ON TIMEKEEPERS. 155 



about four vibrations in an hour, by diminishing the arc in which it 

 vibrated : and when the vibrations were restored to their original magni- 

 tude, the resistance of the air produced a retardation of eight vibrations in 

 the same time. But a heavy pendulum, vibrating in a small arc, is very 

 little affected by this resistance. 



Besides these more essential parts of the watchmaker's art, there are 

 several subordinate considerations which require his attention ; the striking 

 part in particular occupies, in clocks, and in repeating watches, no incon- 

 siderable portion of the bulk of the machine. But the apparatus employed 

 on these occasions requires neither refinement of invention nor delicacy of 

 execution. In old clocks, the number of hours struck is usually deter- 

 mined by the revolution 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 pro- 

 duced 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 

 pendulum, and enlarges the diameter of the circle of which the pendulum 

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

 the clock. Sometimes, however, an unsteady support may be 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 effect on 

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

 varying 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 vibrations 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 com- 

 panion 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 influence each other's vibrations in such a manner 

 as to produce exactly the same note when they sound together. 



* Ellicott, Ph. Tr. 1739, p. 126, describes the interference of two pendulums 

 tbe one set the other in motion the one stopped the other, &c. 



