378 POPULAR LECTURES AND ADDRESSES. 



the distance between them, or, starting with them 

 very wide apart, we gradually diminish the dis- 

 tance, a certain critical arrangement is approached 

 from cither direction, and the gaining rate in the 

 former case, or the losing rate in the latter case, is 

 augmented. This critical arrangement is such that 

 the period of vibration of the suspended chrono- 

 meter, when set to vibrate by an external disturb- 

 ance, is approximately equal to the period of 

 vibration of the balance-wheel. When the upper 

 points of support are adjusted to produce it, and 

 the chronometer, going, is left to itself, the action 

 of the internal prime mover will bring the whole 

 into a state of vibration, which may be either the 

 first fundamental mode (balance-wheel and frame- 

 work vibrating in the same direction), in which 

 case the chronometer will have a losing rate, or the 

 second fundamental mode (balance-wheel and 

 frame vibrating in opposite directions), in which 

 case the chronometer will have a gaining rate. 

 The gain or loss may amount to as much as one 

 second in sixty or eighty with an ordinary ship 

 chronometer, taken off its gimbals, or a pocket 



