We shall now proceed to describe a repeating watch with a horizontal or cylindrical 'scapement of Graham's.
What has been said concerning the repetitions in pendulum clocks, and the simple or plain watch, being once well understood, the reader will have no difficulty in comprehending the mechanism of a repeating watch, which is only on a small scale what the clock is on a great scale.
Fig. I. of Plate CCCVI. represents the wheel-work of the movement and of the repetition, and all the pieces which are put within the frame-plates. There is a dis tinction here between the wheels,--those of the movement, or which serve to measure the time, as the wheels B, C, D, E, F, and those of the repetition, which serve to regu late the interval between the blows of the hammer : such are the wheels a, b, c, d, e J, whose assemblage is called the little wheel work or runners.
The spring of the movement is contained in the bar rel A ; B is the great or fusee wheel ; C the centre or second wheel, whose arbor prolonged carries the cannon pinion on which the minute hand is fitted and adjusted ; D is the third wheel ; E the fourth wheel ; and F the cylinder, 'scapement, or balance wheel. The fusee I is adjusted on the great wheel B, a spring-tight collet and pin keeping the wheel to its place on the fusee ; the chain is wound round on the fusee, and holds likewise of the barrel. The hook 0 of the fusee serves to stop the hand, on the watch being full wound up, by the hook stopping against the end of the guard de gut stop (the name it got before the chain was put to the fusee ; the modern name of it is the fusee stop,) C (Fig. 2.) attached to the other plate ; its effect is the same as in the plain watch. Fig. 3. of Plate CCCII. represents the cylinder 'scape ment, of which a description has already been given in p. 484.
B is the balance fixed on the cylinder ; F is the cylin der wheel, which is represented as to act on the cylinder, and cause vibrations to be made by the balance. None of the pieces are drawn here, such as the cock, slide, curb, and pendulum or spiral spring, as they would have rather made the 'scapement part obscure. The
wheel work, or runners of the repetition, is composed of five wheels, a, b. c, d, e, and of the pinion f, and of four other pinions. The effect of this wheel work is to regu late the interval between each blow of the hammer ; so that if the first wheel a is made to have 42 teeth, the second b 36, the third c 33, the fourth d 30, and the fifth e 25 ; and moreover, if !I the pinions into which these wheels pitch have six leav's or teeth ; then, in the time that the first wheel a mak• s a turn, the pinion f will make 4812 revolutions; but the ratchet R, which the first wheel a carries, is commonly divided into 24 parts, the half of which are afterwards taken away, in order that there may remain only 12 to strike 12 blows for the 12 hours. If, then, we divide 4812 by 24, we shall have the num5er or turns that the filth pinion makes for each blow of the hammer, which gives 2ouL turns of the pinion for one tooth of the ratchet R.
The first wheel a, or great wheel of the striking part, carries a click and a spring, on act a small ratchet, put under the ratchet wheel R, which forms click and ratchet work, like what has been seen in the first wheel of the repetition (Plate CCCV. Fig. I.) which has the same use ; that is to say, when we push the pendant or pusher, the ratchet R retrogrades, without the wheel a turning ; and the spring which.is in the barrel B (Fig. 2.) bringing back the ratchet R, on whose arbor g, the inner end of the spring is hooked ; the small ratchet comes butt against the click, and turns the wheel a ; and the ratchet R makes the hammer M to strike, whose arm in is.engag ed with the teeth of this ratchet.
The spring r attached to the plate (Fig. 2 ) acts on the small part n of the arm in (Fig. I.) The effect of this spring is to press the arm in against the teeth of the ratchet ; so that when we make the watch to repeat, the ratchet R retrogrades, and the spring r brings always back the arm in, in order that the teeth of the ratchet may make the hammer to strike.—Let us now pass on to the description of the motion work.