190 



HELMHOLTZ 



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 from a to M, and does not stop there, but ascends to 

 b, in opposition to the action of gravity, is only to be sought 

 in its velocity. The velocity which it has acquired in mov- 

 ing from the height A a is capable of again raising it to an 

 equal height, B b. The velocity of the moving mass, M, is 

 thus capable of raising this mass; that is to say, in the 

 language of mechanics, of performing work. This would 

 also be the case if we had imparted such a velocity to the 

 suspended weight by a blow. 



From this we learn further how to measure the working 



