ON THE INTERACTION OF NATURAL FOECES. 161 



represent solely the force of the motion as distinguished 

 from the state of unchanged rest — from the gravity of a 

 motionless body, for example, which produces an incessant 

 pressure against the surface which supports it, but does 

 not produce any motion. 



In the case before us, therefore, we had first power in 

 the form of a falling mass of water, then in the form of 

 a lifted hammer, and thirdly in the form of the living 

 force of the falling hammer. We should transform the 

 third form into the second, if we, for example, permitted 

 the hammer to fall upon a highly elastic steel beam 

 strong enough to resist the shock. The hammer would 

 rebound, and in the most favourable case would reach a 

 height equal to that from which it fell, but would never 

 rise higher. In this way its mass would ascend ; and at 

 the moment when its highest point has been attained it 

 would represent the same number of raised foot-pounds 

 as before it fell, never a greater number ; that is to say, 

 living force can generate the same amount of work as 

 that expended in its production. It is therefore equiva- 

 lent to this quantity of work. 



Our clocks are driven by means of sinking weights, 

 and our watches by means of the tension of springs. A 

 weight which lies on the ground, an elastic spring which 

 is without tension, can produce no effects : to obtain such 

 we must first raise the weight or impart tension to the 

 spring, which is accomplished when we wind up our 

 clocks and watches. The man who winds the clock or 

 watch communicates to the weight or to the spring a 

 certain amount of power, and exactly so much as is thus 

 communicated is gradually given out again during- the 

 following twenty-four hours, the original force being thus 

 slowly consumed to overcome the friction of the wheels 

 and the resistance which the pendulum encounters from 

 the air. The wheelwork of the clock therefore developes 



