EXPERIMENTS ON THE LAWS OF MOTION. 51 



upon the upper one of the two weights P, and is so ad- 

 justed as to overcome the resistance of friction ; it 

 produces therefore no motion in the apparatus by its 

 own weight, but maintains against friction any motion 

 produced by a light touch of the finger. This can 

 obviously only be the case if the friction weight is 

 placed upon that weight which is the descending one ; 

 hence we shall always take the weight P on the right 

 hand of the figure as that which descends, and upon 

 which the friction 'weight is placed. The friction 

 increases if the suspended weights are heavier ; three 

 friction weights will therefore be required, one for the 

 case of 70^ being suspended on each side, a second for 

 70 + 98 = 168 8 *, and a third for 70 + 98 + 98 = 266 gr 

 on each side. 



First Experiment. The brass plate is placed 2 deci- 

 metres below the zero of the scale. The two weights of 

 one gramme each are placed upon the left-hand weight, 

 the four grammes weight, besides the friction weight, 

 upon the right-hand weight ; there are thus 72^ on the 

 left, and 74^ on the right side. Raise the upper edge of 

 the right-hand weight to the zero of the scale, after 

 ! starting the clock or pendulum. By gently pressing 

 with the finger upon the wheel, the apparatus is kept at 

 i rest, and started exactly at the beat of the pendulum, 

 taking care not to push the wheel in the act of with- 

 drawing the finger. If the experiment is successful, the 

 extra weight will strike the brass plate precisely two 

 seconds (2 s ) after starting. A space of 2 decimetres has 

 thus been passed over in 2 s , under the action of a force 

 of 2 gr . For as there are 72 gr on the left side, and 74 gr 

 on the right, the moving force is represented by 2 81 . 



E 2 



