COMMON THINGS CLOCKS AND WATCHES. 



freely. Among the series, there will he found a certain point, 

 which will separate those which are moving slower than their 

 natural rate, from those which are moving faster than their 

 natural rate ; and a ball placed at this point would vibrate exactly 

 as it would do if no other balls were placed either above or below 

 it. Such a ball would, as it were, be the centre which would 

 divide those which are accelerated from those which are re- 

 tarded. 



Such a point has, therefore, been denominated the centre of 

 osoillation. 



It is evident then, that a pendulous mass, of magnitude 

 more or less considerable, will vibrate in the same time as it 

 would do if the entire mass were concentrated at its centre 

 of oscillation, and formed there a material point of insensible 

 magnitude. 



By the length of a pendulum, no matter what be its form, 

 is always to be understood the distance of its centre of oscillation 

 from its point of suspension. 



It will be seen from what has been explained above, that 

 by varying the distance of the centre of gravity of the pen- 

 dulum from the point of suspension, the centre of oscillation, 

 and therefore the virtual length of the pendulum, and con- 

 sequently its time of vibration, may be varied. The instru- 

 ment may therefore be so adjusted, that the time of its vibration 

 shall be a second, or any fraction of a second, that may be 

 desired. 



19. Supposing, then, the pendulum to be so adjusted, that it 

 shall make its vibrations at any required rate, one per second for 

 example, let us see how the motion of the indicating hands is 

 governed by such vibrations. 



Upon the axis on which the pendulum oscillates is fixed a piece 

 of metal in the form of an anchor, such as D B A c (fig. 5), so that 

 this piece shall swing alternately right and left with the pendulum. 

 Two short pieces, m and m', called pallets, project inwards at 

 right angles to it from its extremities A and c. 



The form and dimensions of the anchor ABC are accommodated 

 to those of the escapement- wheel, w w, which is part of the 

 clockwork, and which, in common with the other wheels forming 

 the train, is moved in the direction indicated by the arrow by 

 the weight or main-spring. "When the anchor swings to the right 

 the pallet m enters between two teeth of the wheel, the lower of 

 which coming against it, the motion of the wheel is for the 

 moment arrested. When it swings to the left, the pallet m is 

 withdrawn from between the teeth, and the wheel is allowed to 

 move, but only for a moment, for the other pallet m' enters 

 14 



