HERMANN VON HELMHOLTZ 279 



when once set in motion, moves onward eternally with undiminished 

 velocity. Thus we know that the planetary bodies have moved with- 

 out change, through space, for thousands of years. Only by resisting 

 forces can motion be diminished or destroyed. A moving body, such 

 as the hammer or the musket-ball, when it strikes against another, 

 presses the latter together, or penetrates it, until the sum of the re- 

 sisting forces which the body struck presents to its pressure, or to the 

 separation of its particles, is sufficiently great to destroy the motion of 

 the hammer or of the bullet. The motion of a mass regarded as tak- 

 ing the place of working force is called the living force (vis viva) of 

 the mass. The word "living" has of course here no reference what- 

 ever to living beings, but is intended to 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 in- 

 cessant 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 as- 

 cend : 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 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 ef- 

 fects ; 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 



