WATCH

That part of the machine which, by its inertia, regulates the beat and produces le motion. It is formed of a wheel or ring, having its weight principally accumulated in its rim, and connected with a spiral spring in such a. manner that when drawn from the position of rest il acquires U oscillatory i from the alternate contraction and expansion of the spring. The balance answers the same purpose in watch work as (he pendulum in clock-work. ■••din a similar manner by variations of temperature. Supposing the length of the spring to remain constant, the time of vibration is directly proportional to the distance of the centre Df gyration from the axis of the balance; consequently the duration of the vibration Is Increased by heat and diminished by cold. To remedy this Inconvenience, i contrivances have been applied: but that which is most illy adopted la the expansion or compensation balance, the principle of which depends on the unequal expansion of two different metals. It may be constructed in the following manner: The rim contwo I imiu.v. the inner of steel and the outer of bra , united by fusion. After being turned 10 the proper size, the rim is cut in. A, B, and C; and •ich of the parts thus formed being fixed to an arm of the balance, the other is left at liberty to move inwards or outwards according to the variation of the relative lengths of the two metals. I), E, F, are three equal weights placed near the free ends, and which, when adjusted at the proper distances, are fixed in their places by means of screws. Three heavy-headed screws, <;. II, I, enter the arms of the balance, and serve to adjust its centre of gravity to the axis of vibration, and also to increase or diminish the mean rate of motion. Now, suppose the balance to receive an increase of temperature, this will tend to lengthen the arms, and consequently to throw the centre of gyration further from the axis, and thereby diminish the velocity of vibration; but as the bra expands more than the steel, the compound bars will at the same time be bent inwards, and the weights I), E, F, thereby thrown nearer the axis, by which the velocity will be increased. When the temperature is diminished, the arms are shortened; but the weights are now thrown to a greater distance from the axis by reason of the contraction of the bra being greater than that of the steel, and the consequent tendency of the compound liars lo a ume a le curved form. It is easy to see •hat in so small a machine the adjustment of the parts to procure exact compensation ia matter of great nicety, and requiring much practical skill; in fact, it is only po i compensation bv repeated trials. (See Thomson on Time and Timekeepers; Dent on the Construction and Management of Chronometers, Watches, and Clocks. «fcc.) [s. 131]