THE PENDULUM. 129 



sensible influence upon the time of vibration, and the 

 bullet is so small that all its points have approximately 

 the same distance from the points of suspension, and 

 hence vibrate with equal velocities. But in the com- 

 pound pendulum the rod is of considerable weight; the 

 portions nearer to the point of suspension would oscillate 

 more rapidly if they were moving independently of the 

 lower portions ; but, being compelled by the cohesion of 

 the several parts to swing together, the result is that a 

 compound pendulum vibrates more rapidly than a 

 simple pendulum of equal length. 



The pendulum used in the experiments on the laws of motion 

 (fig. 44) needs to be heavy, in order that the small striking weight 

 attached to it may be moved without sensibly altering the time of 

 vibration. For the preceding experiments, balls such as can be made 

 in a common bullet-mould are sufficient. Bullets of lead and gypsum 

 have been used, because they are easily formed by means of a mould. 

 Gypsum, a well-known mineral, loses its water when heated, and 

 crumbles down to a white powder, which is sold under the name of 

 ' plaster of Paris.' If the dry powder be made into a thin paste with 

 water, and be poured into the mould, it will become solid after the 

 lapse of an hour or two, eventually becoming as hard as the original 

 gypsum. The hooks are inserted in the mould in the manner pre- 

 viously explained (p. 56, fig. 46). 



The simple pendulum possesses another important 

 property. The plane in which its vibrations are per- 

 formed, remains fixed, even if the thread or wire, and 

 with it the suspended bullet, are made to rotate about 

 he point of suspension. 



A wire, from 8 to 10 cm long, is bent into a small hook at one end ; 

 he other end is passed upwards through a vertical hole made in the 

 liddle of the cross-bar of the frame, just wide enough to allow the 

 'ire to turn easily, but not more. When the hook is pretty near to 

 ie bar, the wire which projects above is bent first horizontally, and 



m farther on vertically, so as to form a small handle for turning, 

 'he pendulum is then suspended from the hook, the utmost care 



