WATCH

That part of the machine which, by its inertia, regulates the beat and produces equable motion. It is formed of a wheel or ring, having.'17 BALANCE OF POWER. its weight principally accumulated in its rim, and connected with a spiral spring in such a manner that when drawn aside from the position of rest it acquires an oscillatory motion from the alternate contraction and expansion of the spring. The balance answers the same purpose in watch-work as the pendulum in clock-work, and is affected in 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 of 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, various contrivances have been applied; but that which is most generally adopted is the expansion or compensation balance, the principle of which depends on the unequal expansion of two diffierent metals. It may be constructed in the following manner: The rim consists of two laminae, the inner of steel and the outer of bra , united by fusion. After being turned to the proper size, the rim is cut in three places. A, B, and C; and one end of each 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. D, 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 heav3-headed screws, G, H.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 expajids more than the steel, the compoimd bars will at the same time be bent inwards, and the weights D, 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 bars to a ume ale curved form. It is "easy to see that in so small a machine the adjustment of the parts to procure exact com i)ensation is a matter of great nicety, and requiring much practical skill; in fact, It is only po ible to obtain a tolerable compensation by repeated trials. [s. 130]