160 ON THE INTERACTION OF NATUEAL FOECES. 



beneath it. But why does the falling hammer here exer- 

 cise a greater force than when it is permitted simply to 

 press with its own weight on the mass of metal ? Why is 

 its power greater 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 performed by the 

 hammer is determined by its velocity. In other cases, 

 also, the velocity of moving masses is a means of pro- 

 ducing 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 windmill, 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, &c., so that the motion is 

 incessantly weakened and finally arrested. A body, how- 

 ever, 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 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 

 resisting forces presented by the body struck to pres- 

 sure, 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 taking the 

 place of working force is called the living force (vis 

 viva) of the mass. The word 6 living ' has of course here 

 no reference whatever to living beings, but is intended to 



