WATCH. Timepieces moved by a spiral spring instead of a weight were made as early as the 16th century, though the law which governs the mechanical theory of springs was first enunciated by Huygens in the 17th century (ut tensio sic vis); this, however, is not invariable.

Fig. 1 shows the general arrangement of a watch or chronometer (both of which are here considered together). The barrel and fusee will be re-cognized at once. The fusee is a sort of grooved hollow-sided cone; the more rapid swell towards the thick end is re-quired, because one turn of the fusee, when the chain is at that end, takes much more of it off the barrel than at the thin end; and on the assumption that the force of the spring varies as its tension the radius of the cone must increase more rapidly, in order to make the increase of leverage keep pace with the decrease in the force of the spring as it unwinds with an increasing velocity off the thick end of the fusee. The fusee itself is con-nected with the great wheel by a ratchet and click and going ratchet (of which the spring and click are strongly shown in the figure), just as described under CLOCKS (vol. vi. p. 22). Something is also required to prevent the watch from being overwound, or the chain strained so as to break. This is done by means of a hooked lever, set on a hinge in the upper frame-plate (which is taken off in this drawing); and when the watch is nearly wound up the chain moving upwards reaches this lever, and moves it into such a posi-tion that its hook catches hold of the long tooth projecting from the thin end of the fusee; and thus the winding is stopped without any strain on the chain by the sudden check.
By far the greater number of watches now made both on the Continent and in America have the mechanism known as the going barrel in substitution for the chain and fusee. In the going barrel the mainspring is of great length, and only a few coils of it are brought into action. To the going barrel itself the main wheel is attached, and thus the force of the mainspring is transmitted direct to the escapement. The general adoption of the going barrel mechanism is due to the introduction of keyless winding, which can only be adapted to the fusee watches with diffi-culty, and to the greater cheapness of the arrangement. Moreover, it is found that for three or four coils of a long spring, at a certain degree of winding up, the tension varies very little. The going barrel is not used in the best English work, in which absolute uniformity of motion is aimed at; but for ordinary purposes it is not of so much consequence that the same rate from hour to hour should be maintained provided the daily rate is uniform.
In watches without a fusee the apparatus for preventing overwinding is different from that in the old form of watch; it goes by the name of the Geneva stop, and the principle of it is simple. If two wheels work together, of which one has the spaces between some two or more adjacent teeth filled up, it is evident that that wheel cannot be turned quite round. And it will be the same thing if one of the wheels is only a cylinder with a single tooth in it, and the other has a certain number of notches, not going all round, through which that tooth can pass. If, therefore, a one-toothed wheel of this kind is fixed to the barrel arbor, which is turned by the key, and works into a wheel with only 4 or 5 notches in it and a blank space through which the tooth cannot pass, it will evidently allow the barrel to be wound up the 4 or 5 turns and no more; and as it unwinds it turns the stopping wheel back again with it.

The other parts of a watch do not differ from those of a clock, except in size, and the position in which they are arranged, to bring them within the circle of the dial, until we eome to the escapement; and there a different state of things arises, mainly from the fact that the balance of a watch revolves through sometimes as much as 270°, while a clock pendulum only vibrates through 4° or 5°. The balance being common to all the watch escapements, it will be proper first to describe that, and the conditions to which it is subject. The two equal arms, with equal weights at each end, in fig. 3 of article CLOCKS (vol. vi. p. 17) are really a balance just as much as the wheel which is commonly used as the more convenient form. But in that figure there is not to be seen that essential element of a modern balance—the thin spiral spring, opening and closing itself at every vibration. The outer end of this spring is attached to the frame by a cock R (fig. 2), and the inner to the balance at S; and the time of vibration depends only on the strength of the spring and the moment of _inertia of the balance, and not at all upon < the extent or angle of the vibration. And, as the force of a spring varies (ap-proximately) inversely as its length, this suggests a ready method of regulating the watch; for it is easy to make a pointer or index, or " regulator" PT, turning on a ring fixed to the watch plate, concentric with the balance, and having two pins in it at P, called curb pins, just close enough together to embrace the spring, so that, as the index is moved one way or the other, the length of the spring which is free to vibrate may become shorter or longer. When the regulator has been moved as far as it can go towards fast, suppose, and the watch still loses, the spring has to be shortened at the cock R into which its outer end is pinned; and, in order that the balance may be capable of alteration, so as still to stand square with the escapement when the spring is in its neutral state, the other end is not actually pinned to the balance, but the cock S is on a small ring which is set on the axis or verge of the balance pretty tight by friction, but capable of being turned by hand. An index-plate like that in fig. 2 enables one to see smaller movements of the index than radial marks.

It is almost impossible to move such, a regulator little enough, and with sufficient accuracy, for a very small variation of rate. One way of obtaining greater accuracy, and probably the best, is to make the regulator movable by a tangent screw acting £>n its end, and capable of being turned by the watch key. Another mode of giving a small motion to the regulator is by putting a portion of a wheel with teeth on it, and turning it by a small pinion with the index attached to it, so that the motion of the index exceeds that of the regulator itself as much as the radius of that wheel exceeds the radius of the pinion. Chronometers are regulated in a different way altogether. It is not expedient to alter the effective length of the spring after its length is once fixed. For it has long been ascertained that a spring does not give isochronous vibra-tions at all lengths, but only at certain intervals; and therefore it is necessary in an accurate timekeeper to use only one of those lengths of the spring which are found to be isochronous for different arcs of vibration ; and, that being fixed, the timing of the balance can only be done by altering its moment of inertia, and this is done in chronometers by screws with heavy heads in the rim of the balance, set farther in or out as it is wanted to go faster or slower. In marine chronometers, where there is plenty of room for it, the balance-spring is generally made in a cylindrical form, with the coils all of the same diameter, instead of the flat spiral used in watches,—though it does not seem to be quite clear that the cylindrical form is materially better than the other.
The timing of a watch for position, as it is called, is a matter which requires some attention. If the balance is not exactly poised on its axis, it will have a tendency to take one position when the watch is carried vertically, as it always is in the pocket; and the time of vibration will be affected by its disposition thus to act as a pendulum. The watch ought therefore to be tried with XII, IX, VI, and III successively upwards, and if it does not keep the same rate the balance is not properly poised. Marine chronometers, indeed, being set in gimbals (a ring with the two pivots into the box at right angles with the pivots which carry the chronometer), will remain horizontal, though not without some degree of motion under the motion of the ship ; and this gives the balance the further advantage of having its weight resting only on the end of the axis or verge, a position in which there is much less friction than that of a watch carried in the pocket; but there it is not of so much consequence, because the balance is so much lighter than a chronometer balance.
Compensated Balances.—A pendulum requires scarcely any compensation except for its own elongation by heat; but a balance re-quires compensation, not only for its own expansion, which increases its moment of inertia just like the pendulum, but far more on account of the decrease in the strength of the spring under increased heat. Dent, in a pamphlet on compensation balances, gave the following results of some experiments with a glass balance, which he used for the purpose on account of its less expansibility than a metal one :—at 32° F., 3606 vibrations in an hour ; at 66°, 3598-5 ; and at 100°, 3599. If therefore it had been adjusted to go right (or 3600 times in an hour) at 32°, it would have lost 7J and 8J seconds an hour, or more than three minutes a-day, for each successive in-crease of .34°, which is about fifteen times as much as a common wire pendulum would lose under the same increase of heat; and if a metal balance had been used instead of a glass one the difference would have been still greater.
The necessity for this large amount of compensation having arisen from the variation of the elasticity of the spring, the first attempts at correcting it were by acting on the spring itself in the manner of a common regulator. Harrison's compensation consisted of a compound bar of brass and steel soldered together, having one end fixed t^ the watch-frame and the other carrying two curb pins which embraced the spring, as was described at fig. 2. As the brass expands more than the steel, any increase of heat made the bar bend ; and so, if it was set the right way, it carried the pins along the spring, so as to shorten it. This contrivance is called a com-pensation curb ; and it has often been reinvented, or applied in a modified form. But there are two objections to it: the motion of the curb pins does not correspond accurately enough to the varia-tions in the force of the spring, and it disturbs the isochronism, which only subsists at certain definite lengths of the spring.
The compensation which was next invented left the spring un-touched, and provided for the variations of temperature by the construction of the balance itself. Fig. 3 shows the plan of the ordinary compensation balance. Each portion of the rim of the balance is composed of an inner bar of steel with an outer one of brass soldered upon it, and carrying the weights b,b, which are screwed to it. As tin-temperature increases, the brass expanding must bend the steel inwards, and so carries the weights farther in, and diminishes the moment of inertia of the balance. The metals are generally soldered together by pouring melted brass round a solid steel disk, and the whole is afterwards turned and filed away till it leaves only the crossbar in the middle lying flat and the two portions of the rim standing edgeways. The first person who practised this method of uniting them appears to have been Thomas Earnshaw, who brought the chronometer to the state in which it has remained for the last century, with scarcely any alteration except more com-plete compensation.
The adjustment of a balance for compensation can only be done by trial, and requires a good deal of time. It must be done in-dependently of that for time,—the former by shifting the weights, because the nearer they are to the crossbar the less distance they will move over as the rim bends with them. The timing is done by screws with heavy heads (t,t, fig. 3), which are just opposite to the ends of the crossbar, and consequently not affected by the bend-ing of the rim. The compensation may be done approximately by the known results of previous experience with similar balances ; and many watches are sold with compensation balances which have never been tried or adjusted, and sometimes with a mere sham compensation balance, not even cut through.
Secondary Compensation. —When chronometers had been brought to great perfection it was perceived that there was a residuary error, which was due to changes of temperature, but which no adjust-ment of the compensation would correct. For, if the compensation was adjusted for two extreme temperatures, such as 32° and 100°, then the chronometer gained at mean temperatures; and, if ad-justed for any two mean temperatures, it would lose for all beyond them. This error was observed, and attempts were made to correct it before anybody had pointed out how it arose: this appears to have been first done in a paper in the Nautical Magazine by E. J. Dent in 1833; and he gave the following illustration of it. The variation of the force of the spring proceeds uniformly in proportion to the temperature, and therefore may be represented by a straight line inclined at some angle to another straight line divided into degrees of temperature. But the inertia of a balance of the common construction cannot be made to vary uniformly according to the temperature, but will vary more rapidly in cold than heat ; and con-sequently its rate of variation can only be represented by a curve, and the curve can only be made to coincide with the straight line representing the rate of variation of the spring in two points, —either two extremes, or two means, or one extreme and one mean point.
The same thing may be shown mathematically, as follows. Let r be the distance of the compensation weights 6, b, in fig. 3 (which we may assume for convenience to be the whole mass M of the balance) from the centre at some mean temperature, and let dr be their increase of distance due to a decrease of some given number of degrees of heat, under the action of the compensation bars. Then the new moment of inertia will be M (r2 + 2rdr + dr2), and the ratio
of the new to the old will be 1 + + ' antl the term ^
is now too large to be disregarded, as it might be in pendulums, dl
where the compensation -j- is only required to be about ^jth (dr \
of the ( — J in a balance. It is found that an equal increase
of temperature will produce an equal or rather a less motion ( - dr)
of the weights towards the centre than from it at any given point;
inertia to the original one will be 1 -
but, calling it only equal, the ratio of the decreased moment of
2dr , (dry* .
— +1 — I , so that the
— ) ; in other words, the moment of inertia of the balance varies
less in passing from mean to hot temperature than from mean to cold; and consequently if it is adjusted for mean and cold it will not have decreased enough at an equal increase from mean to hot, or the chronometer will lose, and if adjusted for the two extremes it will gain at mean temperatures.
The correction of this error is called the secondary compensation,
increase and the decrease from the mean amount differ by twice


We shall give a short description of the principal classes of inven-tions for this purpose.
The first that was disclosed »vas Eiffo's (sometimes called Moly-neux's), communicated to the astronomer-royal in 1835. In one of several methods proposed by him a compensation curb was used ; and, though, for the reasons given before, this will not answer for the primary compensation, it may for the secondary, where the motion required is very much smaller. In another the primary compensation bar, or a screw in it, was made to reach a spring set within it with a small weight attached at some mean temperature, and, as it bent farther in, it carried this secondary compensation weight along with it. The obvious objection to this is that it is discontinuous ; but the whole motion is so small, not more than the thickness of a piece of paper, that this and other compensations on the same principle appear to have been on some occasions quite successful. Molyneux took a patent for a secondary compensation exactly the same as this of Eiffe's, then before the astronomer-royal.

Another large class of balances, all more or less alike, may be represented by E. J. Dent's, which came next in order of time. He described several forms of his invention ; the following descrip-tion applies to the one he thought the best. In fig. 4 the flat crossbar rr is itself a compensation bar which bends upwards under increased heat; so that, if the weights i>, v were merely set upon up-right stems rising from the ends of the cross-bar, they would approach the axis when that, bar bends upwards. But, instead of the stems rising from the crossbar, they rise from the two secondary compensation pieces stu, in the form of staples, which are set on the crossbar; and, as these secondary pieces themselves also bend upwards, they make the weights approach the axis more rapidly as the heat increases ; and by a proper adjustment of the height of the weights on the stems the moment of inertia of the balance can be made to vary in the proper ratio to the variation of the intensity of the spring. The cylindrical spring stands above the crossbar and between the staples.

Fig. 5 represents Loseby's mercurial compensation balance.
Besides the weights D, D, set near the end of the primary compen-
sation bars B, B, there are small
bent tubes FE, FE with mercury
in them, like a thermometer, the
bulbs being at F, F. As the heat
increases, not only do the primary
weights D, D and the bulbs F, F
approach the centre of the bal-JjcUj
ance, but some of the mercury is M'
driven along the tube, thus carry-
ing some more of the weight
towards the centre, at a ratio in-
creasing more rapidly than the
temperature. The tubes are sealed
at the thin end, with a little air JIJ„ 5
included. The action is here
equally continuous with Dent's, and the adjustments for primary and secondary compensation are apparently more independent of each other; and this modification of Le Roy's use of mercury for compensated balances (which does not appear to have answered) is certainly very elegant and ingenious. Nevertheless an analysis of the Greenwich lists for seven years of Mr Loseby's trials proved that the advantage of this method over the others was more theoretical than practical; Dent's compensation was the most successful of all in three years out of the seven, and Loseby's in only one. Loseby's method has never been adopted by any other chronometer-maker, whereas the principles both of Eiffe's and of Dent's methods have been adopted by several other makers.
A few chronometers have been made with glass balance-springs, which have the advantage of requiring very little primary and no secondary compensation, on account of the very small variation in their elasticity, compared with springs of steel or any other metal.
greater when it is being curved by heat than when it is pulled straighter by cold, which is exactly what is wanted. The difference is not quite so great as it ought to be for complete secondary com-pensation for a very wide range of temperature, but it is enough to give the requisite compensation for all ordinary variations of tem-
Dent also invented a very different method of effecting the primary and secondary compensation at once, and without any additional appendage to the balance or addition to the cost. He called it the prismatic balance, from the shape of the steel rim, of which the section is shown in fig. 6, BG being the brass, and the dark triangle within it the section of the steel part of the rim. A prism of cast-steel will bend more easily from the edge than the other way, and consequently the motion is perature, and chronometers so compensated were found to be also more than usually steady in their rate, for even in the best chrono. meters there appear every now and then quite capricious variations.
Several other forms of secondary compensation have been invented and found successful, all on the same principle of so arranging the compound bars that the weights move more under any given change of temperature in hot weather, or when they are nearest to the axis of the balance, than in cold when they are farthest off. Notices of these may be seen in various volumes of the Horological Journal.
The best chronometers, with all these improvements, cannot be relied on to keep a rate equal to that of a good astronomical clock of the usual kind, though they occasionally do so for a short time.
Watch Escapements.-—The escapements in practical use are—(1) the old vertical escapement, now almost disused ; (2) the lever, very much the most common in English watches ; (3) the horizontal or cylinder, wdiich is equally common in foreign watches, though it was of English invention ; (4) the duplex, which used to be more in fashion for first-rate watches than it is now ; and (5) the detached or chronometer escapement, so called because it is always used in marine chronometers.

The vertical escapement is simply the original clock escapement (see CLOCKS, fig. 3) adapted to the position of the wheels in a watch and the balance, in the manner exhibited in fig. 7. As it requires considerable thickness in the watch, it is inferior in going to all the others, and no cheaper than the lever escapement can now be made; and lor these reasons it has gone out of use.

The lever escapement, as it is now uni-versally made, was invented late in the last century by Thomas Mudge. Fig. 8 shows its action. The position of the lever with reference to the pallets is immaterial in prin-ciple, and is only a question of convenience in the arrangement; but it is generally such as we have given it. The principle is the same as in the dead-beat escapement clock (see CLOCKS, fig. 5), with the advantage that there is no friction on the dead faces of the pallets beyond what is necessary for locking. The reason why this friction cannot be avoided witli a pendulum is that its are of vibration is so small that the requisite depth of intersection cannot be got between the two circles described by the end S of the lever and any pin in the pendulum which would work into it; whereas, in a watch, the pin P, which is set in a cylinder on the verge of the balance, does not generally slip out of the nick in the end of the lever until the balance has got 15° past its middle position. The pallets are under-cut a little, as it is called, i.e., the dead faces are so sloped as to give a little recoil the wrong way, or slightly to resist the unlocking, because otherwise there would be a risk that a shake of the watch would let a tooth escape while the pin is disengaged from the lever. There is also a further provision added for safety. In the cylinder which carries the impulse pin P there is a notch just in front of P, into which the other pin S on the lever fits as they pass; but when the notch has got past the cylinder it would pre-vent the lever from returning, because the safety-pin S cannot pass except through the notch, which is only in the position for letting it pass at the same time that the impulse-pin is engaged in the lever. The pallets in a lever escapement (except bad and cheap ones) are always jewelled, and the scape-wheel is of brass. The stall'of the lever also has jewelled pivot-holes in expensive watches, and the scape-wheel has in all good ones. The holes for the balance-pivots are now always jewelled, if nothing else is. The scape-wheel in this and most of the watch escapements generally beats five times in a second, in large chronometers four times ; and the wheel next to the scape-wheel carries the seconds-hand. Macdowall's single-pin escapement was adapted to watches exactly as the dead escapement of clocks is turned into the lever escapement of watches.

Fig. 9 is a plan of the horizontal or cylinder escapement, cutting through the cylinder, which is on the verge of the balance, at the level of the tops of the teeth of the escape-wheel; for the trian-gular pieces A, B are not flat projections in the same plane as the teeth, but are raised on short stems above the plane of the wheel; and still more of the cylinder than the portion shown at ACD is cut away where the wheel itself has to pass. The author of this escapement was Graham, and it resembles his dead escapement in clocks in principle more than the lever escapement does, though much less in appearance, because in this escapement there is the dead friction of the teeth against the cylinder, first on the outside, as here represented, and then on the inside, as shown by the dotted lines, during the whole vibration of the balance, except that portion which belongs to the impulse. The impulse is given by the oblique outside edges Ao, B6

of the teeth against the edges A, D of the cylinder alternately. The portion of the cylinder which is cut away at the point of action is about 30° less than the semicircle. The cylinder itself is made either of steel or ruby, and, from the small quantity of it which is left at the level of the wheel, it is evidently a very delicate affair ; and probably this has been the main reason why, although it is an English invention, it has been almost entirely abandoned by the English watchmakers in favour of the lever, which was originally a French invention, though very much improved by Mudge, for before his invention the lever had a rack or portion of a toothed wheel on its end, working into a pinion on the balance verge, and consequently it was affected by the dead friction, and that of this wheel and pinion besides. This used to be called the rack lever, and Mudge's the de-tached lever ; but, the rack lever being now quite obsolete, the word "detached" has become detached from the lever escapement, and confined to the chronometer, to which it is more appropriate, as will be seen presently. The Swiss watches have almost universally the horizontal escapement. It is found that—for some reason which is apparently unknown, as the rule certainly does not hold in cases seemingly analogous—a steel scape-wheel acts better in this escapement than a brass one, although in some other cases steel upon steel, or even upon a ruby, very soon throws off a film of rust, unless they are kept well oiled, while brass and steel, or stone, will act with scarcely any oil at all, and in some eases with none.

The duplex escapement (fig. 10) is probably so called because there is a double set of teeth in the scape-wheel,—the long ones (like those of the lever escapement in shape) for locking only, and short ones (or rather upright pins on the rim of the wheel) for giving the impulse to the pallet P on the verge of the balance. It is a single-beat escapement; i.e., the balance only receives the impulse one way, or at every alternate beat, as in the chrono-meter escapement, and in a few clock escapements which have never come into use. When the balance is turning in the direction marked by the arrow, and arrives at the position in which the dotted tooth b has its point against the triangular notch V", the tooth end slips into the notch, and, as the verge turns farther round, the tooth goes on with it till at last it escapes when the tooth has got into the position A ; and by that time the long tooth or pallet which projects from the verge has moved from p to P, and just come in front of the pin T, which stands on the rim of the scape-wheel, and which now begins to push against P, and so gives the impulse until it also escapes when it has arrived at t; and the wheel is then stopped by the next tooth B having got into the position b, with its point resting against the verge, and there is evidently what we have called dead friction between them ; but, as the verge is smaller than the cylinder of the horizontal escapement, and is also made of a jewel, the friction does not seriously affect the motion of the balance. The impulse is also given very directly across the line of centres, and therefore with very little friction, as in the three-legged dead escapement for clocks and in the chronometer escape-ment. A little impulse is also received from the long teeth on the notch ; but the greatest part of that motion is wasted. As the balance turns back, the nick V goes past the end of the tooth b, and in consequence of its smallness, it passes without visibly affecting the motion of the scape-wheel, though of course it does produce a very slight shake in passing. It is evident that, if it did not pass, the tooth could not get into the nick for the next escape. The objection to this escapement is that it requires very great delicacy of adjustment, and the watch also requires to be worn carefully; for, if by accident the balance is once stopped from swinging back far enough to carry the nick V past the tooth end, it will stop alto-gether, as it will lose still more of its vibration the next time from receiving no impulse. The performance of this escapement, when well made, and its independence of oil, are nearly equal to those of the detached escapement; but, as lever watches are now made suffi-ciently good for all but astronomical purposes, for which chrono-meters are used, and they are cheaper both to make and to mend than duplex ones, the manufacture of duplex watches has almost disappeared.
The chronometer or detached escapement is shown at fig. 11, in the form to which it was brought by Earnshaw nearly a century ago, and in which it has remained ever since, with the very slight difference that the pallet P, on which the impulse is given (corre-sponding exactly to the pallet P in the duplex escapement), is now generally set in a radial direction from the verge, whereas Earn-shaw made it sloped backward, or undercut, like the scape-wheel teeth. The early history of escapements on this principle does not seem to be very clear. They appear to have originated in France; but there is no doubt that they were considerably improved by the first Arnold, who died in 17§9. Earnshaw's watches, however, generally beat his in trials.
In fig. 11 the small tooth or cam V, on the verge of the balance, is just on the point of unlocking the detent DT from the tooth T of the scape-wheel; and the tooth A will immediately begin to give the impulse on the pallet P, which, in good chronometers, is always a jewel set in the cylinder ; the tooth V is also a jewel. This part of the action is so evident as to requir no further notice. When the balance returns, the tooth V has to get past the end of thei detent, without disturbing it; for, as soon as it has been unlocked, it falls against the banking-' pin E, and is ready to receive the next tooth B, and must stay there until it is again un-locked. It ends, or rather begins, in a stiffish spring, which is screwed to the block D on the watch frame, so that it moves without any friction of pivots, like a pendulum. The passing is done by means of another spring TV, called the passing spring, which can be pushed away from the body of the detent towards the left, but cannot be pushed the other way without carrying the detent with it. In the back vibration, therefore, as in the duplex escapement, the balance receives no impulse, and it has to overcome the slight resistance of the passing spring besides ; but it has no other friction, and is entirely detached from the scape-wheel the whole time, except when receiving the impulse. That is also the case in the lever escape-ment ; but the impulse in that escapement is given obliquely, and consequently with a good deal of friction ; and, besides, the scape-wheel only acts on the balance through the intervention of the lever, which has the friction of its own pivots and of the impulse pin. The locking-pallet T is undercut a little for safety, and is also a jewel in the best chronometers ; and the passing spring is of gold, as steel will rust. In the duplex and detached escapements, the timing of the action of the different parts requires great care, i.e., the adjusting them so that each may be ready to act exactly at the right time ; and it is curious that the arrangement which would be geometrically correct, or suitable for a very slow motion of the balance, will not do for the real motion. If the pallet P were really set so as just to point to the tooth A in both escapements at the moment of unlocking (as it has been drawn, because otherwise it would look as if it could not act at all), it would run away some distance before the tooth could catch it, because in the duplex escapement the scape-wheel is then only moving slowly, and in the detached it is not moving at all, and has to start from rest. The pallet P is therefore, in fact, set a little farther back, so that it may arrive at the tooth A just at the time when A is ready for it, without wasting time and force in running after it. This, however, seems now to be doubted in practice. The detached escapement has also been made on the duplex plan of having long teeth for the locking and short ones or pins nearer the centre for the impulse; but the advantages do not appear to be worth the additional trouble, and the force required for unlocking is not sensibly diminished by the arrangement, as the spring D must in any case be pretty stiff, to provide against the watch being carried in the position in which the weight of the detent helps to unlock it.
An escapement called the lever chronometer has been several tinie3 reinvented, which implies that it has never come into general use. It is a combination of the lever as to the locking and the chrono-meter as to the impulse. It involves a little drop and therefore waste of force as a tooth of the wheel just escapes at the '' passing " beat where no impulse is given. But it should be understood (as it is not by some wdio write on clock-work) that a single-beat escape-ment involves no more loss of force and the escape of no more teeth than a double one, except the slight drop in the duplex and this lever chronometer or others on the same principle.
There have been several contrivances for remontoire escapements; but there are defects in all of them ; and there is not the same ad-vantage to be obtained by giving the impulse to a watch-balance by means of some other spring instead of the mainspring as there is in turret-clocks, where the force of the train is liable to very much greater variations than in chronometers or small clocks.
Tourbillon escapements, and a few other things not necessary to notice here, are described in the 7th edition of Sir E. Beckett's (now Lord Grimthorpe) Rudimentary Treatise on Clocks, Watches, and Bells.
Repeaters, Keyless Watches, &e.—Repeating-watches, i.e., watches which strike the hours and quarters on pushing in the handle, are now scarcely ever made in England, and with very good reason, for it is almost impossible to crowd into the space of even a large-sized watch the quantity of wheels and other things required for the repeating work without unduly interfering with the going part, and, besides that, the striking work itself is very liable to get out of order.
The winding of watches without a key is an object for which there have been several inventions, and it possesses a considerablo advantage, besides the mere convenience of being independent jf t key, for, as there is then no occasion to open it, the case may bo made to fit more closely, and the aii is more completely excluded.

and consequently the watch will go longer without cleaning ; and it also saves the thickness and the cost of a double back to the case. The first plan of the kind was that of pulling out the knob of the handle, which went into the watch, and had a gathering click attached to it which wound up the fusee, or the barrel, by means of a ratchet. But this was not found to answer: it was liable to get out of order; and, moreover, at every time of winding fresh air was pumped into the watch, which soon produced injurious effects. A far better plan is that of combining the two objects of winding and setting the hands by means of the handle, in the manner we shall now describe. In fig. 12 d is a wheel on the barrel, with bevelled teeth, and there is an-other small bevelled wheel on a spindle b, which ends in a milled head a, within the handle or pendant; these two wheels cannot conveniently be arranged so as to work into each other without the inter-vention of a third between them, which is marked e in the left-hand section. It is eai,y to see that turn-ing the milled head will wind up the barrel. The same arrangement might of course be applied to the fusee, though it would increase the size ; but in fact these watches are made without one, and the practice is increasing. The winding wheel d is also made with the well-known contrivance of Breguet, known by the name of the " tipsy key " wdien applied to a common winding key, which enables you to turn the handle the wrong way without doing anything except moving a ratchet-wheel over its click, and consequently without straining the watch in attempting to wind it the wrong way. The same handle and wheels are also made use of to set the hands, thus :—there is a small wheel / which turns on a stud at the end of the lever fgh, and as the lever turns on a pivot at g, when its end h, which just projects through the rim of the watch, is pushed on one side, the wheel/will then be thrown into gear with the winding wheel d and the hour pinion in the middle of the watch ; and consequently, if the handle is then turned, it will alter the hands, just as they are usually altered from the back by a key in foreign watches, so that the face need never be opened. Of course, while this is doing, you do at the same time wind up the watch a little if the hand has to be turned the way for winding; but that is of no consequence, except that you cannot put the hands forward immediately after you have completely wound up the watch. There are various other arrangements for winding and setting by the handle substantially on the same principle. The ring is difficult to fix firmly when flexible on a stud perforated for the winding arbor, as no screw can go through it. The ring and the stud should be made in one piece. There is no use in it being hinged, as usual.
In the chronograph watch there is, in addition to the centre seeonds-hand, an independent seconds-hand which, when not in operation, stands at zero. Pressure on the crown-piece acts succes-sively (1) on a starting motion, (2) on a stopping motion, and (3) on a motion which sends the hand back by the shortest path to zero.
Watches are also made with what are called split seconds-hands, —the two hands being in their ordinary state together and appear-ing as one, but when you push in a knob one of them is stopped, while the other goes on ; the time shown by the stopped one is of course the time of the observation. Sometimes this is done by merely connecting the hands by a very slight spiral spring, which will allow itself to be untwisted one or two coils without stopping the watch; and, as it cannot be of any use to stop the seconds-hand longer than a minute, this seems to answer. There is, how-ever, another plan, in which these two hands, or at least the socket of one and the arbor of the other, are connected by a pair of disks set obliquely on the arbor and the socket respectively, so that, whenever the spring which keeps them together is allowed to act, it brings the loose hand up to the hand fixed on the arbor; and it does not signify how long it may be stopped by throwing the disks out of contact. One of the disks is heart-shaped, and is connected with the other by a spring, forming what is called a. jumper.
For the use of electrical engineers and others who are brought within the influence of powerful electrical machinery, it has been found necessary to introduce non-magnetizable watches. At present this is best secured by making the balance of silver or platinum alio}7, and the balance spring of gold or palladium. The use of steel in moving parts of the works is carefully avoided, and thus fairly good timekeepers indifferent to magnetic influences are pro-duced.
The introduction of machinery for the manufacture of watch movements has had the effect of greatly cheapening the commoner class of watches, and yet supplying a fairly satisfactory timekeeper.
It is in America that the application of machinery to watchmaking has found its greatest development, and its success has enabled the American manufacturers to obtain considerable foothold- in the European market for cheap watches. But watch movements are also now very extensively made by machinery in Birmingham, Coventry, and several Lancashire towns.
Under the auspices of the Royal Society a department has been
established at Kew observatory for the testing and certifying of
watches in respect of their compensation for variations of tempera-
ture, and their uniformity of motion in different positions.
Watches which obtain certificates of the first class have also
awarded to them merit marks up to 40 for complete absence of
variation in daily rate, 40 for absolute freedom from change of rate
in different positions, and 20 for perfect compensation for variations
of temperature. The testing establishment has only been insti-
tuted for a few years; but its services are being largely sought by
manufacturers of the high class of watches, who have laboured
with equal zeal and success to keep the English-made watch un-
approached by the product of any other nation. (G.)