INFLUENCE OF SUSPENSION ON WATCH. 363 



being guided by a cross-head and slides, and the 

 crank and fly-wheel making one revolution for 

 every backward and forward movement of the 

 piston. The balance-wheel of a watch, vibrating 

 to and fro through a certain angle, performs very 

 approximately a simple harmonic motion. The 

 longer the hair-spring is, the more nearly it will 

 approach to simple harmonic motion, and it will 

 keep time the more accurately. 



Now, against every change of motion of a body 

 there is a certain reaction, and every motion to and 

 fro of the balance-wheel of a watch or chrono- 

 meter reacts upon the case of the watch or chrono- 

 meter ; and if the case is so suspended as to be 

 free to vibrate, the motion of the balance-wheel 

 will generate a vibration of the whole, so that we 

 have two motions to consider one, that of the 

 balance-wheel inside the watch ; the other, that of 

 the whole watch except the balance-wheel. Upon 

 the mode of suspension of the watch or chrono- 

 meter will depend the nature of the vibration 

 which it takes up and the resultant effect upon the 

 rate. The rate is accelerated or retarded according 



