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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. 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