This simple apparatus is all which is required for measuring time, the other mechanism of the watch being devoted to two objects : first, to give a small impulse to the balance at each vibration, to over come the friction and resistance of the air, and cause the balance to describe equal arcs : and, secondly, to register the num ber of vibrations the balance has made.
The first of these objects requires a power which shall be in constant readi ness to act upon the balance. This is accomplished by the re-action of a spiral steel spring, A, (fig. 8) which when at rest and liberty assumes that position and size ; it is coiled up closer, and put into a brass box, a, called the spring barrel ; a small hook which is at the outer end of the spring being put through a hole in the side of the box, a, small arbor, B, is put into the centre of the box, and the cover or lid of the box, D, is shut in : the arbor has a hook projecting from it, which en ters a hole in the inner end of the spring A : its pivots project through the barrel at each side, and enter holes in the two plates E E, (fig. 5) which tbrms the frame of the watch ; the lower pivot passes through the plate, and has a small racket wheel, b, (fig. 3 and 5) fixed upon it, a click entering the teeth thereof pre vents the arbor turning round ; a small steel chain is hooked to the spring-bar rel, a, (fig. 1 and 5) ; at one end it passes round the barrel several times, then round the fusee, d, and is hooked to it by its end. The fusee, d, is a conical piece of brass, with a spiral groove cut there on to receive the chain : it is mounted on pivots which turn in holes, in the two plates, E E, and one of the pivots, e, pro jects a considerable distance, and is cut square. Now if a key is applied to this square, and the fusee, d, by that means turned round so as to wind the chain up on it, the spring barrel will be turned round, and the outer end of the spring, A, being hooked to the barrel, will be turn ed round also; as the inner end is im moveable, by being fixed to the arbor, B, the spring will be coiled up into a closer spiral than it was when at liberty, and will consequently exert a re-action upon the chain, and by that means upon the fusee, which will be turned round thereby when the key is removed. To prevent too much chain being wound up on the fusee, and by that means break ing the chain, or overstraining the spring, a contrivance called a guard is added : it is a small lever, x (fig. 1) moving on a stud fixed to the upper plate of the watch, and pressed downwards by a small spring, : as the chain is wound upon the fusee, it rises in the spiral groove, and lifts up the lever, x, until it touches the upper plate : it is then in a position to intercept the edge of the spi ral piece of metal seen on the top of the fusee, and thus stop it from being wound up any further.
The power of the spring is transmit ted to the balance by means of several cog wheels : the first, f, is upon the fu see ; it is shown separated from the fu see in fig. 6, having a hole through the
centre to receive the arbor of the fusee, and a projecting ring upon its surface ; the under surface of the fusee is shown in fig. 7, having a circular groove cut in it to receive the corresponding ring upon the great wheel, fig. 6 : the inner edge of the groove is cut with teeth to form a rachet wheel ; when the wheel and fu see are put together, a small click, 8., (fig 6) takes into the teeth of the rachet : as the fusee is turned by the key, to wind up the watch, this click slips over the sloping sides of the teeth without turning the great wheel ; but when the fusee is turned the other way by the chain, the click catches the teeth of the rachet wheel, and causes the cog wheel to turn with the fusee ; the great wheel,f, has forty-eight teeth, and turns a pinion of twelve teeth on an arbor in the centre of the watch, which carries the minute hand : a wheel, It, of fifty-four teeth, called the centre wheel, is fixed upon this arbor, and turns a pinion on the same arbor with the third wheel, k, of forty-eight teeth, which turns the pinion of the contrate wheel, 1, of forty-eight teeth ; the contrate wheel gives motion to a pinion of six teeth, and to the balance wheel, m, which has fifteen large teeth, which stop against two small pallets upon the arbor of the balance, or verge, r: these pallets are two small teeth, projecting from the verge at right angles to each other ; one engages the upper side of the wheel, and the other takes the lower. By the action of the mainspring, a, the wheels are all turned; and the balance wheel, ta, if there was no obstruction, would turn with great velocity until all the chain was wound off the fusee ; but one of the pel lets of the verge is always engaged with one of the teeth of the,wheel, suppose one of the teeth on the lower side ; now, by the balance turning round to make a vibration, the pallet allows the tooth to slip off, and the wheel begins to run down by the action of the main-spring, Marking the vibrations by moving the hands, G H; it is, however, stopped im• med;ately, by the next tooth at the top of the wheel meeting the upper pallet of the verge: the balance and pallet was at that time just. beginning to return, and the top of the wheel moving in contrary di ruction to the bottom, the tooth presses against the pallet, and assists the balance to maintain the same arc in its vibration : when the balance is about to raurn, the upper tooth of the wheel slips off the pallet, and the lower one catches on the lower pallet, and assists the balance as before : one of the pivots of the balance wheel works in a small frame, y, called the pottance; the lower pivot of the verge works in the bottom of it also, and the tipper pivot turns in a cock, F, screw ed to the plate, E, and covering the ba lance to defend from injury.