go* LECTURE XVir. 



In old clocks, the number of hours struck is usually determined by the revo- 

 lution of a certain portion of a wheel, which supports an arm, and allows the 

 hammer to strike, until at a proper time it falls into a notch. In watches, 

 and in more modern clocks, the same effect is produced by means of a spiral 

 of 12 teeth, revolving once in 12 hours. 



It is of considerable importance to the accurate performance of a good 

 clock, that it should be firmly fixed to a solid support. Any unsteadiness in 

 the support causes the point of suspension to follow the motion of the pen- 

 dulum, and enlarges the diameter of the circle of which the pendulum de- 

 scribes an arc; it must, therefore, tend in general to retard the motion of the 

 clock. Sometimes, however, an unsteady support may he of such a nature as 

 to accelerate the motion; and an observation of this kind, made by 

 Berthoud, has suggested to Bernoulli a theory of compound vibrations, 

 which may perhaps be true in some cases, but is by no means universally 

 applicable to every case. On account of some circumstances of this 

 kind, it happens that when two clocks are placed near each other, and rest 

 in some degree on the same support, they have often a remarkable etfect on 

 each other's vibrations, so as to continue going for several days, without va- 

 rying a single second, even when they would have differed considerably if 

 otherwise situated: and it sometimes happens that the clock which goes the 

 more slowly of the two will set the other in motion, and then stop itself; a 

 circumstance which has been explained from the greater frequency of the vi- 

 brations of a circular pendulum when confined to a smaller arc, the tendency 

 of the pendulums to vibrate in the same time causing the shorter to describe 

 an arc continually larger and larger, and the longer to contract its vibrations, 

 until at last its motion entirely ceases. This sympathy has some resemblance 

 to the alternate vibrations of two scales hanging on the same beam, one of 

 which may often be observed to stop its vibrations when the other begins to 

 move, and to resume its motion when its companion is at rest ; but it is still 

 more analogous to the mutual Influence of two strings, or even two organ 

 pipes, which, though not separately tuned to a perfect unison, still influenx-e 

 each other's vibrations in such a manner as to produce exactly the same note 

 when they sound together. 



fUV 



