy, <a href="#Page_137">137</a>;<br />
+<span style="margin-left: 1em;">advance of, <a href="#Page_145">145</a>;</span><br />
+<span style="margin-left: 1em;">receiver, <a href="#Page_140">140</a>, <a href="#Page_141">141</a>;</span><br />
+<span style="margin-left: 1em;">syntonic, <a href="#Page_143">143</a>;</span><br />
+<span style="margin-left: 1em;">transmitter, <a href="#Page_138">138</a>, <i><a href="#Page_139">139</a></i>.</span><br />
+<br />
+<br />
+Yale lock, <i><a href="#Page_436">436</a></i>, <i><a href="#Page_437">437</a></i>.<br />
+<br />
+Yellow spot, in eye, <a href="#Page_251">251</a>.<br />
+<br />
+<br />
+Zech, Jacob, <a href="#Page_414">414</a>.<br />
+<br />
+Zeiss field-glasses, <a href="#Page_260">260</a>.<br />
+</p>
+
+
+<p class="section">THE END.</p>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of How it Works, by Archibald Williams
+
+*** END OF THIS PROJECT GUTENBERG EBOOK HOW IT WORKS ***
+
+***** This file should be named 28553-h.htm or 28553-h.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/2/8/5/5/28553/
+
+Produced by Steven Gibbs, Greg Bergquist and the Online
+Distributed Proofreading Team at https://www.pgdp.net
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions means that no
+one owns a United States copyright in these works, so the Foundation
+(and you!) can copy and distribute it in the United States without
+permission and without paying copyright royalties.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need are critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
+
+
+</pre>
+
+</body>
+</html>
diff --git a/28553-h/images/image1.jpg b/28553-h/images/image1.jpg
new file mode 100644
index 0000000..616d73f
--- /dev/null
+++ b/28553-h/images/image1.jpg
diff --git a/28553-h/images/image10.jpg b/28553-h/images/image10.jpg
new file mode 100644
index 0000000..01cbd37
--- /dev/null
+++ b/28553-h/images/image10.jpg
diff --git a/28553-h/images/image100.jpg b/28553-h/images/image100.jpg
new file mode 100644
index 0000000..a931ecd
--- /dev/null
+++ b/28553-h/images/image100.jpg
diff --git a/28553-h/images/image101.jpg b/28553-h/images/image101.jpg
new file mode 100644
index 0000000..c3d989a
--- /dev/null
+++ b/28553-h/images/image101.jpg
diff --git a/28553-h/images/image102.jpg b/28553-h/images/image102.jpg
new file mode 100644
index 0000000..8a10379
--- /dev/null
+++ b/28553-h/images/image102.jpg
diff --git a/28553-h/images/image103.jpg b/28553-h/images/image103.jpg
new file mode 100644
index 0000000..15197df
--- /dev/null
+++ b/28553-h/images/image103.jpg
diff --git a/28553-h/images/image104.jpg b/28553-h/images/image104.jpg
new file mode 100644
index 0000000..be34d3a
--- /dev/null
+++ b/28553-h/images/image104.jpg
diff --git a/28553-h/images/image105.jpg b/28553-h/images/image105.jpg
new file mode 100644
index 0000000..b75a758
--- /dev/null
+++ b/28553-h/images/image105.jpg
diff --git a/28553-h/images/image106.jpg b/28553-h/images/image106.jpg
new file mode 100644
index 0000000..4d1f485
--- /dev/null
+++ b/28553-h/images/image106.jpg
diff --git a/28553-h/images/image107.jpg b/28553-h/images/image107.jpg
new file mode 100644
index 0000000..9dbcf03
--- /dev/null
+++ b/28553-h/images/image107.jpg
diff --git a/28553-h/images/image108.jpg b/28553-h/images/image108.jpg
new file mode 100644
index 0000000..105e8a0
--- /dev/null
+++ b/28553-h/images/image108.jpg
diff --git a/28553-h/images/image109.jpg b/28553-h/images/image109.jpg
new file mode 100644
index 0000000..700f1e0
--- /dev/null
+++ b/28553-h/images/image109.jpg
diff --git a/28553-h/images/image11.jpg b/28553-h/images/image11.jpg
new file mode 100644
index 0000000..7b5d093
--- /dev/null
+++ b/28553-h/images/image11.jpg
diff --git a/28553-h/images/image110.jpg b/28553-h/images/image110.jpg
new file mode 100644
index 0000000..bf9acbf
--- /dev/null
+++ b/28553-h/images/image110.jpg
diff --git a/28553-h/images/image111.jpg b/28553-h/images/image111.jpg
new file mode 100644
index 0000000..d21fb1d
--- /dev/null
+++ b/28553-h/images/image111.jpg
diff --git a/28553-h/images/image112.jpg b/28553-h/images/image112.jpg
new file mode 100644
index 0000000..87d9786
--- /dev/null
+++ b/28553-h/images/image112.jpg
diff --git a/28553-h/images/image113.jpg b/28553-h/images/image113.jpg
new file mode 100644
index 0000000..894b2a0
--- /dev/null
+++ b/28553-h/images/image113.jpg
diff --git a/28553-h/images/image114.jpg b/28553-h/images/image114.jpg
new file mode 100644
index 0000000..1b565b0
--- /dev/null
+++ b/28553-h/images/image114.jpg
diff --git a/28553-h/images/image115.jpg b/28553-h/images/image115.jpg
new file mode 100644
index 0000000..7649fc4
--- /dev/null
+++ b/28553-h/images/image115.jpg
diff --git a/28553-h/images/image116.jpg b/28553-h/images/image116.jpg
new file mode 100644
index 0000000..0a7c11c
--- /dev/null
+++ b/28553-h/images/image116.jpg
diff --git a/28553-h/images/image117.jpg b/28553-h/images/image117.jpg
new file mode 100644
index 0000000..4b044b2
--- /dev/null
+++ b/28553-h/images/image117.jpg
diff --git a/28553-h/images/image118.jpg b/28553-h/images/image118.jpg
new file mode 100644
index 0000000..cce9eb6
--- /dev/null
+++ b/28553-h/images/image118.jpg
diff --git a/28553-h/images/image119.jpg b/28553-h/images/image119.jpg
new file mode 100644
index 0000000..9cdb4ff
--- /dev/null
+++ b/28553-h/images/image119.jpg
diff --git a/28553-h/images/image12.jpg b/28553-h/images/image12.jpg
new file mode 100644
index 0000000..2c5179b
--- /dev/null
+++ b/28553-h/images/image12.jpg
diff --git a/28553-h/images/image120.jpg b/28553-h/images/image120.jpg
new file mode 100644
index 0000000..9998687
--- /dev/null
+++ b/28553-h/images/image120.jpg
diff --git a/28553-h/images/image121.jpg b/28553-h/images/image121.jpg
new file mode 100644
index 0000000..3c87c1f
--- /dev/null
+++ b/28553-h/images/image121.jpg
diff --git a/28553-h/images/image122.jpg b/28553-h/images/image122.jpg
new file mode 100644
index 0000000..30d1bbf
--- /dev/null
+++ b/28553-h/images/image122.jpg
diff --git a/28553-h/images/image123.jpg b/28553-h/images/image123.jpg
new file mode 100644
index 0000000..da22255
--- /dev/null
+++ b/28553-h/images/image123.jpg
diff --git a/28553-h/images/image124.jpg b/28553-h/images/image124.jpg
new file mode 100644
index 0000000..99d3d8f
--- /dev/null
+++ b/28553-h/images/image124.jpg
diff --git a/28553-h/images/image125.jpg b/28553-h/images/image125.jpg
new file mode 100644
index 0000000..1e885ff
--- /dev/null
+++ b/28553-h/images/image125.jpg
diff --git a/28553-h/images/image126.jpg b/28553-h/images/image126.jpg
new file mode 100644
index 0000000..0c9481e
--- /dev/null
+++ b/28553-h/images/image126.jpg
diff --git a/28553-h/images/image127.jpg b/28553-h/images/image127.jpg
new file mode 100644
index 0000000..2ff0709
--- /dev/null
+++ b/28553-h/images/image127.jpg
diff --git a/28553-h/images/image128.jpg b/28553-h/images/image128.jpg
new file mode 100644
index 0000000..064984b
--- /dev/null
+++ b/28553-h/images/image128.jpg
diff --git a/28553-h/images/image129.jpg b/28553-h/images/image129.jpg
new file mode 100644
index 0000000..29081bb
--- /dev/null
+++ b/28553-h/images/image129.jpg
diff --git a/28553-h/images/image13.jpg b/28553-h/images/image13.jpg
new file mode 100644
index 0000000..f59f4be
--- /dev/null
+++ b/28553-h/images/image13.jpg
diff --git a/28553-h/images/image130.jpg b/28553-h/images/image130.jpg
new file mode 100644
index 0000000..9f8d06e
--- /dev/null
+++ b/28553-h/images/image130.jpg
diff --git a/28553-h/images/image131.jpg b/28553-h/images/image131.jpg
new file mode 100644
index 0000000..e6d5287
--- /dev/null
+++ b/28553-h/images/image131.jpg
diff --git a/28553-h/images/image132.jpg b/28553-h/images/image132.jpg
new file mode 100644
index 0000000..d73dccb
--- /dev/null
+++ b/28553-h/images/image132.jpg
diff --git a/28553-h/images/image133.jpg b/28553-h/images/image133.jpg
new file mode 100644
index 0000000..851e2f1
--- /dev/null
+++ b/28553-h/images/image133.jpg
diff --git a/28553-h/images/image134.jpg b/28553-h/images/image134.jpg
new file mode 100644
index 0000000..6c118c9
--- /dev/null
+++ b/28553-h/images/image134.jpg
diff --git a/28553-h/images/image135.jpg b/28553-h/images/image135.jpg
new file mode 100644
index 0000000..e97817a
--- /dev/null
+++ b/28553-h/images/image135.jpg
diff --git a/28553-h/images/image136.jpg b/28553-h/images/image136.jpg
new file mode 100644
index 0000000..80f20a7
--- /dev/null
+++ b/28553-h/images/image136.jpg
diff --git a/28553-h/images/image137.jpg b/28553-h/images/image137.jpg
new file mode 100644
index 0000000..24d148a
--- /dev/null
+++ b/28553-h/images/image137.jpg
diff --git a/28553-h/images/image138.jpg b/28553-h/images/image138.jpg
new file mode 100644
index 0000000..ac2e4c4
--- /dev/null
+++ b/28553-h/images/image138.jpg
diff --git a/28553-h/images/image139.jpg b/28553-h/images/image139.jpg
new file mode 100644
index 0000000..0a88e8d
--- /dev/null
+++ b/28553-h/images/image139.jpg
diff --git a/28553-h/images/image14.jpg b/28553-h/images/image14.jpg
new file mode 100644
index 0000000..d940c41
--- /dev/null
+++ b/28553-h/images/image14.jpg
diff --git a/28553-h/images/image140.jpg b/28553-h/images/image140.jpg
new file mode 100644
index 0000000..d8f7f38
--- /dev/null
+++ b/28553-h/images/image140.jpg
diff --git a/28553-h/images/image141.jpg b/28553-h/images/image141.jpg
new file mode 100644
index 0000000..8e755cc
--- /dev/null
+++ b/28553-h/images/image141.jpg
diff --git a/28553-h/images/image142.jpg b/28553-h/images/image142.jpg
new file mode 100644
index 0000000..0d22a31
--- /dev/null
+++ b/28553-h/images/image142.jpg
diff --git a/28553-h/images/image143.jpg b/28553-h/images/image143.jpg
new file mode 100644
index 0000000..cbe1042
--- /dev/null
+++ b/28553-h/images/image143.jpg
diff --git a/28553-h/images/image144.jpg b/28553-h/images/image144.jpg
new file mode 100644
index 0000000..943a0e6
--- /dev/null
+++ b/28553-h/images/image144.jpg
diff --git a/28553-h/images/image145.jpg b/28553-h/images/image145.jpg
new file mode 100644
index 0000000..eb7c783
--- /dev/null
+++ b/28553-h/images/image145.jpg
diff --git a/28553-h/images/image146.jpg b/28553-h/images/image146.jpg
new file mode 100644
index 0000000..8ce9370
--- /dev/null
+++ b/28553-h/images/image146.jpg
diff --git a/28553-h/images/image147.jpg b/28553-h/images/image147.jpg
new file mode 100644
index 0000000..9d079a8
--- /dev/null
+++ b/28553-h/images/image147.jpg
diff --git a/28553-h/images/image148.jpg b/28553-h/images/image148.jpg
new file mode 100644
index 0000000..9d9dfb0
--- /dev/null
+++ b/28553-h/images/image148.jpg
diff --git a/28553-h/images/image149.jpg b/28553-h/images/image149.jpg
new file mode 100644
index 0000000..dd22f98
--- /dev/null
+++ b/28553-h/images/image149.jpg
diff --git a/28553-h/images/image15.jpg b/28553-h/images/image15.jpg
new file mode 100644
index 0000000..dadbd60
--- /dev/null
+++ b/28553-h/images/image15.jpg
diff --git a/28553-h/images/image150.jpg b/28553-h/images/image150.jpg
new file mode 100644
index 0000000..b68a405
--- /dev/null
+++ b/28553-h/images/image150.jpg
diff --git a/28553-h/images/image151.jpg b/28553-h/images/image151.jpg
new file mode 100644
index 0000000..0fa4dd0
--- /dev/null
+++ b/28553-h/images/image151.jpg
diff --git a/28553-h/images/image152.jpg b/28553-h/images/image152.jpg
new file mode 100644
index 0000000..1444c23
--- /dev/null
+++ b/28553-h/images/image152.jpg
diff --git a/28553-h/images/image153.jpg b/28553-h/images/image153.jpg
new file mode 100644
index 0000000..e813263
--- /dev/null
+++ b/28553-h/images/image153.jpg
diff --git a/28553-h/images/image154.jpg b/28553-h/images/image154.jpg
new file mode 100644
index 0000000..7f6f06a
--- /dev/null
+++ b/28553-h/images/image154.jpg
diff --git a/28553-h/images/image155.jpg b/28553-h/images/image155.jpg
new file mode 100644
index 0000000..9934f64
--- /dev/null
+++ b/28553-h/images/image155.jpg
diff --git a/28553-h/images/image156.jpg b/28553-h/images/image156.jpg
new file mode 100644
index 0000000..57f9f76
--- /dev/null
+++ b/28553-h/images/image156.jpg
diff --git a/28553-h/images/image157.jpg b/28553-h/images/image157.jpg
new file mode 100644
index 0000000..fc07843
--- /dev/null
+++ b/28553-h/images/image157.jpg
diff --git a/28553-h/images/image158.jpg b/28553-h/images/image158.jpg
new file mode 100644
index 0000000..ad6619d
--- /dev/null
+++ b/28553-h/images/image158.jpg
diff --git a/28553-h/images/image159.jpg b/28553-h/images/image159.jpg
new file mode 100644
index 0000000..2c34cf1
--- /dev/null
+++ b/28553-h/images/image159.jpg
diff --git a/28553-h/images/image16.jpg b/28553-h/images/image16.jpg
new file mode 100644
index 0000000..46406a8
--- /dev/null
+++ b/28553-h/images/image16.jpg
diff --git a/28553-h/images/image160.jpg b/28553-h/images/image160.jpg
new file mode 100644
index 0000000..70d8a44
--- /dev/null
+++ b/28553-h/images/image160.jpg
diff --git a/28553-h/images/image161.jpg b/28553-h/images/image161.jpg
new file mode 100644
index 0000000..4b40335
--- /dev/null
+++ b/28553-h/images/image161.jpg
diff --git a/28553-h/images/image162.jpg b/28553-h/images/image162.jpg
new file mode 100644
index 0000000..76d642d
--- /dev/null
+++ b/28553-h/images/image162.jpg
diff --git a/28553-h/images/image163.jpg b/28553-h/images/image163.jpg
new file mode 100644
index 0000000..8142abe
--- /dev/null
+++ b/28553-h/images/image163.jpg
diff --git a/28553-h/images/image164.jpg b/28553-h/images/image164.jpg
new file mode 100644
index 0000000..871b6e7
--- /dev/null
+++ b/28553-h/images/image164.jpg
diff --git a/28553-h/images/image165.jpg b/28553-h/images/image165.jpg
new file mode 100644
index 0000000..bd30a79
--- /dev/null
+++ b/28553-h/images/image165.jpg
diff --git a/28553-h/images/image166.jpg b/28553-h/images/image166.jpg
new file mode 100644
index 0000000..504b83f
--- /dev/null
+++ b/28553-h/images/image166.jpg
diff --git a/28553-h/images/image167.jpg b/28553-h/images/image167.jpg
new file mode 100644
index 0000000..690540e
--- /dev/null
+++ b/28553-h/images/image167.jpg
diff --git a/28553-h/images/image168.jpg b/28553-h/images/image168.jpg
new file mode 100644
index 0000000..eca2446
--- /dev/null
+++ b/28553-h/images/image168.jpg
diff --git a/28553-h/images/image169.jpg b/28553-h/images/image169.jpg
new file mode 100644
index 0000000..3d4c268
--- /dev/null
+++ b/28553-h/images/image169.jpg
diff --git a/28553-h/images/image17.jpg b/28553-h/images/image17.jpg
new file mode 100644
index 0000000..1c13281
--- /dev/null
+++ b/28553-h/images/image17.jpg
diff --git a/28553-h/images/image170.jpg b/28553-h/images/image170.jpg
new file mode 100644
index 0000000..1a014da
--- /dev/null
+++ b/28553-h/images/image170.jpg
diff --git a/28553-h/images/image171.jpg b/28553-h/images/image171.jpg
new file mode 100644
index 0000000..6aa1235
--- /dev/null
+++ b/28553-h/images/image171.jpg
diff --git a/28553-h/images/image172.jpg b/28553-h/images/image172.jpg
new file mode 100644
index 0000000..c3c73b9
--- /dev/null
+++ b/28553-h/images/image172.jpg
diff --git a/28553-h/images/image173.jpg b/28553-h/images/image173.jpg
new file mode 100644
index 0000000..d2758f1
--- /dev/null
+++ b/28553-h/images/image173.jpg
diff --git a/28553-h/images/image174.jpg b/28553-h/images/image174.jpg
new file mode 100644
index 0000000..a1ef666
--- /dev/null
+++ b/28553-h/images/image174.jpg
diff --git a/28553-h/images/image175.jpg b/28553-h/images/image175.jpg
new file mode 100644
index 0000000..b8f6e2e
--- /dev/null
+++ b/28553-h/images/image175.jpg
diff --git a/28553-h/images/image176.jpg b/28553-h/images/image176.jpg
new file mode 100644
index 0000000..9ea0cae
--- /dev/null
+++ b/28553-h/images/image176.jpg
diff --git a/28553-h/images/image177.jpg b/28553-h/images/image177.jpg
new file mode 100644
index 0000000..f1d8a6c
--- /dev/null
+++ b/28553-h/images/image177.jpg
diff --git a/28553-h/images/image178.jpg b/28553-h/images/image178.jpg
new file mode 100644
index 0000000..b359ce0
--- /dev/null
+++ b/28553-h/images/image178.jpg
diff --git a/28553-h/images/image179.jpg b/28553-h/images/image179.jpg
new file mode 100644
index 0000000..9d9c2b4
--- /dev/null
+++ b/28553-h/images/image179.jpg
diff --git a/28553-h/images/image18.jpg b/28553-h/images/image18.jpg
new file mode 100644
index 0000000..b16f9f5
--- /dev/null
+++ b/28553-h/images/image18.jpg
diff --git a/28553-h/images/image180.jpg b/28553-h/images/image180.jpg
new file mode 100644
index 0000000..4955e1c
--- /dev/null
+++ b/28553-h/images/image180.jpg
diff --git a/28553-h/images/image181.jpg b/28553-h/images/image181.jpg
new file mode 100644
index 0000000..618363c
--- /dev/null
+++ b/28553-h/images/image181.jpg
diff --git a/28553-h/images/image182.jpg b/28553-h/images/image182.jpg
new file mode 100644
index 0000000..fa62158
--- /dev/null
+++ b/28553-h/images/image182.jpg
diff --git a/28553-h/images/image183.jpg b/28553-h/images/image183.jpg
new file mode 100644
index 0000000..fac6fc7
--- /dev/null
+++ b/28553-h/images/image183.jpg
diff --git a/28553-h/images/image184.jpg b/28553-h/images/image184.jpg
new file mode 100644
index 0000000..d209469
--- /dev/null
+++ b/28553-h/images/image184.jpg
diff --git a/28553-h/images/image185.jpg b/28553-h/images/image185.jpg
new file mode 100644
index 0000000..fa28126
--- /dev/null
+++ b/28553-h/images/image185.jpg
diff --git a/28553-h/images/image186.jpg b/28553-h/images/image186.jpg
new file mode 100644
index 0000000..a4cf4f0
--- /dev/null
+++ b/28553-h/images/image186.jpg
diff --git a/28553-h/images/image187.jpg b/28553-h/images/image187.jpg
new file mode 100644
index 0000000..1a33fca
--- /dev/null
+++ b/28553-h/images/image187.jpg
diff --git a/28553-h/images/image188.jpg b/28553-h/images/image188.jpg
new file mode 100644
index 0000000..bfa6374
--- /dev/null
+++ b/28553-h/images/image188.jpg
diff --git a/28553-h/images/image189.jpg b/28553-h/images/image189.jpg
new file mode 100644
index 0000000..e4f2bf6
--- /dev/null
+++ b/28553-h/images/image189.jpg
diff --git a/28553-h/images/image19.jpg b/28553-h/images/image19.jpg
new file mode 100644
index 0000000..25f6fae
--- /dev/null
+++ b/28553-h/images/image19.jpg
diff --git a/28553-h/images/image190.jpg b/28553-h/images/image190.jpg
new file mode 100644
index 0000000..ce78580
--- /dev/null
+++ b/28553-h/images/image190.jpg
diff --git a/28553-h/images/image191.jpg b/28553-h/images/image191.jpg
new file mode 100644
index 0000000..6cd891d
--- /dev/null
+++ b/28553-h/images/image191.jpg
diff --git a/28553-h/images/image192.jpg b/28553-h/images/image192.jpg
new file mode 100644
index 0000000..d56d494
--- /dev/null
+++ b/28553-h/images/image192.jpg
diff --git a/28553-h/images/image193.jpg b/28553-h/images/image193.jpg
new file mode 100644
index 0000000..6b8398a
--- /dev/null
+++ b/28553-h/images/image193.jpg
diff --git a/28553-h/images/image194.jpg b/28553-h/images/image194.jpg
new file mode 100644
index 0000000..5fcc423
--- /dev/null
+++ b/28553-h/images/image194.jpg
diff --git a/28553-h/images/image195.jpg b/28553-h/images/image195.jpg
new file mode 100644
index 0000000..fd5e5b2
--- /dev/null
+++ b/28553-h/images/image195.jpg
diff --git a/28553-h/images/image196.jpg b/28553-h/images/image196.jpg
new file mode 100644
index 0000000..5dc3a40
--- /dev/null
+++ b/28553-h/images/image196.jpg
diff --git a/28553-h/images/image197.jpg b/28553-h/images/image197.jpg
new file mode 100644
index 0000000..ef98e6e
--- /dev/null
+++ b/28553-h/images/image197.jpg
diff --git a/28553-h/images/image198.jpg b/28553-h/images/image198.jpg
new file mode 100644
index 0000000..6dee218
--- /dev/null
+++ b/28553-h/images/image198.jpg
diff --git a/28553-h/images/image199.jpg b/28553-h/images/image199.jpg
new file mode 100644
index 0000000..d660ad8
--- /dev/null
+++ b/28553-h/images/image199.jpg
diff --git a/28553-h/images/image2.jpg b/28553-h/images/image2.jpg
new file mode 100644
index 0000000..84973fa
--- /dev/null
+++ b/28553-h/images/image2.jpg
diff --git a/28553-h/images/image20.jpg b/28553-h/images/image20.jpg
new file mode 100644
index 0000000..6a58fa6
--- /dev/null
+++ b/28553-h/images/image20.jpg
diff --git a/28553-h/images/image200.jpg b/28553-h/images/image200.jpg
new file mode 100644
index 0000000..9cccc77
--- /dev/null
+++ b/28553-h/images/image200.jpg
diff --git a/28553-h/images/image201.jpg b/28553-h/images/image201.jpg
new file mode 100644
index 0000000..09cd6bf
--- /dev/null
+++ b/28553-h/images/image201.jpg
diff --git a/28553-h/images/image202.jpg b/28553-h/images/image202.jpg
new file mode 100644
index 0000000..0945b77
--- /dev/null
+++ b/28553-h/images/image202.jpg
diff --git a/28553-h/images/image203.jpg b/28553-h/images/image203.jpg
new file mode 100644
index 0000000..6615d28
--- /dev/null
+++ b/28553-h/images/image203.jpg
diff --git a/28553-h/images/image204.jpg b/28553-h/images/image204.jpg
new file mode 100644
index 0000000..31768d0
--- /dev/null
+++ b/28553-h/images/image204.jpg
diff --git a/28553-h/images/image205.jpg b/28553-h/images/image205.jpg
new file mode 100644
index 0000000..b193340
--- /dev/null
+++ b/28553-h/images/image205.jpg
diff --git a/28553-h/images/image206.jpg b/28553-h/images/image206.jpg
new file mode 100644
index 0000000..2112086
--- /dev/null
+++ b/28553-h/images/image206.jpg
diff --git a/28553-h/images/image207.jpg b/28553-h/images/image207.jpg
new file mode 100644
index 0000000..d9e4f33
--- /dev/null
+++ b/28553-h/images/image207.jpg
diff --git a/28553-h/images/image208.jpg b/28553-h/images/image208.jpg
new file mode 100644
index 0000000..8d2c611
--- /dev/null
+++ b/28553-h/images/image208.jpg
diff --git a/28553-h/images/image209.jpg b/28553-h/images/image209.jpg
new file mode 100644
index 0000000..ada2d67
--- /dev/null
+++ b/28553-h/images/image209.jpg
diff --git a/28553-h/images/image21.jpg b/28553-h/images/image21.jpg
new file mode 100644
index 0000000..d0a123c
--- /dev/null
+++ b/28553-h/images/image21.jpg
diff --git a/28553-h/images/image210.jpg b/28553-h/images/image210.jpg
new file mode 100644
index 0000000..430e8d2
--- /dev/null
+++ b/28553-h/images/image210.jpg
diff --git a/28553-h/images/image211.jpg b/28553-h/images/image211.jpg
new file mode 100644
index 0000000..bd761e7
--- /dev/null
+++ b/28553-h/images/image211.jpg
diff --git a/28553-h/images/image212.jpg b/28553-h/images/image212.jpg
new file mode 100644
index 0000000..6839b32
--- /dev/null
+++ b/28553-h/images/image212.jpg
diff --git a/28553-h/images/image213.jpg b/28553-h/images/image213.jpg
new file mode 100644
index 0000000..e70b3ba
--- /dev/null
+++ b/28553-h/images/image213.jpg
diff --git a/28553-h/images/image214.jpg b/28553-h/images/image214.jpg
new file mode 100644
index 0000000..8ed240f
--- /dev/null
+++ b/28553-h/images/image214.jpg
diff --git a/28553-h/images/image215.jpg b/28553-h/images/image215.jpg
new file mode 100644
index 0000000..d539c90
--- /dev/null
+++ b/28553-h/images/image215.jpg
diff --git a/28553-h/images/image216.jpg b/28553-h/images/image216.jpg
new file mode 100644
index 0000000..d2505dc
--- /dev/null
+++ b/28553-h/images/image216.jpg
diff --git a/28553-h/images/image217.jpg b/28553-h/images/image217.jpg
new file mode 100644
index 0000000..4739ee1
--- /dev/null
+++ b/28553-h/images/image217.jpg
diff --git a/28553-h/images/image218.jpg b/28553-h/images/image218.jpg
new file mode 100644
index 0000000..eb60a82
--- /dev/null
+++ b/28553-h/images/image218.jpg
diff --git a/28553-h/images/image219.jpg b/28553-h/images/image219.jpg
new file mode 100644
index 0000000..24450ed
--- /dev/null
+++ b/28553-h/images/image219.jpg
diff --git a/28553-h/images/image22.jpg b/28553-h/images/image22.jpg
new file mode 100644
index 0000000..6348bbf
--- /dev/null
+++ b/28553-h/images/image22.jpg
diff --git a/28553-h/images/image220.jpg b/28553-h/images/image220.jpg
new file mode 100644
index 0000000..77c38d0
--- /dev/null
+++ b/28553-h/images/image220.jpg
diff --git a/28553-h/images/image221.jpg b/28553-h/images/image221.jpg
new file mode 100644
index 0000000..aff20b6
--- /dev/null
+++ b/28553-h/images/image221.jpg
diff --git a/28553-h/images/image222.jpg b/28553-h/images/image222.jpg
new file mode 100644
index 0000000..c8456dd
--- /dev/null
+++ b/28553-h/images/image222.jpg
diff --git a/28553-h/images/image223.jpg b/28553-h/images/image223.jpg
new file mode 100644
index 0000000..5adb542
--- /dev/null
+++ b/28553-h/images/image223.jpg
diff --git a/28553-h/images/image224.jpg b/28553-h/images/image224.jpg
new file mode 100644
index 0000000..731d494
--- /dev/null
+++ b/28553-h/images/image224.jpg
diff --git a/28553-h/images/image225.jpg b/28553-h/images/image225.jpg
new file mode 100644
index 0000000..9e0ee60
--- /dev/null
+++ b/28553-h/images/image225.jpg
diff --git a/28553-h/images/image226.jpg b/28553-h/images/image226.jpg
new file mode 100644
index 0000000..4b87981
--- /dev/null
+++ b/28553-h/images/image226.jpg
diff --git a/28553-h/images/image227.jpg b/28553-h/images/image227.jpg
new file mode 100644
index 0000000..791febc
--- /dev/null
+++ b/28553-h/images/image227.jpg
diff --git a/28553-h/images/image228.jpg b/28553-h/images/image228.jpg
new file mode 100644
index 0000000..6ca6a9d
--- /dev/null
+++ b/28553-h/images/image228.jpg
diff --git a/28553-h/images/image229.jpg b/28553-h/images/image229.jpg
new file mode 100644
index 0000000..7cec2ad
--- /dev/null
+++ b/28553-h/images/image229.jpg
diff --git a/28553-h/images/image23.jpg b/28553-h/images/image23.jpg
new file mode 100644
index 0000000..2456d5d
--- /dev/null
+++ b/28553-h/images/image23.jpg
diff --git a/28553-h/images/image230.jpg b/28553-h/images/image230.jpg
new file mode 100644
index 0000000..2cc93c5
--- /dev/null
+++ b/28553-h/images/image230.jpg
diff --git a/28553-h/images/image231.jpg b/28553-h/images/image231.jpg
new file mode 100644
index 0000000..821aed7
--- /dev/null
+++ b/28553-h/images/image231.jpg
diff --git a/28553-h/images/image232.jpg b/28553-h/images/image232.jpg
new file mode 100644
index 0000000..f2a50a2
--- /dev/null
+++ b/28553-h/images/image232.jpg
diff --git a/28553-h/images/image233.jpg b/28553-h/images/image233.jpg
new file mode 100644
index 0000000..9f168bd
--- /dev/null
+++ b/28553-h/images/image233.jpg
diff --git a/28553-h/images/image234.jpg b/28553-h/images/image234.jpg
new file mode 100644
index 0000000..0a313f2
--- /dev/null
+++ b/28553-h/images/image234.jpg
diff --git a/28553-h/images/image235.jpg b/28553-h/images/image235.jpg
new file mode 100644
index 0000000..9215fb1
--- /dev/null
+++ b/28553-h/images/image235.jpg
diff --git a/28553-h/images/image236.jpg b/28553-h/images/image236.jpg
new file mode 100644
index 0000000..6deede2
--- /dev/null
+++ b/28553-h/images/image236.jpg
diff --git a/28553-h/images/image237.jpg b/28553-h/images/image237.jpg
new file mode 100644
index 0000000..40ed71b
--- /dev/null
+++ b/28553-h/images/image237.jpg
diff --git a/28553-h/images/image238.jpg b/28553-h/images/image238.jpg
new file mode 100644
index 0000000..0ccf4fc
--- /dev/null
+++ b/28553-h/images/image238.jpg
diff --git a/28553-h/images/image239.jpg b/28553-h/images/image239.jpg
new file mode 100644
index 0000000..0f3482a
--- /dev/null
+++ b/28553-h/images/image239.jpg
diff --git a/28553-h/images/image24.jpg b/28553-h/images/image24.jpg
new file mode 100644
index 0000000..bdaa0fc
--- /dev/null
+++ b/28553-h/images/image24.jpg
diff --git a/28553-h/images/image240.jpg b/28553-h/images/image240.jpg
new file mode 100644
index 0000000..9893b22
--- /dev/null
+++ b/28553-h/images/image240.jpg
diff --git a/28553-h/images/image25.jpg b/28553-h/images/image25.jpg
new file mode 100644
index 0000000..4c31538
--- /dev/null
+++ b/28553-h/images/image25.jpg
diff --git a/28553-h/images/image26.jpg b/28553-h/images/image26.jpg
new file mode 100644
index 0000000..2ba2948
--- /dev/null
+++ b/28553-h/images/image26.jpg
diff --git a/28553-h/images/image27.jpg b/28553-h/images/image27.jpg
new file mode 100644
index 0000000..eb0c7a5
--- /dev/null
+++ b/28553-h/images/image27.jpg
diff --git a/28553-h/images/image28.jpg b/28553-h/images/image28.jpg
new file mode 100644
index 0000000..1132a97
--- /dev/null
+++ b/28553-h/images/image28.jpg
diff --git a/28553-h/images/image29.jpg b/28553-h/images/image29.jpg
new file mode 100644
index 0000000..79b6b65
--- /dev/null
+++ b/28553-h/images/image29.jpg
diff --git a/28553-h/images/image3.jpg b/28553-h/images/image3.jpg
new file mode 100644
index 0000000..117684b
--- /dev/null
+++ b/28553-h/images/image3.jpg
diff --git a/28553-h/images/image30.jpg b/28553-h/images/image30.jpg
new file mode 100644
index 0000000..4f4d064
--- /dev/null
+++ b/28553-h/images/image30.jpg
diff --git a/28553-h/images/image31.jpg b/28553-h/images/image31.jpg
new file mode 100644
index 0000000..1a182f3
--- /dev/null
+++ b/28553-h/images/image31.jpg
diff --git a/28553-h/images/image32.jpg b/28553-h/images/image32.jpg
new file mode 100644
index 0000000..907f0ba
--- /dev/null
+++ b/28553-h/images/image32.jpg
diff --git a/28553-h/images/image33.jpg b/28553-h/images/image33.jpg
new file mode 100644
index 0000000..af6f8a7
--- /dev/null
+++ b/28553-h/images/image33.jpg
diff --git a/28553-h/images/image34.jpg b/28553-h/images/image34.jpg
new file mode 100644
index 0000000..d73210d
--- /dev/null
+++ b/28553-h/images/image34.jpg
diff --git a/28553-h/images/image35.jpg b/28553-h/images/image35.jpg
new file mode 100644
index 0000000..b56d77a
--- /dev/null
+++ b/28553-h/images/image35.jpg
diff --git a/28553-h/images/image36.jpg b/28553-h/images/image36.jpg
new file mode 100644
index 0000000..d246498
--- /dev/null
+++ b/28553-h/images/image36.jpg
diff --git a/28553-h/images/image37.jpg b/28553-h/images/image37.jpg
new file mode 100644
index 0000000..dd1ef31
--- /dev/null
+++ b/28553-h/images/image37.jpg
diff --git a/28553-h/images/image38.jpg b/28553-h/images/image38.jpg
new file mode 100644
index 0000000..d1a06c5
--- /dev/null
+++ b/28553-h/images/image38.jpg
diff --git a/28553-h/images/image39.jpg b/28553-h/images/image39.jpg
new file mode 100644
index 0000000..2da6acc
--- /dev/null
+++ b/28553-h/images/image39.jpg
diff --git a/28553-h/images/image4.jpg b/28553-h/images/image4.jpg
new file mode 100644
index 0000000..bfb6945
--- /dev/null
+++ b/28553-h/images/image4.jpg
diff --git a/28553-h/images/image40.jpg b/28553-h/images/image40.jpg
new file mode 100644
index 0000000..9b033a5
--- /dev/null
+++ b/28553-h/images/image40.jpg
diff --git a/28553-h/images/image41.jpg b/28553-h/images/image41.jpg
new file mode 100644
index 0000000..4fc8072
--- /dev/null
+++ b/28553-h/images/image41.jpg
diff --git a/28553-h/images/image42.jpg b/28553-h/images/image42.jpg
new file mode 100644
index 0000000..e7bbfb8
--- /dev/null
+++ b/28553-h/images/image42.jpg
diff --git a/28553-h/images/image43.jpg b/28553-h/images/image43.jpg
new file mode 100644
index 0000000..6b00d4b
--- /dev/null
+++ b/28553-h/images/image43.jpg
diff --git a/28553-h/images/image44.jpg b/28553-h/images/image44.jpg
new file mode 100644
index 0000000..8b1f72a
--- /dev/null
+++ b/28553-h/images/image44.jpg
diff --git a/28553-h/images/image45.jpg b/28553-h/images/image45.jpg
new file mode 100644
index 0000000..547fc38
--- /dev/null
+++ b/28553-h/images/image45.jpg
diff --git a/28553-h/images/image46.jpg b/28553-h/images/image46.jpg
new file mode 100644
index 0000000..5e718b3
--- /dev/null
+++ b/28553-h/images/image46.jpg
diff --git a/28553-h/images/image47.jpg b/28553-h/images/image47.jpg
new file mode 100644
index 0000000..baaa07a
--- /dev/null
+++ b/28553-h/images/image47.jpg
diff --git a/28553-h/images/image48.jpg b/28553-h/images/image48.jpg
new file mode 100644
index 0000000..06710e7
--- /dev/null
+++ b/28553-h/images/image48.jpg
diff --git a/28553-h/images/image49.jpg b/28553-h/images/image49.jpg
new file mode 100644
index 0000000..d555fdd
--- /dev/null
+++ b/28553-h/images/image49.jpg
diff --git a/28553-h/images/image5.jpg b/28553-h/images/image5.jpg
new file mode 100644
index 0000000..028f201
--- /dev/null
+++ b/28553-h/images/image5.jpg
diff --git a/28553-h/images/image50.jpg b/28553-h/images/image50.jpg
new file mode 100644
index 0000000..db3ea74
--- /dev/null
+++ b/28553-h/images/image50.jpg
diff --git a/28553-h/images/image51.jpg b/28553-h/images/image51.jpg
new file mode 100644
index 0000000..bf470c3
--- /dev/null
+++ b/28553-h/images/image51.jpg
diff --git a/28553-h/images/image52.jpg b/28553-h/images/image52.jpg
new file mode 100644
index 0000000..ac28be8
--- /dev/null
+++ b/28553-h/images/image52.jpg
diff --git a/28553-h/images/image53.jpg b/28553-h/images/image53.jpg
new file mode 100644
index 0000000..3d1fc32
--- /dev/null
+++ b/28553-h/images/image53.jpg
diff --git a/28553-h/images/image54.jpg b/28553-h/images/image54.jpg
new file mode 100644
index 0000000..c80db2f
--- /dev/null
+++ b/28553-h/images/image54.jpg
diff --git a/28553-h/images/image55.jpg b/28553-h/images/image55.jpg
new file mode 100644
index 0000000..4389355
--- /dev/null
+++ b/28553-h/images/image55.jpg
diff --git a/28553-h/images/image56.jpg b/28553-h/images/image56.jpg
new file mode 100644
index 0000000..8af5d42
--- /dev/null
+++ b/28553-h/images/image56.jpg
diff --git a/28553-h/images/image57.jpg b/28553-h/images/image57.jpg
new file mode 100644
index 0000000..64d22ab
--- /dev/null
+++ b/28553-h/images/image57.jpg
diff --git a/28553-h/images/image58.jpg b/28553-h/images/image58.jpg
new file mode 100644
index 0000000..9ef533a
--- /dev/null
+++ b/28553-h/images/image58.jpg
diff --git a/28553-h/images/image59.jpg b/28553-h/images/image59.jpg
new file mode 100644
index 0000000..af1bbde
--- /dev/null
+++ b/28553-h/images/image59.jpg
diff --git a/28553-h/images/image6.jpg b/28553-h/images/image6.jpg
new file mode 100644
index 0000000..506a04c
--- /dev/null
+++ b/28553-h/images/image6.jpg
diff --git a/28553-h/images/image60.jpg b/28553-h/images/image60.jpg
new file mode 100644
index 0000000..7a09c20
--- /dev/null
+++ b/28553-h/images/image60.jpg
diff --git a/28553-h/images/image61.jpg b/28553-h/images/image61.jpg
new file mode 100644
index 0000000..cdef313
--- /dev/null
+++ b/28553-h/images/image61.jpg
diff --git a/28553-h/images/image62.jpg b/28553-h/images/image62.jpg
new file mode 100644
index 0000000..b30ddc3
--- /dev/null
+++ b/28553-h/images/image62.jpg
diff --git a/28553-h/images/image63.jpg b/28553-h/images/image63.jpg
new file mode 100644
index 0000000..ccb0813
--- /dev/null
+++ b/28553-h/images/image63.jpg
diff --git a/28553-h/images/image64.jpg b/28553-h/images/image64.jpg
new file mode 100644
index 0000000..321ecd5
--- /dev/null
+++ b/28553-h/images/image64.jpg
diff --git a/28553-h/images/image65.jpg b/28553-h/images/image65.jpg
new file mode 100644
index 0000000..387e24b
--- /dev/null
+++ b/28553-h/images/image65.jpg
diff --git a/28553-h/images/image66.jpg b/28553-h/images/image66.jpg
new file mode 100644
index 0000000..ce84c04
--- /dev/null
+++ b/28553-h/images/image66.jpg
diff --git a/28553-h/images/image67.jpg b/28553-h/images/image67.jpg
new file mode 100644
index 0000000..f44f0d1
--- /dev/null
+++ b/28553-h/images/image67.jpg
diff --git a/28553-h/images/image68.jpg b/28553-h/images/image68.jpg
new file mode 100644
index 0000000..c4be869
--- /dev/null
+++ b/28553-h/images/image68.jpg
diff --git a/28553-h/images/image69.jpg b/28553-h/images/image69.jpg
new file mode 100644
index 0000000..cf87111
--- /dev/null
+++ b/28553-h/images/image69.jpg
diff --git a/28553-h/images/image7.jpg b/28553-h/images/image7.jpg
new file mode 100644
index 0000000..778cd8a
--- /dev/null
+++ b/28553-h/images/image7.jpg
diff --git a/28553-h/images/image70.jpg b/28553-h/images/image70.jpg
new file mode 100644
index 0000000..d59921c
--- /dev/null
+++ b/28553-h/images/image70.jpg
diff --git a/28553-h/images/image71.jpg b/28553-h/images/image71.jpg
new file mode 100644
index 0000000..0c63f7d
--- /dev/null
+++ b/28553-h/images/image71.jpg
diff --git a/28553-h/images/image72.jpg b/28553-h/images/image72.jpg
new file mode 100644
index 0000000..feb75cb
--- /dev/null
+++ b/28553-h/images/image72.jpg
diff --git a/28553-h/images/image73.jpg b/28553-h/images/image73.jpg
new file mode 100644
index 0000000..4921d8c
--- /dev/null
+++ b/28553-h/images/image73.jpg
diff --git a/28553-h/images/image74.jpg b/28553-h/images/image74.jpg
new file mode 100644
index 0000000..708df4e
--- /dev/null
+++ b/28553-h/images/image74.jpg
diff --git a/28553-h/images/image75.jpg b/28553-h/images/image75.jpg
new file mode 100644
index 0000000..e71fbbe
--- /dev/null
+++ b/28553-h/images/image75.jpg
diff --git a/28553-h/images/image76.jpg b/28553-h/images/image76.jpg
new file mode 100644
index 0000000..c69233e
--- /dev/null
+++ b/28553-h/images/image76.jpg
diff --git a/28553-h/images/image77.jpg b/28553-h/images/image77.jpg
new file mode 100644
index 0000000..365a108
--- /dev/null
+++ b/28553-h/images/image77.jpg
diff --git a/28553-h/images/image78.jpg b/28553-h/images/image78.jpg
new file mode 100644
index 0000000..20ed863
--- /dev/null
+++ b/28553-h/images/image78.jpg
diff --git a/28553-h/images/image79.jpg b/28553-h/images/image79.jpg
new file mode 100644
index 0000000..c3f9584
--- /dev/null
+++ b/28553-h/images/image79.jpg
diff --git a/28553-h/images/image8.jpg b/28553-h/images/image8.jpg
new file mode 100644
index 0000000..91c1448
--- /dev/null
+++ b/28553-h/images/image8.jpg
diff --git a/28553-h/images/image80.jpg b/28553-h/images/image80.jpg
new file mode 100644
index 0000000..9964b4b
--- /dev/null
+++ b/28553-h/images/image80.jpg
diff --git a/28553-h/images/image81.jpg b/28553-h/images/image81.jpg
new file mode 100644
index 0000000..f2a6c2c
--- /dev/null
+++ b/28553-h/images/image81.jpg
diff --git a/28553-h/images/image82.jpg b/28553-h/images/image82.jpg
new file mode 100644
index 0000000..bd4c513
--- /dev/null
+++ b/28553-h/images/image82.jpg
diff --git a/28553-h/images/image83.jpg b/28553-h/images/image83.jpg
new file mode 100644
index 0000000..fc26e6d
--- /dev/null
+++ b/28553-h/images/image83.jpg
diff --git a/28553-h/images/image84.jpg b/28553-h/images/image84.jpg
new file mode 100644
index 0000000..96faf4c
--- /dev/null
+++ b/28553-h/images/image84.jpg
diff --git a/28553-h/images/image85.jpg b/28553-h/images/image85.jpg
new file mode 100644
index 0000000..44b3202
--- /dev/null
+++ b/28553-h/images/image85.jpg
diff --git a/28553-h/images/image86.jpg b/28553-h/images/image86.jpg
new file mode 100644
index 0000000..13a4f90
--- /dev/null
+++ b/28553-h/images/image86.jpg
diff --git a/28553-h/images/image87.jpg b/28553-h/images/image87.jpg
new file mode 100644
index 0000000..6909422
--- /dev/null
+++ b/28553-h/images/image87.jpg
diff --git a/28553-h/images/image88.jpg b/28553-h/images/image88.jpg
new file mode 100644
index 0000000..986480e
--- /dev/null
+++ b/28553-h/images/image88.jpg
diff --git a/28553-h/images/image89.jpg b/28553-h/images/image89.jpg
new file mode 100644
index 0000000..ba7b849
--- /dev/null
+++ b/28553-h/images/image89.jpg
diff --git a/28553-h/images/image9.jpg b/28553-h/images/image9.jpg
new file mode 100644
index 0000000..3a1a160
--- /dev/null
+++ b/28553-h/images/image9.jpg
diff --git a/28553-h/images/image90.jpg b/28553-h/images/image90.jpg
new file mode 100644
index 0000000..be97d50
--- /dev/null
+++ b/28553-h/images/image90.jpg
diff --git a/28553-h/images/image91.jpg b/28553-h/images/image91.jpg
new file mode 100644
index 0000000..b20ddd8
--- /dev/null
+++ b/28553-h/images/image91.jpg
diff --git a/28553-h/images/image92.jpg b/28553-h/images/image92.jpg
new file mode 100644
index 0000000..79f8c09
--- /dev/null
+++ b/28553-h/images/image92.jpg
diff --git a/28553-h/images/image93.jpg b/28553-h/images/image93.jpg
new file mode 100644
index 0000000..4a0862a
--- /dev/null
+++ b/28553-h/images/image93.jpg
diff --git a/28553-h/images/image94.jpg b/28553-h/images/image94.jpg
new file mode 100644
index 0000000..d76e45a
--- /dev/null
+++ b/28553-h/images/image94.jpg
diff --git a/28553-h/images/image95.jpg b/28553-h/images/image95.jpg
new file mode 100644
index 0000000..2ecfb0e
--- /dev/null
+++ b/28553-h/images/image95.jpg
diff --git a/28553-h/images/image96.jpg b/28553-h/images/image96.jpg
new file mode 100644
index 0000000..0bf5fe9
--- /dev/null
+++ b/28553-h/images/image96.jpg
diff --git a/28553-h/images/image97.jpg b/28553-h/images/image97.jpg
new file mode 100644
index 0000000..7b7ba84
--- /dev/null
+++ b/28553-h/images/image97.jpg
diff --git a/28553-h/images/image98.jpg b/28553-h/images/image98.jpg
new file mode 100644
index 0000000..e452cc1
--- /dev/null
+++ b/28553-h/images/image98.jpg
diff --git a/28553-h/images/image99.jpg b/28553-h/images/image99.jpg
new file mode 100644
index 0000000..73355d1
--- /dev/null
+++ b/28553-h/images/image99.jpg
diff --git a/28553.txt b/28553.txt
new file mode 100644
index 0000000..ae9ff0c
--- /dev/null
+++ b/28553.txt
@@ -0,0 +1,9549 @@
+The Project Gutenberg EBook of How it Works, by Archibald Williams
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: How it Works
+       Dealing in simple language with steam, electricity, light,
+       heat, sound, hydraulics, optics, etc., and with their
+       applications to
+
+Author: Archibald Williams
+
+Release Date: April 10, 2009 [EBook #28553]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK HOW IT WORKS ***
+
+
+
+
+Produced by Steven Gibbs, Greg Bergquist and the Online
+Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+Transcriber's Note
+
+The punctuation and spelling from the original text have been faithfully
+preserved. Only obvious typographical errors have been corrected.
+Subscripts are represented as X_1. Superscripts are represented by X^1.
+
+
+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.
+
+[Illustration: 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
+
+
+
+
+PREFACE.
+
+
+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 on the magnetic needle--Method
+of reversing the current--Sounding instruments--Telegraphic
+relays--Recording telegraphs--High-speed telegraphy      127
+
+
+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 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      322
+
+
+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 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      410
+
+
+
+
+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?
+
+If ice be heated above 32 deg. Fahrenheit, its molecules lose their
+cohesion, and move freely round one another--the ice is turned into
+water. Heat water above 212 deg. 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.
+
+[Illustration: FIG. 1.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 4.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 1/2 to 3/4 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.
+
+[Illustration: 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.
+
+[Illustration: 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_ (CO_2). 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 CO_2 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 deg. Fahrenheit or more, and the poor stokers have
+a very bad time of it.
+
+[Illustration: FIG. 10.--Sketch showing how the "forced draught" is
+produced in a stokehold and how it affects the furnaces.]
+
+[Illustration: 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 3-1/2 miles. In the case of a Lancashire
+boiler at equal pressure the distance would be 2-1/2 miles; of a
+locomotive boiler, at 125 lbs., 1-1/2 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.
+
+[Illustration: 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 P^1. The cocks T^1 T^2 are normally open, allowing the
+ingress of steam and water respectively to the tube. Cock T^3 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 P^2
+P^3 are removed.
+
+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.
+
+[Illustration: FIG. 12.--Section of a water-gauge.]
+
+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 Schaeffer-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.
+
+[Illustration: FIG. 13.--Showing the principle of the steam-gauge.]
+
+[Illustration: 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 Schaeffer-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.
+
+[Illustration: FIG. 15.--Diagram illustrating the principle of a
+steam-injector.]
+
+[Illustration: 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.
+
+
+Having 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 18.--Sectional plan of a horizontal engine.]
+
+
+DOUBLE-CYLINDER ENGINES.
+
+[Illustration: FIG. 19.]
+
+[Illustration: 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, C_1,
+of a double-cylinder engine is at a "dead point," the other, C_2, has
+reached a position at which the piston exerts the maximum of turning
+power. In Fig. 20 each crank is at 45 deg. 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.
+
+[Illustration: 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_, G^1, 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.
+
+[Illustration: 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.
+
+
+[Illustration: 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 C^2,
+the centre of the shaft, and C^1, 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.
+
+[Illustration: FIG. 24.]
+
+[Illustration: 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 deg. 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.
+
+[Illustration: 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 deg. in advance--that is, more than what the engineers call
+_square_. Fig. 26 shows such an arrangement. The angle between E and
+E^1 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.
+
+[Illustration: FIG. 27.--A slide-valve with "lap."]
+
+[Illustration: 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 x (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 E^2.
+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.
+
+[Illustration: 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 deg., 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.
+
+[Illustration: 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. E^1 and E^2 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 E^1 has pushed the lower end of the link fully
+back; E^2 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 E^1 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 E^2 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
+versa_. 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 34.]
+
+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 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.
+
+[Illustration: 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.
+
+
+More 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.
+
+[Illustration: FIG. 36.--The wheel and nozzles of a De Laval turbine.]
+
+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 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 8-1/2 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 44-1/2 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.
+
+[Illustration: 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, D^1, D^2,
+D^3, 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.
+
+[Illustration: FIG. 38.--Blades or vanes of a Parsons turbine.]
+
+[Illustration: 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 D^1 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
+D^3, 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, P^1, P^2, P^3.
+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 P^1 to the left with the same pressure as
+it pushes the blades of D^1 to the right. After completing the first
+stage it fills the passage C, which communicates with the second piston,
+P^2, and the pressure on that piston negatives the thrust on D^2.
+Similarly, the passage E causes the steam to press equally on P^3 and
+the vanes of D^3. 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.
+
+[Illustration: 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 1/2 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."
+
+In 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.
+
+[Illustration]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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, C^1, 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.
+
+[Illustration: 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 deg. 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 25-1/3 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 1-1/2 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?
+
+Of 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.
+
+[Illustration: FIG. 50.--Permanent magnet, and the "lines of force"
+emanating from it.]
+
+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.
+
+[Illustration: FIG. 51.--Electro-magnet: A, armature; B, battery.]
+
+Having 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 must be put a crank to alter the direction of 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 T^1, 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.
+
+[Illustration: 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.
+
+[Illustration: 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 T^1. 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 Roentgen 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 Roentgen 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.
+
+
+Take 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.
+
+[Illustration: FIG. 54.--Sketch of the side elevation of a Wheatstone
+needle instrument.]
+
+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 right, and comes to rest in a
+normal position for a moment, the letter A is signified;
+right-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.
+
+[Illustration: FIGS. 55, 56.--The coils of a needle instrument. The
+arrows show the direction taken by the current.]
+
+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 are at the right end of
+the coils, the S. poles 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.
+
+[Illustration: 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, A^1
+B^1. Crossing these at right angles (under A A^1 and over B B^1)
+are the flat brass springs, L R, L^1 R^1, having buttons at their
+lower ends, and fixed at their upper ends to baseboards. When at rest
+they all press upwards against the plates A and A^1 respectively. R
+and L^1 are connected with the line circuit, in which are the coils of
+dials 1 and 2, one at each station. L and R^1 are connected with the
+earth-plates E E^1. 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 L^1 A^1 and
+R to earth-plate E^1, 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 versa_
+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.
+
+[Illustration: FIG. 58.--Section of a telegraph wire insulator on its
+arm. The shaded circle is the line wire, the two blank circles indicate
+the wire which ties the line wire to the insulator.]
+
+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 produced. The
+ribbon-feeding apparatus is set in motion automatically by the current,
+and continues to pull the ribbon along until the message is completed.
+
+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.
+
+
+In 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 aerial
+wires hang. At their upper and lower ends respectively the earth and
+aerial 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 aerial 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]
+
+[Illustration: FIG. 59.--Sketch of the transmitter of a wireless
+telegraphy outfit.]
+
+[Illustration: 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.
+
+[Illustration: 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 aerial 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 aerial 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 R^1 R^2.
+
+
+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 aerial 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 versa_, selective wireless telegraphy becomes possible.
+
+
+ADVANCE OF WIRELESS TELEGRAPHY.
+
+The history of wireless telegraphy may be summed up as follows:--
+
+1842.--Professor Morse sent aerial 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 1/2 mile.
+
+1885.--Sir William Preece signalled from Lavernock Point, near Cardiff,
+to Steep Holm, an island in the Bristol Channel. Distance about 5-1/2
+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; 17-1/2 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.
+
+
+For 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.
+
+[Illustration: 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.
+
+[Illustration: 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 T^1 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.
+
+[Illustration: 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 P_2 of induction coils. The
+transmitters are in the line circuit, which includes the secondary
+windings S S_2 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 B_2, 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 T_2 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 65.--The headdress of an operator at a telephone
+exchange. The receiver is fastened over one ear, and the transmitter to
+the chest.]
+
+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 telephonic
+connection, though three exchanges were in the 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 S_2 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 S_3, 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.
+
+
+In 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.
+
+[Illustration: 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 C^1. Two small
+brushes, B B^1, 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.
+
+[Illustration: FIG. 67.]
+
+[Illustration: 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 C^1 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.
+
+[Illustration: 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 C^1, 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 B^1 on to segment C^1. 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 71.]
+
+[Illustration: 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.
+
+[Illustration: 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 deg.
+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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 77.--Sketch showing a "series" winding.]
+
+[Illustration: 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.
+
+[Illustration: 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 versa_. 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+
+In 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.
+
+[Illustration: FIG. 85.--Vacuum brake "off."]
+
+[Illustration: 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.
+
+[Illustration: FIG. 87.--Guard's valve for applying the Vacuum brake.]
+
+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 train pipe through the valve. It is thus 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_.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+
+Under 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.
+
+[Illustration: FIG. 91.--Distant and home signals.]
+
+
+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 reinforced by an "advance starting" signal 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.
+
+[Illustration: 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 versa_, 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 94.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 96.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 98.--Points open to main line.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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
+
+[Illustration: 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.
+
+
+Light 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.
+
+[Illustration: 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).
+
+[Illustration: FIG. 104.--Six forms of lenses.]
+
+
+THE CAMERA.
+
+[Illustration: FIG. 105.]
+
+[Illustration: 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.
+
+[Illustration: 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 A^1 and B^1. In reality a countless number
+of rays would be transmitted from every point of the object and
+collected to form the image.
+
+[Illustration: 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.
+
+[Illustration: 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,
+F^1, 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.
+
+[Illustration: 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 B^1, denotes the principal
+focal length of the lens, and A C, or A C^1, denotes twice the focal
+length. We will take the positions in order:--
+
+_Position I._ Object further away than 2_f_. Inverted image _smaller_
+than object, at distance somewhat exceeding _f_.
+
+_Position II._ Object at distance = 2_f_. Inverted image at distance =
+2_f_, and of size equal to that of object.
+
+_Position III_ Object nearer than 2_f_. Inverted image further away than
+2_f_; _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 2_f_ from the
+optical centre of the lens. And it follows from this that the further he
+can rack his camera out beyond 2_f_ 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.
+
+[Illustration: FIG. 111.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 113.]
+
+[Illustration: FIG. 114.]
+
+
+DISTORTION OF IMAGE.
+
+[Illustration: FIG. 115.--Section of a rectilinear lens.]
+
+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 this kind are called _rectilinear_,
+or 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?
+
+[Illustration: 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.
+
+[Illustration: FIG. 117_a_.]
+
+[Illustration: FIG. 117_b_.]
+
+[Illustration: FIG. 118_a_.]
+
+[Illustration: FIG. 118_b_.]
+
+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 117_a_, 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. 117_b_) 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. 118_a_ 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. 118_b_, 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.
+
+
+In 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_.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 121.]
+
+[Illustration: FIG. 122.]
+
+
+USE OF THE SIMPLE MICROSCOPE IN THE TELESCOPE.
+
+[Illustration: 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:--
+
+  Magnification = focal length of lens
+                  --------------------
+                      _d_
+
+[Illustration: FIG. 124.]
+
+In Fig. 124 we have interposed between the eye and the object a small
+magnifying glass of 2-1/2-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 x 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
+  2-1/2
+
+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.
+
+[Illustration: FIG. 125.]
+
+
+THE GALILEAN TELESCOPE.
+
+[Illustration: 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.
+
+[Illustration: FIG. 127.]
+
+
+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 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 A^1, in
+which the lines D _b_, E _c_ meet when produced backwards.
+
+[Illustration: 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.
+
+
+Sound 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.
+
+[Illustration: 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).
+
+[Illustration: 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.
+
+[Illustration: 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, B^1, 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.
+
+In 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.
+
+[Illustration: FIG 136.--Showing how the harmonics of a "stopped" pipe
+are formed.]
+
+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 the pipe. Blow very
+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 A^1 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.
+
+[Illustration: FIG. 137.--Showing how harmonics of an open pipe are
+formed, B, B^1, and C are "nodes." The arrows indicate the distance
+travelled by a sound impulse from a node to a node.]
+
+
+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.
+
+
+WHERE OVERTONES ARE USED.
+
+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.
+
+[Illustration: FIG. 138.--Section of an ordinary wooden "flue" pipe.]
+
+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.
+
+[Illustration: FIG. 139.--The table of a sound-board.]
+
+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_, one for each 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
+pipes. The grooves under the big pipes are wider than those under the
+small pipes, as they have to pass more 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.
+
+[Illustration: FIG. 140.]
+
+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 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.
+
+[Illustration: FIG. 141.]
+
+
+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.
+
+[Illustration: FIG. 142.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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, [Greek: akouein], "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.
+
+
+In 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 148.--Perspective view of a phonograph recorder.]
+
+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 much larger than others (Fig. 151, _a_). A 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.
+
+[Illustration: FIG. 149.--Section of the reproducer of an Edison Bell
+phonograph.]
+
+[Illustration: FIG. 150.--Perspective view of a phonograph reproducer.]
+
+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 note.
+
+
+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. 151_b_). 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.
+
+[Illustration: FIG. 151_a._]
+
+[Illustration: FIG. 151_b._]
+
+[Illustration: FIG. 151_c._--Section of a gramophone reproducer.]
+
+In Fig. 151_c_ 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.
+
+
+When 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 P^1, 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.
+
+[Illustration: 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 4-1/3 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 163.--Divers at work below water with pneumatic
+tools.]
+
+
+AIR-PUMPS.
+
+[Illustration: FIG. 164.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 166.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 7-1/4 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.
+
+[Illustration: 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."
+
+
+In 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.
+
+[Illustration: FIG. 172.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 174.]
+
+
+THE FORCE-PUMP.
+
+[Illustration: FIG. 175. Force-pump; suction stroke.]
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 179.]
+
+[Illustration: FIG. 180.--The cylinder and ram of a hydraulic press.]
+
+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.
+
+Among these, the most used is the tap, 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 182.--An automatic ball-valve.]
+
+
+THE WATER-METER.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 72-1/2 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 199.]
+
+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 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.
+
+[Illustration: FIG. 200.--Sketch of the bellows and chambers of a "dry"
+gas meter.]
+
+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 a certain volume of gas has been
+ejected either from them or from the chambers in which they 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.
+
+The 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.
+
+[Illustration: FIG. 201.]
+
+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 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.
+
+[Illustration: FIG. 202.]
+
+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 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 deg.
+would become too long at 80 deg., and slow the clock, and too short at
+40 deg., 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 204.--"Cylinder" watch escapement.]
+
+
+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. When the
+tooth has passed the nearer edge it flies 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 P^1 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.
+
+[Illustration: FIG. 205.--"Lever" watch escapement.]
+
+In the illustration a tooth has just passed under the "impulse face" _b_
+of P^1. 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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 1/3 of 1/4 = 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.
+
+[Illustration: 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.
+
+[Illustration: 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, S^1, which presses on the tail of the tumbler.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: 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!
+
+[Illustration: 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 A^1, B^1, C^1, D^1, E^1,
+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.
+
+[Illustration: 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 _un_lock 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.
+
+[Illustration: 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 = 2-1/2. One turn of the pedal turns the rear wheel 2-1/2 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 2-1/2 x 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 x 28 inches = 84 inches.
+
+A handy formula to remember is, gearing = T/_t_ x 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.
+
+[Illustration: FIG. 222.]
+
+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.
+
+A gain in speed means a loss in power, and _vice versa_. 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.
+
+[Illustration: FIG. 223.]
+
+[Illustration: FIG. 224.]
+
+[Illustration: 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 = 1/4 of a revolution in a clockwise direction, and C 10/30
+= 1/3 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 1/3 of distance round C; A
+advances 1/3 + 1/4 = 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 = 4/5 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.
+
+[Illustration: 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.
+
+[Illustration: 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.
+
+[Illustration: FIG. 228.]
+
+[Illustration: 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.)
+
+[Illustration: FIG. 230.]
+
+[Illustration: 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.
+
+[Illustration: FIG. 232.--Relative positions of sun, moon, and earth at
+"spring" tides.]
+
+[Illustration: FIG. 233.--Relative positions of sun, 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.
+
+  Cylinder, hydraulic press, _363_;
+    steam, _49_.
+
+
+  Danes, 382.
+
+  Dead point, 47.
+
+  De Brouwer stoker, 401.
+
+  Detector in Chubb lock, 435.
+
+  Diving-bell, _332_;
+    simple, _333_, _334_.
+
+  Diving-dress, 335.
+
+  Direction of current in dynamo circuit, 163.
+
+  Diver's feats, 338;
+    helmet, _336_;
+    lamp, _338_.
+
+  Donkey-engines, 68.
+
+  Doorstop, self-closing, 344.
+
+  Double-cylinder engines, 47.
+
+  Draught, forced, 28, _29_;
+    induced, 29.
+
+  Drum and fusee, _414_.
+
+  Durability of motor-car engine, 96.
+
+  D-valve, 67.
+
+  Dynamo, alternating, 164, 174;
+    brushes, _172_;
+    compound, 174;
+    continuous-current, 165;
+    multipolar, 169;
+    series wound, _173_;
+    shunt wound, _173_;
+    simple, 161, _162_.
+
+
+  Ear, the, _271_, _273_;
+    a good, 274, 307;
+    sensitiveness of, 275.
+
+  Eccentric, _52_, 53;
+    setting of, 53.
+
+  Edison, Thomas, 310.
+
+  Edison-Bell phonograph, 310.
+
+  Electricity, current, 115;
+    forms of, 113;
+    nature of, 112;
+    static, 114.
+
+  Electric bell, 119, _120_;
+    signalling, 225;
+    slot, 226.
+
+  Electroplating, 185, _186_.
+
+  Electro-magnets, 117.
+
+  Endolymph, 272.
+
+  Engines, compound, 59;
+    donkey, 68;
+    double-cylinder, 47;
+    internal-combustion, 87, 95;
+    reciprocating, 44.
+
+  Escapement of timepieces, 416;
+    cylinder, _420_;
+    lever, 421, _422_.
+
+  Ether, 270.
+
+  Eustachian tube, 276.
+
+  Eye, human, 246, _247_;
+    self-accommodation of, 248.
+
+  Expansive working of steam, 56.
+
+
+  Faraday, Michael, 159.
+
+  Field, magnetic, 159;
+    magnets, 171;
+    ring, 174.
+
+  Filters, 374;
+    Maignen, _373_;
+    Berkefeld, 374.
+
+  Filtration beds, 372.
+
+  Flute, 308.
+
+  Flying-machines, 348.
+
+  Fly-wheel, use of, 48.
+
+  Focus, meaning of, 237;
+    principal, 238.
+
+  Foci, conjugate, 262.
+
+  Force, lines of, 116.
+
+  Forces, component, 345.
+
+  Free wheel, _440_.
+
+  Furring-up of pipes, 391.
+
+  Fusee, drum and, 414.
+
+
+  Galileo, 259, 325, 416.
+
+  Galilean telescope, _259_.
+
+  Gas, coal, 394;
+    governor, 402;
+    meter, 405;
+    traps, 374;
+    works, 394.
+
+  Gasometer, 397;
+    largest, _398_, 399.
+
+  Gauge, steam, 36, _38_;
+    water, 35, _36_.
+
+  Gear, compensating, 107, _108_.
+
+  Gear-box of motor car, 105.
+
+  Gearing of cycle, 439.
+
+  Glaisher, 348.
+
+  Gland, 50, 363.
+
+  Glass, flint and crown, 242.
+
+  Going-barrel for watches, 415.
+
+  Gooch reversing gear, 65.
+
+  Governors, speed, 67;
+    of motor car, 103, _104_.
+
+  Graham, 418.
+
+  Gramophone, 317;
+    records, 319, 321;
+    reproducer, _318_.
+
+
+  Hairspring, 412.
+
+  Hay-cutter, 451.
+
+  Heart, the, 355;
+    disease, 361;
+    rate of pulsation of, 361;
+    size of, 357.
+
+  Heat of sun, 451.
+
+  Hele, Peter, 412.
+
+  Helmet, diver's, _336_.
+
+  Helmholtz, 274, 308.
+
+  Hero of Alexandria, 74.
+
+  Herschel, 261.
+
+  Hertz, Dr., 138.
+
+  Hertzian waves, 138.
+
+  Hot-water supply, 386.
+
+  Hour-hand train in timepieces, _429_.
+
+  Household water supply, 364.
+
+  Hughes type-printer, 134.
+
+  Hydraulic press, 361, _362_.
+
+  Hydro, 385.
+
+
+  Ignition of charge in motor-car cylinder, 100, _101_.
+
+  Image and object, relative positions of, 239;
+    distortion of, 245.
+
+  Incandescent gas mantle, 407;
+    electric lamp, 179.
+
+  Incus, 272.
+
+  Index mechanism of water-meter, 37.
+
+  Indicator of electric bell, 119.
+
+  Induction coil, 121;
+    uses of, 125.
+
+  Injector, 39;
+    Giffard's, _41_;
+    principle of, 40;
+    self-starting, 42.
+
+  Interlocking of signals, 204, 222.
+
+  Internal-combustion engine, 87.
+
+  Iris of eye, 249;
+    stop, 249.
+
+
+  Kelvin, Lord, 158.
+
+  Keyless winding mechanism, 425, _426_, 428.
+
+  Kite, 345.
+
+
+  Lamp, arc, 182;
+    how it works, 392;
+    incandescent, 179;
+    manufacture of incandescent lamps, 180.
+
+  Lap of slide-valve, _57_, 59.
+
+  Larynx, 306.
+
+  Laxey wheel, _380_, 381.
+
+  Leads, 208.
+
+  Lenses, 231;
+    correction of for colour, 240, _241_;
+    focus of, 236;
+    rectilinear, _245_;
+    spherical aberration in, 243.
+
+  Levers, signal, colours of, 208.
+
+  Limit of error in cylinder, 52.
+
+  Light, electric, 179;
+    nature of, 230;
+    propagation of, 231.
+
+  Li Hung Chang, 157.
+
+  Lindsay, James Bowman, 145.
+
+  Lines of force, 116, 162.
+
+  "Linking up," 65.
+
+  Locks, 430;
+    Barron, 433;
+    Bramah, 437;
+    Chubb, 433, 434;
+    Hobbs, 437;
+    simplest, _431_;
+    tumbler, _432_;
+    Yale, _436_.
+
+  Locking gear for signals, 205.
+
+  Locomotive, electric, 178;
+    advantages of, 179.
+
+  Lungs, 359.
+
+
+  Magic-lantern, 263, _264_.
+
+  Magnet, 115;
+    permanent, 115, 116;
+    temporary, 115.
+
+  Magnetism, 115.
+
+  Magnetic needle, influence of current on, 129.
+
+  Mainspring, invention of, 412.
+
+  Malleus, 272.
+
+  Marconi, 140, 146.
+
+  Marine chronometers, 415;
+    delicacy of, 425.
+
+  Marine speed governor, 71.
+
+  Marine turbine, advantages of, 84.
+
+  Maudslay, Henry, 363.
+
+  Maxim, Sir Hiram, 348.
+
+  Micrometer free wheel, 441.
+
+  Micro-photography, 265.
+
+  Microscope, 254;
+    compound, 261, _263_;
+    in telescope, 257;
+    simple, _254_.
+
+  Mineral oil, 392.
+
+  Mirror, parabolic, 261, _262_;
+    plane, _267_.
+
+  Morse, 132, 145;
+    code, 128;
+    inker, 142;
+    sounder, 132.
+
+  Motor car, the, 92;
+    electric, 177.
+
+  Mouth, 307.
+
+  Mowing-machines, 450.
+
+  Musical sounds, 277.
+
+
+  Nerve, auditory, 272;
+    optic, 246.
+
+  Nodes on a string, 285;
+    column of air, 291.
+
+  Note, fundamental, 285;
+    quality of, 285.
+
+  Niagara Falls, power station at, 174.
+
+
+  Organ, the, 294, _300_;
+    bellows, 303;
+    console, 305;
+    echo, solo, swell, great, and choir, 301;
+    electric and pneumatic, 305;
+    largest in the world, 306;
+    pedals, 298;
+    pipes, 295;
+    pipes, arrangement of, 295;
+    sound-board, _296_;
+    wind-chest, 297.
+
+  Otto cycle, 91.
+
+  Overtones, 285.
+
+
+  Pallets of organ, 297.
+
+  Parallel arrangement of electric lamps, 184.
+
+  Paris, siege of, 265.
+
+  Pedals of organ, 298.
+
+  Pelton wheel, _377_.
+
+  Pendulum, 412;
+    compensating, 418, _419_.
+
+  Perilymph, 272.
+
+  Perry, Professor, 16.
+
+  Petrol, 98.
+
+  Phonograph, 310;
+    governor, _311_;
+    recorder, 312, _313_;
+    records, making of, 319;
+    reproducer, 315;
+    tracings on record of, _317_.
+
+  Pianoforte, 277;
+    sounding-board, 280;
+    striking mechanism, 281;
+    strings, 281.
+
+  Piccolo, 308.
+
+  Pipes, closed, 289;
+    flue, 301;
+    open, 292;
+    organ, 295;
+    reed, 301, _302_;
+    tuning, 302.
+
+  Piston valve, 67.
+
+  Pneumatic tyres, 341.
+
+  Poldhu, signalling station at, 138.
+
+  Points, railway, 208, _210_;
+    and signals in combination, 211.
+
+  Poles of a magnet, 115.
+
+  Popoff, Professor A., 138, 145.
+
+  Power, transmission of, 175.
+
+  Preece, Sir William, 145.
+
+  Primary winding of induction coil, 122.
+
+  Pump, air, 340;
+    bucket, 352, _353_;
+    force, 354;
+    most marvellous, 355;
+    Westinghouse air, 199.
+
+
+  Railway brakes, 187;
+    signalling, 200.
+
+  Rays, converging and diverging, _256_;
+    heat, concentrated by lens, _232_;
+    light, 232, 235, 236, 237.
+
+  Records, master, 319, 320.
+
+  Reciprocation, 51.
+
+  Reed, human, 306;
+    pipes, 301, _302_.
+
+  Reflecting telescope, 260.
+
+  Relays, telegraphic, 133, 141.
+
+  Retina, 247.
+
+  Retorts, 395.
+
+  Reversing gear, 62;
+    Allan, 65;
+    Gooch, 65;
+    radial, 66.
+
+  Rocking bar mechanism for watches, 425.
+
+  Rods of Corti, 274.
+
+  Ruhmkorff coil, 121, _122_.
+
+
+  Safety-valve, 32, _33_, 391.
+
+  Sand-glasses, 411.
+
+  Scissors, action of, _450_.
+
+  Secondary winding of induction coil, 122.
+
+  Series arrangement of electric lamps, 183.
+
+  Series winding of dynamo, _173_.
+
+  Shunt wound dynamo, _173_.
+
+  Sight, long and short, 250.
+
+  Signalling, automatic, 228;
+    electric, 225;
+    pneumatic, 225;
+    power, 225.
+
+  Signal levers, _206_.
+
+  Signals, interlocking of, 204;
+    position of, 202;
+    railway, 200;
+    single line, 215.
+
+  Silencer on motor cars, 109.
+
+  Siphon, _351_.
+
+  Slide-valve, 49, 50, 51;
+    setting of, 53.
+
+  Sliders, 297.
+
+  Sound, nature of, 270;
+    board of organ, 296;
+    board of piano, 280.
+
+  Spagnoletti disc instrument, 212.
+
+  Sparking-plug, _102_.
+
+  Spectacles, use of, 249.
+
+  Spectrum, colours of, 230.
+
+  Speed governors, 67, _68_, _69_;
+    Hartwell, 70;
+    marine, 71.
+
+  Speed of motor cars, 110.
+
+  Spot, blind, in eye, 251;
+    yellow, in eye, 251.
+
+  Spring balance for watches, 419;
+    compensating, 423, _424_.
+
+  Stapes, 272.
+
+  Steam, what it is, 13;
+    energy of, 14;
+    engines, 44;
+    engines, reciprocating, _45_;
+    expansive working of, 59, 81;
+    gauge, 36;
+    gauge, principle of, 37;
+    turbine, 74;
+    turbine, De Laval, 76, _77_;
+    turbine, Hero's, 74;
+    turbine, Parsons, 79, _80_;
+    volume of, as compared with water, 15.
+
+  Stephenson, George, 63, 375.
+
+  Stop, in lens, 244;
+    iris, 249;
+    use of, 244.
+
+  Sun-dial of Ahaz, 410.
+
+  Syntonic transmission of wireless messages, 143.
+
+
+  Talking-machines, 310.
+
+  Tapper in wireless telegraphy receiver, 141.
+
+  Tappet arm, 205.
+
+  Telegraph, electric, 127;
+    insulator, _133_;
+    needle, _128_;
+    recording, 133;
+    sounder, 132.
+
+  Telegraphy, high-speed, 135;
+    wireless, 137.
+
+  Telephone, 147;
+    Bell, _148_;
+    circuit, double-line, 155;
+    circuit, general arrangement, _152_, 153;
+    exchange, _154_, 155.
+
+  Telephony, submarine, 157.
+
+  Telescope, 257;
+    Galilean, _259_;
+    prismatic, _260_;
+    reflecting, 260;
+    terrestrial, _259_.
+
+  Threshing-machine, 447, _448_.
+
+  Thurston, Professor, 31.
+
+  Tides, 452;
+    high, 453;
+    neap and spring, 455.
+
+  Timbre, 285.
+
+  Tompion, Thomas, 412.
+
+  Torricelli, 325.
+
+  Trachea, 306.
+
+  Train staff signalling, 216;
+    single, 216;
+    and ticket, 217;
+    electric, 218.
+
+  Transformation of current, 124, 176.
+
+  Transmission of power, 174, _175_.
+
+  Transmitter, Edison telephone, 150;
+    granular carbon, 150, _151_.
+
+  Triple-valve, 196.
+
+  Trolley arm, 176.
+
+  Turbines, steam, 74.
+
+  _Turbinia_, the, 79.
+
+  Tympanum, 137, 271, 272.
+
+
+  Universal joint, 93.
+
+
+  Vacuum brake, 189, _190_, _191_.
+
+  Vacuum chamber of aneroid barometer, _330_.
+
+  Valve, piston, 67;
+    safety, 32;
+    of internal-combustion engine, 89.
+
+  Valves of the heart, 357.
+
+  Veins, 358;
+    capillary, 358;
+    pulmonary, 361.
+
+  Ventral segments, 291.
+
+  Ventricles, 357.
+
+  Vibration of columns of air, 288, 289;
+    of rods, 287;
+    of strings, 278;
+    of strings, conditions regulating, 278.
+
+  _Viper_, the, 86.
+
+  Virag, Pollak--high-speed telegraphy, 136.
+
+  Vitreous humour, 246.
+
+  Voltage, 121, 161.
+
+  Vowel sounds, 308.
+
+
+  Wasborough, Matthew, 51.
+
+  Watches, first, 412.
+
+  Water cock, _365_;
+    engines, 375;
+    gauge, 35, _36_;
+    jacket, 19, 95;
+    meter, _368_;
+    supply, 371;
+    turbines, 174, 376;
+    wheels, 375.
+
+  Watt, James, 51, 69, 375.
+
+  Welsbach incandescent mantle, 407.
+
+  Westinghouse air-brake, 194, _195_, _197_;
+    George, 194.
+
+  Wheatstone needle instrument, 128, 131;
+    automatic transmitter, 135.
+
+  Wind, why it blows, 323;
+    action of on kites, 345;
+    on sails, 346.
+
+  Windmills, 375.
+
+  Window, oval, in ear, 272;
+    round, in ear, 272.
+
+  Wireless telegraphy, 137;
+    advance of, 145;
+    receiver, 140, 141;
+    syntonic, 143;
+    transmitter, 138, _139_.
+
+
+  Yale lock, _436_, _437_.
+
+  Yellow spot, in eye, 251.
+
+
+  Zech, Jacob, 414.
+
+  Zeiss field-glasses, 260.
+
+
+THE END.
+
+
+
+
+
+End of the Project Gutenberg EBook of How it Works, by Archibald Williams
+
+*** END OF THIS PROJECT GUTENBERG EBOOK HOW IT WORKS ***
+
+***** This file should be named 28553.txt or 28553.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/2/8/5/5/28553/
+
+Produced by Steven Gibbs, Greg Bergquist and the Online
+Distributed Proofreading Team at https://www.pgdp.net
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions means that no
+one owns a United States copyright in these works, so the Foundation
+(and you!) can copy and distribute it in the United States without
+permission and without paying copyright royalties.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need are critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
diff --git a/28553.zip b/28553.zip
new file mode 100644
index 0000000..bd0b2c7
--- /dev/null
+++ b/28553.zip
diff --git a/LICENSE.txt b/LICENSE.txt
new file mode 100644
index 0000000..6312041
--- /dev/null
+++ b/LICENSE.txt
@@ -0,0 +1,11 @@
+This eBook, including all associated images, markup, improvements,
+metadata, and any other content or labor, has been confirmed to be
+in the PUBLIC DOMAIN IN THE UNITED STATES.
+
+Procedures for determining public domain status are described in
+the "Copyright How-To" at https://www.gutenberg.org.
+
+No investigation has been made concerning possible copyrights in
+jurisdictions other than the United States. Anyone seeking to utilize
+this eBook outside of the United States should confirm copyright
+status under the laws that apply to them.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..f94d1ad
--- /dev/null
+++ b/README.md
@@ -0,0 +1,2 @@
+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #28553 (https://www.gutenberg.org/ebooks/28553)