154 LECTURE XVII. 



remained at its original distance from the point of suspension, which was 

 determined hy a fixed plate, transmitting the slender spring, as usual, be- 

 tween two opposite edges. The same effect is produced more simply by 

 suspending the pendulum from the summit of a bar nearly parallel to it, 

 and of the same substance with itself, resting on a fixed support, and either 

 of the same length with the pendulum, or a little longer, accordingly as 

 the distance of the fixed plate from the point of support of the bar, is 

 determined by materials which may be considered as nearly of an inva- 

 riable length, or as liable to a certain degree of expansion. (Plate XVI. 

 Fig. 210.) 



All these methods of compensation are peculiar to clocks ; for watches, 

 it is usual to unite together two metals which differ in expansibility, so as 

 to form a compound plate ; one side of the . plate is commonly of steel, 

 the other of brass, and it is obvious that any increase of temperature, by 

 causing the brass to expand more than the steel, must bend the whole plate. 

 Such a plate is variously applied ; the most accurate method, which is 

 employed by Arnold and other modern artists, is to make it a part of the 

 balance itself, fixing a weight on its extremity, which is brought nearer to 

 the centre, by the increase of curvature of the plate, whenever the ex- 

 pansion of the arms of the balance tends to remove it further off. The 

 best way of making the plate appears to be to turn a ring of steel, and to 

 immerse it in melted brass, and then to turn away what is superfluous of 

 the brass. The magnitude of the weight, and the length of the plate, may 

 easily be so regulated as to compensate not only for the expansion pro- 

 duced by heat, but also for the diminution of the elasticity of the spring. 

 Sometimes also a plate has been applied in such a way as to shorten the 

 spring when the temperature is increased, by an operation similar to that 

 which serves to regulate a common watch, the clip that determines the 

 effective length of the spring, being moved backwards and forwards ; and 

 a similar effect has also been produced by dividing this clip into two parts, 

 one of which is fixed to a compound plate, and is made to approach the 

 other, so as to confine the spring more narrowly and thus diminish its 

 length, upon an increase of temperature. (Plate XVI. Fig. 211.) 



The flexure of a compound plate has also been applied in a simple and 

 elegant manner by Mr. Nicholson to the pendulum of a clock, by causing 

 it to support the upper extremity of the pendulum. The plate is placed 

 horizontally, the brass being uppermost, and carries the pendulum in the 

 middle, while the ends rest on two fixed points, of which the distance may 

 be adjusted with great accuracy, so that when the temperature is in- 

 creased, the curvature of the plate may raise the rod of the pendulum, 

 enough to keep the weight or bob at a constant distance below the fixed 

 point, which determines its upper extremity. (Plate XVI. Fig. 212.) 



The resistance, opposed to the motion of a pendulum by the air, affects 

 in some degree its velocity, and the variation of the density of the atmo- 

 sphere must therefore also produce some irregularities in timekeepers : 

 they are, however, too small to be sensible. Derham* found that fhe 

 resistance of the air accelerated the motion of a half second pendulum 

 * Ph. Tr. 1704, xxiv. 1785, 



