Chapter XX.

VARIOUS MECHANISMS. CLOCKS AND WATCHES:--A short history of timepieces--The construction of timepieces--The driving power--The escapement--Compensating pendulums--The spring balance--The cylinder escapement--The lever escapement--Compensated balance-wheels--Keyless winding mechanism for watches--The hour hand train. LOCKS:--The Chubb lock--The Yale lock. THE CYCLE:--The gearing of a cycle--The free wheel--The change-speed gear. AGRICULTURAL MACHINES:--The threshing-machine--Mowing-machines. SOME NATURAL PHENOMENA:--Why sun-heat varies in intensity--The tides--Why high tide varies daily. CLOCKS AND WATCHES. A SHORT HISTORY OF TIMEPIECES. The oldest device for measuring time is the sun-dial. That of Ahaz mentioned in the Second Book of Kings is the earliest dial of which we have record. The obelisks of the Egyptians and the curious stone pillars of the Druidic age also probably served as shadow-casters. The clepsydra, or water-clock, also of great antiquity, was the first contrivance for gauging the passage of the hours independently of the motion of the earth. In its simplest form it was a measure into which water fell drop by drop, hour levels being marked on the inside. Subsequently a very simple mechanism was added to drive a pointer--a float carrying a vertical rack, engaging with a cog on the pointer spindle; or a string from the float passed over a pulley attached to the pointer and rotated it as the float rose, after the manner of the wheel barometer (Fig. 153). In 807 A.D. Charlemagne received from the King of Persia a water-clock which struck the hours. It is thus described in Gifford's "History of France":--"The dial was composed of twelve small doors, which represented the division of the hours. Each door opened at the hour it was intended to represent, and out of it came a small number of little balls, which fell one by one, at equal distances of time, on a brass drum. It might be told by the eye what hour it was by the number of doors that were open, and by the ear by the number of balls that fell. When it was twelve o'clock twelve horsemen in miniature issued forth at the same time and shut all the doors." Sand-glasses were introduced about 330 A.D. Except for special purposes, such as timing sermons and boiling eggs, they have not been of any practical value. The clepsydra naturally suggested to the mechanical mind the idea of driving a mechanism for registering time by the force of gravity acting on some body other than water. The invention of the _weight-driven clock_ is attributed, like a good many other things, to Archimedes, the famous Sicilian mathematician of the third century B.C.; but no record exists of any actual clock composed of wheels operated by a weight prior to 1120 A.D. So we may take that year as opening the era of the clock as we know it. About 1500 Peter Hele of Nuremberg invented the _mainspring_ as a substitute for the weight, and the _watch_ appeared soon afterwards (1525 A.D.). The pendulum was first adopted for controlling the motion of the wheels by Christian Huygens, a distinguished Dutch mechanician, in 1659. To Thomas Tompion, "the father of English watchmaking," is ascribed the honour of first fitting a _hairspring_ to the escapement of a watch, in or about the year 1660. He also introduced the _cylinder escapement_ now so commonly used in cheap watches. Though many improvements have been made since his time, Tompion manufactured clocks and watches which were excellent timekeepers, and as a reward for the benefits conferred on his fellows during his lifetime, he was, after death, granted the exceptional honour of a resting-place in Westminster Abbey. THE CONSTRUCTION OF TIMEPIECES. A clock or watch contains three main elements:--(1) The source of power, which may be a weight or a spring; (2) the train of wheels operated by the driving force; (3) the agent for controlling the movements of the train--this in large clocks is usually a pendulum, in small clocks and watches a hairspring balance. To these may be added, in the case of clocks, the apparatus for striking the hour. THE DRIVING POWER. _Weights_ are used only in large clocks, such as one finds in halls, towers, and observatories. The great advantage of employing weights is that a constant driving power is exerted. _Springs_ occupy much less room than weights, and are indispensable for portable timepieces. The employment of them caused trouble to early experimenters on account of the decrease in power which necessarily accompanies the uncoiling of a wound-up spring. Jacob Zech of Prague overcame the difficulty in 1525 by the invention of the _fusee_, a kind of conical pulley interposed between the barrel, or circular drum containing the mainspring, and the train of wheels which the spring has to drive. The principle of the "drum and fusee" action will be understood from Fig. 201. The mainspring is a long steel ribbon fixed at one end to an arbor (the watchmaker's name for a spindle or axle), round which it is tightly wound. The arbor and spring are inserted in the barrel. The arbor is prevented from turning by a ratchet, B, and click, and therefore the spring in its effort to uncoil causes the barrel to rotate. [Illustration: FIG. 201.] A string of catgut (or a very fine chain) is connected at one end to the circumference of the drum, and wound round it, the other end being fixed to the larger end of the fusee, which is attached to the driving-wheel of the watch or clock by the intervention of a ratchet and click (not shown). To wind the spring the fusee is turned backward by means of a key applied to the square end A of the fusee arbor, and this draws the string from off the drum on to the fusee. The force of the spring causes the fusee to rotate by pulling the string off it, coil by coil, and so drives the train of wheels. But while the mainspring, when fully wound, turns the fusee by uncoiling the string from the smallest part of the fusee, it gets the advantage of the larger radius as its energy becomes lessened. The fusee is still used for marine chronometers, for some clocks that have a mainspring and pendulum, and occasionally for watches. In the latter it has been rendered unnecessary by the introduction of the _going-barrel_ by Swiss watchmakers, who formed teeth on the edge of the mainspring barrel to drive the train of wheels. This kind of drum is called "going" because it drives the watch during the operation of winding, which is performed by rotating the drum arbor to which the inner end of the spring is attached. A ratchet prevents the arbor from being turned backwards by the spring. The adoption of the going-barrel has been made satisfactory by the improvements in the various escapement actions. THE ESCAPEMENT. [Illustration: FIG. 202.] The spring or weight transmits its power through a train of cogs to the _escapement_, or device for regulating the rate at which the wheels are to revolve. In clocks a _pendulum_ is generally used as the controlling agent. Galileo, when a student at Pisa, noticed that certain hanging lamps in the cathedral there swung on their cords at an equal rate; and on investigation he discovered the principle that the shorter a pendulum is the more quickly will it swing to and fro. As has already been observed, Huygens first applied the principle to the governing of clocks. In Fig. 202 we have a simple representation of the "dead-beat" escapement commonly used in clocks. The escape-wheel is mounted on the shaft of the last cog of the driving train, the pallet on a spindle from which depends a split arm embracing the rod and the pendulum. We must be careful to note that the pendulum _controls_ motion only; it does not cause movement. The escape-wheel revolves in a clockwise direction. The two pallets _a_ and _b_ are so designed that only one can rest on the teeth at one time. In the sketch the sloping end of _b_ has just been forced upwards by the pressure of a tooth. This swings the pallet and the pendulum. The momentum of the latter causes _a_ to descend, and at the instant when _b_ clears its tooth _a_ catches and holds another. The left-hand side of _a_, called the _locking-face_, is part of a circle, so that the escape-wheel is held motionless as long as it touches _a_: hence the term, "dead beat"--that is, brought to a dead stop. As the pendulum swings back, to the left, under the influence of gravity, _a_ is raised and frees the tooth. The wheel jerks round, and another tooth is caught by the locking-face of _b_. Again the pendulum swings to the right, and the sloping end of _b_ is pushed up once more, giving the pendulum fresh impetus. This process repeats itself as long as the driving power lasts--for weeks, months, or years, as the case may be, and the mechanism continues to be in good working order. COMPENSATING PENDULUMS. Metal expands when heated; therefore a steel pendulum which is of the exact length to govern a clock correctly at a temperature of 60° would become too long at 80°, and slow the clock, and too short at 40°, and cause it to gain. In common clocks the pendulum rod is often made of wood, which maintains an almost constant length at all ordinary temperatures. But for very accurate clocks something more efficient is required. Graham, the partner of Thomas Tompion, took advantage of the fact that different kinds of metal have different ratios of expansion to produce a _self-compensating_ pendulum on the principle illustrated by Fig. 203. He used steel for the rod, and formed the _bob_, or weighted end, of a glass jar containing mercury held in a stirrup; the mercury being of such a height that, as the pendulum rod lengthened with a rise of temperature, the mercury expanded _upwards_ sufficiently to keep the distance between the point of suspension and the centre of gravity of the bob always the same. With a fall of temperature the rod shortened, while the mercury sank in the jar. This device has not been improved upon, and is still used in observatories and other places where timekeepers of extreme precision are required. The milled nut S in Fig. 203 is fitted at the end of the pendulum rod to permit the exact adjustment of the pendulum's length. For watches, chronometers, and small clocks THE SPRING BALANCE takes the place of the pendulum. We still have an escape-wheel with teeth of a suitable shape to give impulses to the controlling agent. There are two forms of spring escapement, but as both employ a hairspring and balance-wheel we will glance at these before going further. [Illustration: FIG. 203.] The _hairspring_ is made of very fine steel ribbon, tempered to extreme elasticity, and shaped to a spiral. The inner end is attached to the arbor of the _balance-wheel_, the outer end to a stud projecting from the plate of the watch. When the balance-wheel, impelled by the escapement, rotates, it winds up the spring. The energy thus stored helps the wheel to revolve the other way during the locking of a tooth of the escape-wheel. The time occupied by the winding and the unwinding depends upon the length of the spring. The strength of the impulse makes no difference. A strong impulse causes the spring to coil itself up more than a weak impulse would; but inasmuch as more energy is stored the process of unwinding is hastened. To put the matter very simply--a strong impulse moves the balance-wheel further, but rotates it quickly; a weak impulse moves it a shorter distance, but rotates it slowly. In fact, the principle of the pendulum is also that of the hairspring; and the duration of a vibration depends on the length of the rod in the one case, and of the spring in the other. Motion is transmitted to the balance by one of two methods. Either (1) directly, by a cylinder escapement; or (2) indirectly, through a lever. [Illustration: FIG. 204.--"Cylinder" watch escapement.] THE CYLINDER ESCAPEMENT is seen in Fig. 204. The escape-wheel has sharp teeth set on stalks. (One tooth is removed to show the stalk.) The balance-wheel is mounted on a small steel cylinder, with part of the circumference cut away at the level of the teeth, so that if seen from above it would appear like _a_ in our illustration. A tooth is just beginning to shove its point under the nearer edge of the opening. As it is forced forwards, _b_ is revolved in a clockwise direction, winding up the hairspring. When the tooth has passed the nearer edge it flies forward, striking the inside of the further wall of the cylinder, which holds it while the spring uncoils. The tooth now pushes its way past the other edge, accelerating the unwinding, and, as it escapes, the next tooth jumps forward and is arrested by the outside of the cylinder. The balance now reverses its motion, is helped by the tooth, is wound up, locks the tooth, and so on. THE LEVER ESCAPEMENT is somewhat more complicated. The escape-wheel teeth are locked and unlocked by the pallets P P^1 projecting from a lever which moves on a pivot (Fig. 205). The end of the lever is forked, and has a square notch in it. On the arbor of the balance-wheel is a roller, or plate, R, which carries a small pin, I. Two pins, B B, projecting from the plate of the watch prevent the lever moving too far. We must further notice the little pin C on the lever, and a notch in the edge of the roller. [Illustration: FIG. 205.--"Lever" watch escapement.] In the illustration a tooth has just passed under the "impulse face" _b_ of P^1. The lever has been moved upwards at the right end; and its forked end has given an impulse to R, and through it to the balance-wheel. The spring winds up. The pin C prevents the lever dropping, because it no longer has the notch opposite to it, but presses on the circumference of R. As the spring unwinds it strikes the lever at the moment when the notch and C are opposite. The lever is knocked downwards, and the tooth, which had been arrested by the locking-face _a_ of pallet P, now presses on the impulse face _b_, forcing the left end of the lever up. The impulse pin I receives a blow, assisting the unwinding of the spring, and C again locks the lever. The same thing is repeated in alternate directions over and over again. COMPENSATING BALANCE-WHEELS. The watchmaker has had to overcome the same difficulty as the clockmaker with regard to the expansion of the metal in the controlling agent. When a metal wheel is heated its spokes lengthen, and the rim recedes from the centre. Now, let us suppose that we have two rods of equal weight, one three feet long, the other six feet long. To an end of each we fasten a 2-lb. weight. We shall find it much easier to wave the shorter rod backwards and forwards quickly than the other. Why? Because the weight of the longer rod has more leverage over the hand than has that of the shorter rod. Similarly, if, while the mass of the rim of a wheel remains constant, the length of the spokes varies, the effort needed to rotate the wheel to and fro at a constant rate must vary also. Graham got over the difficulty with a rod by means of the compensating pendulum. Thomas Earnshaw mastered it in wheels by means of the _compensating balance_, using the same principle--namely, the unequal expansion of different metals. Any one who owns a compensated watch will see, on stopping the tiny fly-wheel, that it has two spokes (Fig. 206), each carrying an almost complete semicircle of rim attached to it. A close examination shows that the rim is compounded of an outer strip of brass welded to an inner lining of steel. The brass element expands more with heat and contracts more with cold than steel; so that when the spokes become elongated by a rise of temperature, the pieces bend inwards at their free ends (Fig. 207); if the temperature falls, the spokes are shortened, and the rim pieces bend outwards (Fig. 208).[39] This ingenious contrivance keeps the leverage of the rim constant within very fine limits. The screws S S are inserted in the rim to balance it correctly, and very fine adjustment is made by means of the four tiny weights W W. In ships' chronometers,[40] the rim pieces are _sub_-compensated towards their free ends to counteract slight errors in the primary compensation. So delicate is the compensation that a daily loss or gain of only half a second is often the limit of error. [Illustration: FIG. 206. FIG. 207. FIG. 208. A "compensating" watch balance, at normal, super-normal, and sub-normal temperatures.] KEYLESS WINDING MECHANISM FOR WATCHES. The inconvenience attaching to a key-wound watch caused the Swiss manufacturers to put on the market, in 1851, watches which dispensed with a separate key. Those of our readers who carry keyless watches will be interested to learn how the winding and setting of the hands is effected by the little serrated knob enclosed inside the pendant ring. There are two forms of "going-barrel" keyless mechanism--(1) The rocking bar; (2) the shifting sleeve. The _rocking bar_ device is shown in Figs. 209, 210. The milled head M turns a cog, G, which is always in gear with a cog, F. This cog gears with two others, A and B, mounted at each end of the rocker R, which moves on pivot S. A spring, S P, attached to the watch plate presses against a small stud on the rocking bar, and keeps A normally in gear with C, mounted on the arbor of the mainspring. [Illustration: FIG. 209.--The winding mechanism of a keyless watch.] To wind the watch, M is turned so as to give F an anti-clockwise motion. The teeth of F now press A downwards and keep it in gear with C while the winding is done. A spring click (marked solid black) prevents the spring uncoiling (Fig. 209). If F is turned in a clockwise direction it lifts A and prevents it biting the teeth of C, and no strain is thrown on C. To set the hands, the little push-piece P is pressed inwards by the thumb (Fig. 210) so as to depress the right-hand end of R and bring B into gear with D, which in turn moves E, mounted on the end of the minute-hand shaft. The hands can now be moved in either direction by turning M. On releasing the push-piece the winding-wheels engage again. The _shifting sleeve_ mechanism has a bevel pinion in the place of G (Fig. 209) gearing with the mainspring cog. The shaft of the knob M is round where it passes through the bevel and can turn freely inside it, but is square below. On the square part is mounted a little sliding clutch with teeth on the top corresponding with the other teeth on the under side of the bevel-wheel, and teeth similar to those of G (Fig. 209) at the end. The clutch has a groove cut in the circumference, and in this lies the end of a spring lever which can be depressed by the push-piece. The mechanism much resembles on a small scale the motor car changing gear (Fig. 49). Normally, the clutch is pushed up the square part of the knob shaft by the spring so as to engage with the bevel and the winding-wheels. On depressing the clutch by means of the push-piece it gears with the minute-hand pinion, and lets go of the bevel. [Illustration: FIG. 210.--The hand-setting mechanism in action.] In one form of this mechanism the push-piece is dispensed with, and the minute-wheel pinion is engaged by pulling the knob upwards. THE HOUR-HAND TRAIN. [Illustration: FIG. 211.--The hour-hand train of a clock.] The teeth of the mainspring drum gear with a cog on the minute-hand shaft, which also carries one of the cogs of the escapement train. The shaft is permitted by the escapement to revolve once an hour. Fig. 211 shows diagrammatically how this is managed. The hour-hand shaft A (solid black) can be moved round inside the cog B, driven by the mainspring drum. It carries a cog, C. This gears with a cog, D, having three times as many teeth. The cog E, united to D, drives cog F, having four times as many teeth as E. To F is attached the collar G of the hour-hand. F and G revolve outside the minute-hand shaft. On turning A, C turns D and E, E turns F and the hour-hand, which revolves 1/3 of 1/4 = 1/12 as fast as A.[41] * * * * * LOCKS. On these unfortunately necessary mechanisms a great deal of ingenuity has been expended. With the advance of luxury and the increased worship of wealth, it becomes more and more necessary to guard one's belongings against the less scrupulous members of society. [Illustration: FIG. 212.] The simplest form of lock, such as is found in desks and very cheap articles, works on the principle shown in Fig. 212. The bolt is split at the rear, and the upper part bent upwards to form a spring. The under edge has two notches cut in it, separated by a curved excrescence. The key merely presses the bolt upwards against the spring, until the notch, engaging with the frame, moves it backwards or forwards until the spring drives the tail down into the other notch. This primitive device affords, of course, very little security. An advance is seen in the TUMBLER LOCK. [Illustration: FIG. 213.] The bolt now can move only in a horizontal direction. It has an opening cut in it with two notches (Figs. 213, 214). Behind the bolt lies the _tumbler_ T (indicated by the dotted line), pivoted at the angle on a pin. From the face of the tumbler a stud, S, projects through the hole in the bolt. This stud is forced into one or other of the notches by the spring, S^1, which presses on the tail of the tumbler. [Illustration: FIG. 214.] In Fig. 213 the key is about to actuate the locking mechanism. The next diagram (Fig. 214) shows how the key, as it enters the notch on the lower side of the bolt to move it along, also raises the tumbler stud clear of the projection between the two notches. By the time that the bolt has been fully "shot," the key leaves the under notch and allows the tumbler stud to fall into the rear locking-notch. A lock of this type also can be picked very easily, as the picker has merely to lift the tumbler and move the bolt along. Barron's lock, patented in 1778, had two tumblers and two studs; and the opening in the bolt had notches at the top as well as at the bottom (Fig. 215). This made it necessary for both tumblers to be raised simultaneously to exactly the right height. If either was not lifted sufficiently, a stud could not clear its bottom notch; if either rose too far, it engaged an upper notch. The chances therefore were greatly against a wrong key turning the lock. [Illustration: FIG. 215.--The bolt of a Barron lock.] THE CHUBB LOCK is an amplification of this principle. It usually has several tumblers of the shape shown in Fig. 216. The lock stud in these locks projects from the bolt itself, and the openings, or "gates," through which the stud must pass as the lock moves, are cut in the tumblers. It will be noticed that the forward notch of the tumbler has square serrations in the edges. These engage with similar serrations in the bolt stud and make it impossible to raise the tumbler if the bolt begins to move too soon when a wrong key is inserted. [Illustration: FIG. 216.--Tumbler of Chubb lock.] Fig. 217 is a Chubb key with eight steps. That nearest the head (8) operates a circular revolving curtain, which prevents the introduction of picking tools when a key is inserted and partly turned, as the key slot in the curtain is no longer opposite that in the lock. Step 1 moves the bolt. [Illustration: FIG. 217.--A Chubb key.] In order to shoot the bolt the height of the key steps must be so proportioned to the depth of their tumblers that all the gates in the tumblers are simultaneously raised to the right level for the stud to pass through them, as in Fig. 218. Here you will observe that the tumbler D on the extreme right (lifted by step 2 of the key) has a stud, D S, projecting from it over the other tumblers. This is called the _detector tumbler_. If a false key or picking tool is inserted it is certain to raise one of the tumblers too far. The detector is then over-lifted by the stud D S, and a spring catch falls into a notch at the rear. It is now impossible to pick the lock, as the detector can be released only by the right key shooting the bolt a little further in the locking direction, when a projection on the rear of the bolt lifts the catch and allows the tumbler to fall. The detector also shows that the lock has been tampered with, since even the right key cannot move the bolt until the overlocking has been performed. [Illustration: FIG. 218.--A Chubb key raising all the tumblers to the correct height.] Each tumbler step of a large Chubb key can be given one of thirty different heights; the bolt step one of twenty. By merely transposing the order of the steps in a six-step key it is possible to get 720 different combinations. By diminishing or increasing the heights the possible combinations may be raised to the enormous total of 7,776,000! [Illustration: FIG. 219.--Section of a Yale lock.] THE YALE LOCK, which comes from America, works on a quite different system. Its most noticeable feature is that it permits the use of a very small key, though the number of combinations possible is still enormous (several millions). In our illustrations (Figs. 219, 220, 221) we show the mechanism controlling the turning of the key. The keyhole is a narrow twisted slot in the face of a cylinder, G (Fig. 219), which revolves inside a larger fixed cylinder, F. As the key is pushed in, the notches in its upper edge raise up the pins A^1, B^1, C^1, D^1, E^1, until their tops exactly reach the surface of G, which can now be revolved by the key in Fig. 220, and work the bolt through the medium of the arm H. (The bolt itself is not shown.) If a wrong key is inserted, either some of the lower pins will project upwards into the fixed cylinder F (see Fig. 221), or some of the pins in F will sink into G. It is then impossible to turn the key. [Illustration: FIG. 220.--Yale key turning.] There are other well-known locks, such as those invented by Bramah and Hobbs. But as these do not lend themselves readily to illustration no detailed account can be given. We might, however, notice the _time_ lock, which is set to a certain hour, and can be opened by the right key or a number of keys in combination only when that hour is reached. Another very interesting device is the _automatic combination_ lock. This may have twenty or more keys, any one of which can lock it; but the same one must be used to _un_lock it, as the key automatically sets the mechanism in favour of itself. With such a lock it would be possible to have a different key for every day in the month; and if any one key got into wrong hands it would be useless unless it happened to be the one which last locked the lock. [Illustration: FIG. 221.--The wrong key inserted. The pins do not allow the lock to be turned.] * * * * * THE CYCLE. There are a few features of this useful and in some ways wonderful contrivance which should be noticed. First, THE GEARING OF A CYCLE. To a good many people the expression "geared to 70 inches," or 65, or 80, as the case may be, conveys nothing except the fact that the higher the gear the faster one ought to be able to travel. Let us therefore examine the meaning of such a phrase before going farther. The safety cycle is always "geared up"--that is, one turn of the pedals will turn the rear wheel more than once. To get the exact ratio of turning speed we count the teeth on the big chain-wheel, and the teeth on the small chain-wheel attached to the hub of the rear wheel, and divide the former by the latter. To take an example:--The teeth are 75 and 30 in number respectively; the ratio of speed therefore = 75/30 = 5/2 = 2-1/2. One turn of the pedal turns the rear wheel 2-1/2 times. The gear of the cycle is calculated by multiplying this result by the diameter of the rear wheel in inches. Thus a 28-inch wheel would in this case give a gear of 2-1/2 × 28 = 70 inches. One turn of the pedals on a machine of this gear would propel the rider as far as if he were on a high "ordinary" with the pedals attached directly to a wheel 70 inches in diameter. The gearing is raised or lowered by altering the number ratio of the teeth on the two chain-wheels. If for the 30-tooth wheel we substituted one of 25 teeth the gearing would be-- 75/25 × 28 inches = 84 inches. A handy formula to remember is, gearing = T/_t_ × D, where T = teeth on large chain-wheel; _t_ = teeth on small chain-wheel; and D = diameter of driving-wheel in inches. Two of the most important improvements recently added to the cycle are--(1) The free wheel; (2) the change-speed gear. THE FREE WHEEL is a device for enabling the driving-wheel to overrun the pedals when the rider ceases pedalling; it renders the driving-wheel "free" of the driving gear. It is a ratchet specially suited for this kind of work. From among the many patterns now marketed we select the Micrometer free-wheel hub (Fig. 222), which is extremely simple. The _ratchet-wheel_ R is attached to the hub of the driving-wheel. The small chain-wheel (or "chain-ring," as it is often called) turns outside this, on a number of balls running in a groove chased in the neck of the ratchet. Between these two parts are the _pawls_, of half-moon shape. The driving-wheel is assumed to be on the further side of the ratchet. To propel the cycle the chain-ring is turned in a clockwise direction. Three out of the six pawls at once engage with notches in the ratchet, and are held tightly in place by the pressure of the chain-ring on their rear ends. The other three are in a midway position. [Illustration: FIG. 222.] When the rider ceases to pedal, the chain-ring becomes stationary, but the ratchet continues to revolve. The pawls offer no resistance to the ratchet teeth, which push them up into the semicircular recesses in the chain-ring. Each one rises as it passes over a tooth. It is obvious that driving power cannot be transmitted again to the road wheel until the chain-wheel is turned fast enough to overtake the ratchet. THE CHANGE-SPEED GEAR. A gain in speed means a loss in power, and _vice versâ_. By gearing-up a cycle we are able to make the driving-wheel revolve faster than the pedals, but at the expense of control over the driving-wheel. A high-geared cycle is fast on the level, but a bad hill-climber. The low-geared machine shows to disadvantage on the flat, but is a good hill-climber. Similarly, the express engine must have large driving-wheels, the goods engine small driving-wheels, to perform their special functions properly. In order to travel fast over level country, and yet be able to mount hills without undue exertion, we must be able to do what the motorist does--change gear. Two-speed and three-speed gears are now very commonly fitted to cycles. They all work on the same principle, that of the epicyclic train of cog-wheels, the mechanisms being so devised that the hub turns more slowly than, at the same speed as, or faster than the small chain-wheel,[42] according to the wish of the rider. We do not propose to do more here than explain the principle of the epicyclic train, which means "a wheel on (or running round) a wheel." Lay a footrule on the table and roll a cylinder along it by the aid of a second rule, parallel to the first, but resting on the cylinder. It will be found that, while the cylinder advances six inches, the upper rule advances twice that distance. In the absence of friction the work done by the agent moving the upper rule is equal to that done in overcoming the force which opposes the forward motion of the cylinder; and as the distance through which the cylinder advances is only half that through which the upper rule advances, it follows that the _force_ which must act on the upper rule is only half as great as that overcome in moving the cylinder. The carter makes use of this principle when he puts his hand to the top of a wheel to help his cart over an obstacle. [Illustration: FIG. 223.] [Illustration: FIG. 224.] [Illustration: FIG. 225.] Now see how this principle is applied to the change-speed gear. The lower rule is replaced by a cog-wheel, C (Fig. 223); the cylinder by a cog, B, running round it; and the upper rule by a ring, A, with internal teeth. We may suppose that A is the chain-ring, B a cog mounted on a pin projecting from the hub, and C a cog attached to the fixed axle. It is evident that B will not move so fast round C as A does. The amount by which A will get ahead of B can be calculated easily. We begin with the wheels in the position shown in Fig. 223. A point, I, on A is exactly over the topmost point of C. For the sake of convenience we will first assume that instead of B running round C, B is revolved on its axis for one complete revolution in a clockwise direction, and that A and C move as in Fig. 224. If B has 10 teeth, C 30, and A 40, A will have been moved 10/40 = 1/4 of a revolution in a clockwise direction, and C 10/30 = 1/3 of a revolution in an anti-clockwise direction. Now, coming back to what actually does happen, we shall be able to understand how far A rotates round C relatively to the motion of B, when C is fixed and B rolls (Fig. 225). B advances 1/3 of distance round C; A advances 1/3 + 1/4 = 7/12 of distance round B. The fractions, if reduced to a common denominator, are as 4:7, and this is equivalent to 40 (number of teeth on A): 40 + 30 (teeth on A + teeth on C.) To leave the reader with a very clear idea we will summarize the matter thus:--If T = number of teeth on A, _t_ = number of teeth on C, then movement of A: movement of B:: T + _t_: T. Here is a two-speed hub. Let us count the teeth. The chain-ring (= A) has 64 internal teeth, and the central cog (= C) on the axle has 16 teeth. There are four cogs (= B) equally spaced, running on pins projecting from the hub-shell between A and C. How much faster than B does A run round C? Apply the formula:--Motion of A: motion of B:: 64 + 16: 64. That is, while A revolves once, B and the hub and the driving-wheel will revolve only 64/80 = 4/5 of a turn. To use scientific language, B revolves 20 per cent. slower than A. This is the gearing we use for hill-climbing. On the level we want the driving-wheel to turn as fast as, or faster than, the chain-ring. To make it turn at the same rate, both A and C must revolve together. In one well-known gear this is effected by sliding C along the spindle of the wheel till it disengages itself from the spindle, and one end locks with the plate which carries A. Since B is now being pulled round at the bottom as well as the top, it cannot rotate on its own axis any longer, and the whole train revolves _solidly_--that is, while A turns through a circle B does the same. To get an _increase_ of gearing, matters must be so arranged that the drive is transmitted from the chain-wheel to B, and from A to the hub. While B describes a circle, A and the driving-wheel turn through a circle and a part of a circle--that is, the driving-wheel revolves faster than the hub. Given the same number of teeth as before, the proportional rates will be A = 80, B = 64, so that the gear _rises_ 25 per cent. By means of proper mechanism the power is transmitted in a three-speed gear either (1) from chain-wheel to A, A to B, B to wheel = _low_ gear; or (2) from chain-wheel to A and C simultaneously = solid, normal, or _middle_ gear; or (3) from chain-wheel to B, B to A, A to wheel = _high_ gear. In two-speed gears either 1 or 3 is omitted. * * * * * AGRICULTURAL MACHINES. THE THRESHING-MACHINE. Bread would not be so cheap as it is were the flail still the only means of separating the grain from the straw. What the cream separator has done for the dairy industry (p. 384), the threshing-machine has done for agriculture. A page or two ought therefore to be spared for this useful invention. [Illustration: FIG. 226.--Section of a threshing machine.] In Fig. 226 a very complete fore-and-aft section of the machine is given. After the bands of the sheaves have been cut, the latter are fed into the mouth of the _drum_ A by the feeder, who stands in the feeding-box on the top of the machine. The drum revolves at a very high velocity, and is fitted with fluted beaters which act against a steel concave, or breastwork, B, the grain being threshed out of the straw in passing between the two. The breastwork is provided with open wires, through which most of the threshed grain, cavings (short straws), and chaff passes on to a sloping board. The straw is flung forward on to the shakers C, which gradually move the straw towards the open end and throw it off. Any grain, etc., that has escaped the drum falls through the shakers on to D, and works backwards to the _caving riddles_, or moving sieves, E. The _main blower_, by means of a revolving fan, N, sends air along the channel X upwards through these riddles, blowing the short straws away to the left. The grain, husks, and dust fall through E on to G, over the end of which they fall on to the _chaff riddle_, H. A second column of air from the blower drives the chaff away. The heavy grain, seeds, dust, etc., fall on to I, J, and K in turn, and are shaken until only the grain remains to pass along L to the elevator bottom, M. An endless band with cups attached to it scoops up the grain, carries it aloft, and shoots it into hopper P. It then goes through the shakers Q, R, is dusted by the _back end blower_, S, and slides down T into the open end of the rotary screen-drum U, which is mounted on the slope, so that as it turns the grain travels gradually along it. The first half of the screen has wires set closely together. All the small grain that falls through this, called "thirds," passes into a hopper, and is collected in a sack attached to the hopper mouth. The "seconds" fall through the second half of the drum, more widely spaced, into their sack; and the "firsts" fall out of the end and through a third spout. MOWING-MACHINES. [Illustration: FIG. 227.] The ordinary _lawn--mower_ employs a revolving reel, built up of spirally-arranged knives, the edges of which pass very close to a sharp plate projecting from the frame of the mower. Each blade, as it turns, works along the plate, giving a shearing cut to any grass that may be caught between the two cutting edges. The action is that of a pair of scissors (Fig. 227), one blade representing the fixed, the other the moving knife. If you place a cylinder of wood in the scissors it will be driven forward by the closing of the blades, and be marked by them as it passes along the edges. The same thing happens with grass, which is so soft that it is cut right through. HAY-CUTTER. The _hay-cutter_ is another adaptation of the same principle. A cutter-bar is pulled rapidly backwards and forwards in a frame which runs a few inches above the ground by a crank driven by the wheels through gearing. To the front edge of the bar are attached by one side a number of triangular knives. The frame carries an equal number of spikes pointing forward horizontally. Through slots in these the cutter-bar works, and its knives give a drawing cut to grass caught between them and the sides of the spikes. * * * * * SOME NATURAL PHENOMENA. WHY SUN-HEAT VARIES IN INTENSITY. The more squarely parallel heat-rays strike a surface the greater will be the number that can affect that surface. This is evident from Figs. 228, 229, where A B is an equal distance in both cases. The nearer the sun is to the horizon, the more obliquely do its rays strike the earth. Hence midday is necessarily warmer than the evening, and the tropics, where the sun stands overhead, are hotter than the temperate zones, where, even in summer at midday, the rays fall more or less on the slant. [Illustration: FIG. 228.] [Illustration: FIG. 229.] The atmospheric envelope which encompasses the earth tends to increase the effect of obliquity, since a slanting ray has to travel further through it and is robbed of more heat than a vertical ray. THE TIDES. All bodies have an attraction for one another. The earth attracts the moon, and the moon attracts the earth. Now, though the effect of this attraction is not visible as regards the solid part of the globe, it is strongly manifested by the water which covers a large portion of the earth's surface. The moon attracts the water most powerfully at two points, that nearest to it and that furthest away from it; as shown on an exaggerated scale in Fig. 230. Since the earth and the water revolve as one mass daily on their axis, every point on the circumference would be daily nearest to and furthest from the moon at regular intervals, and wherever there is ocean there would be two tides in that period, were the moon stationary as regards the earth. (It should be clearly understood that the tides are not great currents, but mere thickenings of the watery envelope. The inrush of the tide is due to the temporary rise of level.) [Illustration: FIG. 230.] [Illustration: FIG. 231.] WHY HIGH TIDE VARIES DAILY. The moon travels round the earth once in twenty-eight days. In Fig. 231 the point _a_ is nearest the moon at, say, twelve noon. At the end of twenty-four hours it will have arrived at the same position by the compass, but yet not be nearest to the moon, which has in that period moved on 1/28th of a revolution round the earth.[43] Consequently high tide will not occur till _a_ has reached position _b_ and overtaken the moon, as it were, which takes about an hour on the average. This explains why high tide occurs at intervals of more than twelve hours. [Illustration: FIG. 232.--Relative positions of sun, moon, and earth at "spring" tides.] [Illustration: FIG. 233.--Relative positions of sun, moon, and earth at "neap" tides.] NEAP TIDES AND SPRING TIDES. The sun, as well as the moon, attracts the ocean, but with less power, owing to its being so much further away. At certain periods of the month, sun, earth, and moon are all in line. Sun and moon then pull together, and we get the highest, or _spring_ tides (Fig. 232). When sun and moon pull at right angles to one another--namely, at the first and third quarters--the excrescence caused by the moon is flattened (Fig. 233), and we get the lowest, or _neap_ tides. [39] In both Figs. 207 and 208 the degree of expansion is very greatly exaggerated. [40] As the sun passes the meridian (twelve o'clock, noon) the chronometer's reading is taken, and the longitude, or distance east or west of Greenwich, is reckoned by the difference in time between local noon and that of the chronometer. [41] For much of the information given here about clocks and watches the author is indebted to "The History of Watches," by Mr. J.F. Kendal. [42] We shall here notice only those gears which are included in the hub of the driving-wheel. [43] The original position of the moon is indicated by the dotted circle. INDEX. NOTE.--Figures in italics signify that an illustration of the thing referred to appears on the page. Aberration, spherical, of lens, 243. Acoustics, 294. Achromatic lens, 243. Action carriage of piano, 283. Advancing the spark, 102. Air-gun, _342_. Air-pump for cycle tyres, _340_; for Westinghouse brake, 199. Alternating currents, 164; dynamo, 164. Amperage, 125. Angle of advance, 57, 58; incidence, 268; reflection, 268. Aorta, 360. Arc lamp, 182. Archimedes, 412. Armature, 162. Arteries, 358. Arterial blood, 359. Atmospheric pressure, 350. Auditory nerve, 272. Automatic brakes, 188; signalling, 228; stoker, 399. Backfall, 298. Balance-wheel, 419. Ball cock, 366, _367_. Balloon, fire, 323; gas, 347. Barometer, aneroid, 328, _329_; and weather, 331; Fortin's, _326_; meaning of, 325; simple, _328_; wheel, _327_. Beau de Rochas, 89. Bell, diving, _332_; electric, 119, _120_. Bellows of organ, 303. Bioscope, 266. Blades, turbine, _81_, 83. Block system, 201, 212. Blood, arterial, 359; circulation of, _356_, _357_, 360; venous, 359. Blower-plate, 393, _394_. Boat, sails of, 346. Boiler, Babcock and Wilcox, _21_, 22; explosions, 34, 391; fire-tube, 21; fittings, 31; Lancashire, 25, _26_; locomotive, _20_, 23; multitubular, 21; principle of, 15; stored energy in, 32; vertical, _25_; water supply to, 39; water-tube, 21. Brakes, hydraulic, 188; motor car, 110; railway, 187; vacuum, 189, _190_, _191_; Westinghouse, 194, _195_, _197_. Bramah, 363, 437. Breezes, land and sea, 324. Brushes of dynamo, 161, _172_. Bunsen burner, 409. Burning-glass, 232. Camera, the, 233; pinhole, _234_, _235_. Canals, semicircular, 273. Capillary attraction, 392; veins, 358. Carbon dioxide, 27, 359; monoxide, 27. Carburetter, 98, _99_. Cardan shaft, 93. _Carmania_, the, 83. Centrifugal force, 382. Change-speed gear, 105, 442. Chassis of motor car, 92. Circulation of water in a boiler, _17_, _18_, _19_; of water in a motor car, 95, _97_. Clarionet, 308. Clock, first weight-driven, 412; water, 410. Clutch of motor car, 105. Coal, as fuel, 15; gas, 394; gas making, 394; gas plant, _396_; gas, purification of, 397. Cochlea, 273. Coherer, 140. Coil, Ruhmkorff, 121. Coke, 395. Combinations in Chubb lock, 436; Yale lock, 436. Combustion, 26, 393; perfect, 28. Compensating gear, 107, _108_. Compound engines, 59; arrangement of, 61; invention of, 59. Compound locomotives, 62. Compound microscope, 261. Condenser, marine, 71, _72_; of Ruhmkorff coil, 123. Conduit, 176. Convex lens, image cast by, _236_. Conjugate foci, 262. Cornet, 308. Corti, rods of, 274. Coxwell, 348. Cream separator, 381, _383_. Current, reversal of electric, _130_, 131; transformation of, 124. Cushioning of steam, 55. Cycle, gearing of, 439. Cylinder, hydraulic press, _363_; steam, _49_. Danes, 382. Dead point, 47. De Brouwer stoker, 401. Detector in Chubb lock, 435. Diving-bell, _332_; simple, _333_, _334_. Diving-dress, 335. Direction of current in dynamo circuit, 163. Diver's feats, 338; helmet, _336_; lamp, _338_. Donkey-engines, 68. Doorstop, self-closing, 344. Double-cylinder engines, 47. Draught, forced, 28, _29_; induced, 29. Drum and fusee, _414_. Durability of motor-car engine, 96. D-valve, 67. Dynamo, alternating, 164, 174; brushes, _172_; compound, 174; continuous-current, 165; multipolar, 169; series wound, _173_; shunt wound, _173_; simple, 161, _162_. Ear, the, _271_, _273_; a good, 274, 307; sensitiveness of, 275. Eccentric, _52_, 53; setting of, 53. Edison, Thomas, 310. Edison-Bell phonograph, 310. Electricity, current, 115; forms of, 113; nature of, 112; static, 114. Electric bell, 119, _120_; signalling, 225; slot, 226. Electroplating, 185, _186_. Electro-magnets, 117. Endolymph, 272. Engines, compound, 59; donkey, 68; double-cylinder, 47; internal-combustion, 87, 95; reciprocating, 44. Escapement of timepieces, 416; cylinder, _420_; lever, 421, _422_. Ether, 270. Eustachian tube, 276. Eye, human, 246, _247_; self-accommodation of, 248. Expansive working of steam, 56. Faraday, Michael, 159. Field, magnetic, 159; magnets, 171; ring, 174. Filters, 374; Maignen, _373_; Berkefeld, 374. Filtration beds, 372. Flute, 308. Flying-machines, 348. Fly-wheel, use of, 48. Focus, meaning of, 237; principal, 238. Foci, conjugate, 262. Force, lines of, 116. Forces, component, 345. Free wheel, _440_. Furring-up of pipes, 391. Fusee, drum and, 414. Galileo, 259, 325, 416. Galilean telescope, _259_. Gas, coal, 394; governor, 402; meter, 405; traps, 374; works, 394. Gasometer, 397; largest, _398_, 399. Gauge, steam, 36, _38_; water, 35, _36_. Gear, compensating, 107, _108_. Gear-box of motor car, 105. Gearing of cycle, 439. Glaisher, 348. Gland, 50, 363. Glass, flint and crown, 242. Going-barrel for watches, 415. Gooch reversing gear, 65. Governors, speed, 67; of motor car, 103, _104_. Graham, 418. Gramophone, 317; records, 319, 321; reproducer, _318_. Hairspring, 412. Hay-cutter, 451. Heart, the, 355; disease, 361; rate of pulsation of, 361; size of, 357. Heat of sun, 451. Hele, Peter, 412. Helmet, diver's, _336_. Helmholtz, 274, 308. Hero of Alexandria, 74. Herschel, 261. Hertz, Dr., 138. Hertzian waves, 138. Hot-water supply, 386. Hour-hand train in timepieces, _429_. Household water supply, 364. Hughes type-printer, 134. Hydraulic press, 361, _362_. Hydro, 385. Ignition of charge in motor-car cylinder, 100, _101_. Image and object, relative positions of, 239; distortion of, 245. Incandescent gas mantle, 407; electric lamp, 179. Incus, 272. Index mechanism of water-meter, 37. Indicator of electric bell, 119. Induction coil, 121; uses of, 125. Injector, 39; Giffard's, _41_; principle of, 40; self-starting, 42. Interlocking of signals, 204, 222. Internal-combustion engine, 87. Iris of eye, 249; stop, 249. Kelvin, Lord, 158. Keyless winding mechanism, 425, _426_, 428. Kite, 345. Lamp, arc, 182; how it works, 392; incandescent, 179; manufacture of incandescent lamps, 180. Lap of slide-valve, _57_, 59. Larynx, 306. Laxey wheel, _380_, 381. Leads, 208. Lenses, 231; correction of for colour, 240, _241_; focus of, 236; rectilinear, _245_; spherical aberration in, 243. Levers, signal, colours of, 208. Limit of error in cylinder, 52. Light, electric, 179; nature of, 230; propagation of, 231. Li Hung Chang, 157. Lindsay, James Bowman, 145. Lines of force, 116, 162. "Linking up," 65. Locks, 430; Barron, 433; Bramah, 437; Chubb, 433, 434; Hobbs, 437; simplest, _431_; tumbler, _432_; Yale, _436_. Locking gear for signals, 205. Locomotive, electric, 178; advantages of, 179. Lungs, 359. Magic-lantern, 263, _264_. Magnet, 115; permanent, 115, 116; temporary, 115. Magnetism, 115. Magnetic needle, influence of current on, 129. Mainspring, invention of, 412. Malleus, 272. Marconi, 140, 146. Marine chronometers, 415; delicacy of, 425. Marine speed governor, 71. Marine turbine, advantages of, 84. Maudslay, Henry, 363. Maxim, Sir Hiram, 348. Micrometer free wheel, 441. Micro-photography, 265. Microscope, 254; compound, 261, _263_; in telescope, 257; simple, _254_. Mineral oil, 392. Mirror, parabolic, 261, _262_; plane, _267_. Morse, 132, 145; code, 128; inker, 142; sounder, 132. Motor car, the, 92; electric, 177. Mouth, 307. Mowing-machines, 450. Musical sounds, 277. Nerve, auditory, 272; optic, 246. Nodes on a string, 285; column of air, 291. Note, fundamental, 285; quality of, 285. Niagara Falls, power station at, 174. Organ, the, 294, _300_; bellows, 303; console, 305; echo, solo, swell, great, and choir, 301; electric and pneumatic, 305; largest in the world, 306; pedals, 298; pipes, 295; pipes, arrangement of, 295; sound-board, _296_; wind-chest, 297. Otto cycle, 91. Overtones, 285. Pallets of organ, 297. Parallel arrangement of electric lamps, 184. Paris, siege of, 265. Pedals of organ, 298. Pelton wheel, _377_. Pendulum, 412; compensating, 418, _419_. Perilymph, 272. Perry, Professor, 16. Petrol, 98. Phonograph, 310; governor, _311_; recorder, 312, _313_; records, making of, 319; reproducer, 315; tracings on record of, _317_. Pianoforte, 277; sounding-board, 280; striking mechanism, 281; strings, 281. Piccolo, 308. Pipes, closed, 289; flue, 301; open, 292; organ, 295; reed, 301, _302_; tuning, 302. Piston valve, 67. Pneumatic tyres, 341. Poldhu, signalling station at, 138. Points, railway, 208, _210_; and signals in combination, 211. Poles of a magnet, 115. Popoff, Professor A., 138, 145. Power, transmission of, 175. Preece, Sir William, 145. Primary winding of induction coil, 122. Pump, air, 340; bucket, 352, _353_; force, 354; most marvellous, 355; Westinghouse air, 199. Railway brakes, 187; signalling, 200. Rays, converging and diverging, _256_; heat, concentrated by lens, _232_; light, 232, 235, 236, 237. Records, master, 319, 320. Reciprocation, 51. Reed, human, 306; pipes, 301, _302_. Reflecting telescope, 260. Relays, telegraphic, 133, 141. Retina, 247. Retorts, 395. Reversing gear, 62; Allan, 65; Gooch, 65; radial, 66. Rocking bar mechanism for watches, 425. Rods of Corti, 274. Ruhmkorff coil, 121, _122_. Safety-valve, 32, _33_, 391. Sand-glasses, 411. Scissors, action of, _450_. Secondary winding of induction coil, 122. Series arrangement of electric lamps, 183. Series winding of dynamo, _173_. Shunt wound dynamo, _173_. Sight, long and short, 250. Signalling, automatic, 228; electric, 225; pneumatic, 225; power, 225. Signal levers, _206_. Signals, interlocking of, 204; position of, 202; railway, 200; single line, 215. Silencer on motor cars, 109. Siphon, _351_. Slide-valve, 49, 50, 51; setting of, 53. Sliders, 297. Sound, nature of, 270; board of organ, 296; board of piano, 280. Spagnoletti disc instrument, 212. Sparking-plug, _102_. Spectacles, use of, 249. Spectrum, colours of, 230. Speed governors, 67, _68_, _69_; Hartwell, 70; marine, 71. Speed of motor cars, 110. Spot, blind, in eye, 251; yellow, in eye, 251. Spring balance for watches, 419; compensating, 423, _424_. Stapes, 272. Steam, what it is, 13; energy of, 14; engines, 44; engines, reciprocating, _45_; expansive working of, 59, 81; gauge, 36; gauge, principle of, 37; turbine, 74; turbine, De Laval, 76, _77_; turbine, Hero's, 74; turbine, Parsons, 79, _80_; volume of, as compared with water, 15. Stephenson, George, 63, 375. Stop, in lens, 244; iris, 249; use of, 244. Sun-dial of Ahaz, 410. Syntonic transmission of wireless messages, 143. Talking-machines, 310. Tapper in wireless telegraphy receiver, 141. Tappet arm, 205. Telegraph, electric, 127; insulator, _133_; needle, _128_; recording, 133; sounder, 132. Telegraphy, high-speed, 135; wireless, 137. Telephone, 147; Bell, _148_; circuit, double-line, 155; circuit, general arrangement, _152_, 153; exchange, _154_, 155. Telephony, submarine, 157. Telescope, 257; Galilean, _259_; prismatic, _260_; reflecting, 260; terrestrial, _259_. Threshing-machine, 447, _448_. Thurston, Professor, 31. Tides, 452; high, 453; neap and spring, 455. Timbre, 285. Tompion, Thomas, 412. Torricelli, 325. Trachea, 306. Train staff signalling, 216; single, 216; and ticket, 217; electric, 218. Transformation of current, 124, 176. Transmission of power, 174, _175_. Transmitter, Edison telephone, 150; granular carbon, 150, _151_. Triple-valve, 196. Trolley arm, 176. Turbines, steam, 74. _Turbinia_, the, 79. Tympanum, 137, 271, 272. Universal joint, 93. Vacuum brake, 189, _190_, _191_. Vacuum chamber of aneroid barometer, _330_. Valve, piston, 67; safety, 32; of internal-combustion engine, 89. Valves of the heart, 357. Veins, 358; capillary, 358; pulmonary, 361. Ventral segments, 291. Ventricles, 357. Vibration of columns of air, 288, 289; of rods, 287; of strings, 278; of strings, conditions regulating, 278. _Viper_, the, 86. Virag, Pollak--high-speed telegraphy, 136. Vitreous humour, 246. Voltage, 121, 161. Vowel sounds, 308. Wasborough, Matthew, 51. Watches, first, 412. Water cock, _365_; engines, 375; gauge, 35, _36_; jacket, 19, 95; meter, _368_; supply, 371; turbines, 174, 376; wheels, 375. Watt, James, 51, 69, 375. Welsbach incandescent mantle, 407. Westinghouse air-brake, 194, _195_, _197_; George, 194. Wheatstone needle instrument, 128, 131; automatic transmitter, 135. Wind, why it blows, 323; action of on kites, 345; on sails, 346. Windmills, 375. Window, oval, in ear, 272; round, in ear, 272. Wireless telegraphy, 137; advance of, 145; receiver, 140, 141; syntonic, 143; transmitter, 138, _139_. Yale lock, _436_, _437_. Yellow spot, in eye, 251. Zech, Jacob, 414. Zeiss field-glasses, 260. THE END. *** END OF THE PROJECT GUTENBERG EBOOK HOW IT WORKS *** Updated editions will replace the previous one—the old editions will be renamed. 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