ON THE INTERACTION OF NATURAL FORCES. 143 



as the height from which it falls is increased, and the greater 

 therefore the velocity of its fall? We find, in fact, that the 

 work pel-formed by the hammer is determined by its velocity. 

 In other cases, also, the velocity of moving masses is a means 

 of producing great effects. I only remind you of the destruc- 

 tive effects of musket-bullets, which in a state of rest are the 

 most harmless things in the world. I remind you of the wind- 

 mill, which derives its force from the moving air. It may 

 appear surprising that motion, which we are accustomed to 

 regard as a non-essential and transitory endowment of bodies, 

 can produce such great effects. But the fact is, that motion 

 appears to us, under ordinary circumstances, transitory, because 

 the movement of all terrestrial bodies is resisted perpetually 

 by other forces, friction, resistance of the air, <fec., so that the 

 motion is incessantly weakened and finally arrested. A body, 

 however, which is opposed by no resisting force, when once set 

 in motion moves onward eternally with undiminished velocity. 

 Thus we know that the planetary bodies have moved without 

 change through space for thousands of years. Only by resist- 

 ing 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 resisting forces presented by the body 

 struck to pressure, or to the separation of its particles, is suffi- 

 ciently great to destroy the motion of the hammer 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 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 



