MEASURING THE VELOCITY. 



39 



Fig. 13. 



The two weights are first made to 

 balance each other, when one of 

 them is raised nearly to the wheel 

 at c, and a small weight in the form 

 of a short rod is placed across it. 

 It immediately descends with ac- 

 celerated or increasing velocity un- 

 til it reaches the hole in the shelf a, 

 through which the weight passes, 

 but the rod is caught and retained. 

 The motion is now no longer accel- 

 erated, because the weights have 

 become equal, and the descending 

 one continues to fall uniformly ait 

 the same rate that it passed through 

 the hole in the shelf, until it strikes 

 the bottom. Durmg this time its 

 velocity may be accurately meas- 

 ured by means of the clock and pen- 

 dulum attached to the instrument. 

 By sliding the shelf up or down, the 

 velocity, after falling through different spaces to reach 

 the shelf, may be accurately determined. When the 

 shelf is placed very near the top, so that but little ve- 

 locity is acquired, the descending weight will move 

 very slowly all the way down ; but when placed low- 

 er, the weight continues downward more rapidly. It 

 is necessary that the wheel turn with extreme ease, to 

 effect which, friction-wheels, described hereafter, are 

 usually employed. 



There are many instances showing the accelerated 

 motion and increased force of fallinsr bodies. Wlien a 



Atwood's Machine for 

 measuring the de- 

 scent of bodies. 



