218 INTERACTION OF NATURAL FORCES. 



it strikes against another, presses the latter together, or pene- 

 trates it, until the sum of the resisting forces which the body 

 struck presents to its pressure, or to the separation of its par- 

 ticles, is sufficiently great to destroy the motion of the ham- 

 mer or of the bullet. The motion of a mass regarded as 

 taking the place of working force is called the living force (vis 

 viva) of the mass. The word " living " has of course here 

 no reference whatever to living beings, but is intended to rep- 

 resent 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 

 fallen 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 ex- 

 pended in its production. It is therefore equivalent 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 



