333 SECTIONMECHANICS. 



the dead-beat of Graham, invented early last century ; the detached 

 escapement invented by Sir Geo. Airy some thirty or forty years ago, 

 and the more modern gravity escapements. 



The compensation of the pendulum for changes of temperature is 

 effected -in various ways, but the principle is the same in all. Two 

 metals, one expanding more freely than the other, are used, and so 

 applied that the expansion of the one counteracts and nullifies the ex- 

 pansion of the other, so that the centre of oscillation of the pendulum 

 is maintained at the same point. 



Originally, brass and steel were the metals employed, but mercury 

 and steel or iron, or zinc and steel or iron are now generally used so we 

 will at once pass to them. 



A mercurial pendulum consists of a steel rod at the lower end of 

 which hangs a jar or vessel of glass or iron containing the mercury. 

 Now the jar of mercury forms the bob of the pendulum, and is at the 

 same time the compensating element. The action in changes of 

 temperature is as follows. In heat, the steel rod lengthens and the jar 

 descends, but the same cause which produces this, makes the mercury 

 expand and rise in the jar, so that one expansion compensates for the 

 other. In cold, the metals contract similarly, and the same effect is 

 produced by the contrary action. 



In a zinc and steel compensation, as now made, the construction is 

 somewhat more complicated, but still is easily explained. The bob or 

 principal mass of the pendulum is of lead, and does not form part of the 

 compensation. It is supported by a steel tube, to the lower end of 

 which it is fastened at its centre. The upper end of this tube is fitted 

 with a collar which rests on the top of a zinc tube, this in its turn is 

 supported by a nut on the lower end of the central steel rod. In heat, 

 the steel tube and rod expand and would allow the bob to descend, 

 but the zinc expanding raises it up an equal amount, so that the same 

 theoretical length is preserved : the length of the zinc tube being such, 

 that its longiditudinal expansion in any given temperature exactly 

 equals the expansion of the steel rod and tube together. A drawing 

 of a zinc and steel compensation pendulum is here. 



Now, for astronomical clocks in which the greatest accuracy is re- 

 quired, the mercurial pendulum so long stood unrivalled that it may 

 fairly be asked how it is that it now seems likely to be superseded by 



