Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/423

W A T C H Fig. 10. 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 Mudgo, 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 cases 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, arc 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 1799. Earnshaw s watches, however, generally beat his in trials. Fi 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. hi 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 times 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 who 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 rcnwwfoiYe 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, il-c. 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 considerable advantage, besides the mere convenience of being independent of a key, for, as there is then no occasion to open it, the case may be

made to fit more closely, and the air is more completely excluded,