292 



ON THE CONSERVATION OF FORCE. 



The weight is thereby raised to the height A a. Hence my 

 arm must exert a certain force to bring the weight to a. Gravity 

 resists this motion, and endeavours to bring back the weight to 

 M, the lowest point which it can reach. 



Now, if after I have brought the weight to a I let it go, it 

 obeys this force of gravity and returns to M, arrives there with 

 a certain velocity, and no longer remains quietly hanging at M as 

 it did before, but swings beyond M towards b, where its motion 



stops as soon as it has traversed on the side of B an arc equal 

 in length to that on the side of A, and after it has risen to a 

 distance B b above the horizontal line, which is equal to the 

 height A a, to which my arm had previously raised it. In b the 

 pendulum returns, swings the same way back through M towards 

 a, and so on, until its oscillations are gradually diminished, 

 and ultimately annulled by the resistance of the air and by 

 friction. 



You see here that the reason why the weight, when it comes 



