LAWS OF FALLING BODIES. 59 



portioned, that the second one turns sixty times slower than the first, and 

 this will be the minute hand ; a third wheel moving twelve times slower than 

 the last will constitute the hour hand. 



„ J A watch ditl'ers from a clock in havina; a vibrating whed in- 



How does * , ~ o J 



watch differ stead of a vibrating pendulum. This wheel, called the baHance- 

 fromaclock? wheel, is moved by a .^^rin.^ which is always forcing it to a 

 middle position of rest, but does not fix it there, because the velocity ac- 

 YlQ.^ 31, quired during its approach from 



either sidt- to the middle position, 

 carries it just as far past on th* 

 other side, and the spring has to 

 begin its work again. The balr 

 ance-ioheel at each vibration allows 

 one tooth of the adjoining wheel to 

 pass, as the pendulum does in a clock, and the record of the beats is pre- 

 served by the wheels wliich follow, as alreadj^ explained for the clock. 



Fig. 31 represents the arrangement used to keep up the motion in a watch. 

 The barrel, or wheel A, incloses a spring, which, when compressed by wind- 

 ing up, tends to liberate itself^ or unwind, in virtue of its elasticity. This 

 effort to unwind, turns the barrel upon its axis, and thus, by means of a chain 

 coiled round it, motion is communicated to the other wheels of the watch. 



. „ 117. The leno'th of a pendulum influences 



•What influ- . „ . ^., . •'^ . - 



ence has the the time 01 its vibi'ation : the Ioniser the pen- 



lengthof apen- , .... ■'• 



duium on its clulum tho slowei' are its vibrations. 



tiou? The reason why long pendulums vibrate more slowly than 



short ones is, that in corresponding arcs, or paths, the ball of 

 the long pendulum has a greater journey to perform, without having a steeper 

 Une of descent. 



■What is th ^^^' ^^'^^ *^^® ^ pendulum rod, Fig. 32, A D, having balls 



centpf of oscii- upon it at C and D, and cause it to vibrate, the ball, B, being 

 ktion in a pen- nearer to the point of suspension, will tend to perform 

 its oscillations more quickly than the ball C. In like 

 manner, every other point on the pendulum rod tends to complete its 

 oscillations in a different time ; but as they are all connected together 

 inflexibly, all are compelled to perform their oscillations in the same 

 time. But the action of the portions of the rod near to the ball, B, 

 is to accelerate the motion of the pendulum, and the action of the fi q 

 portions of the rod near to the ball C, is to retard it ; therefore a point 

 mny he found where all these counteractions will balance one an- 

 other, or be neutralized, and this point is termed the Centeu of Os- 

 cillation, and the sum of the momenta of all the portions of the g A 

 rod on each side of this point will balance. The center of oscillation 

 docs not correspond ■vsnth the center of gravitv, but is always a little 

 below it ; the practical method of bringing them near together, is to 

 make the rod hght, and the termination of the pendulum heavy. ^ 



Aq 



