TETTE FUNCTION OF MACHINES. 217 



time is required to raise it to the same height ; so that, how- 

 ever we may alter, by the interposition of machinery, the in- 

 tensity of the acting force, still in a certain time, during which 

 the mill-stream furnishes us with a definite quantity of water, 

 a certain definite quantity of work, and no more, can be per- 

 formed. 



Our machinery, therefore, has, in the first place, done 

 nothing more than make use of the gravity of the falling wa- 

 ter in order to overpower the gravity of the hammer, and to 

 raise the latter. When it has lifted the hammer to the neces- 

 sary height, it again liberates it, and the hammer falls upon 

 the metal mass which is pushed beneath it. But why does 

 the falling hammer here exercise 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 ? We find, in fact, that the work per- 

 formed by the hammer is determined by its velocity. In 

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

 producing great eifects. I only remind you of the destructive 

 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 pro- 

 duce 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, etc., so that motion 

 is incessantly weakened and finally neutralized. 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 resisting forces can motion be diminished or destroyed. 

 A moving body, such as the hammer or the musket-ball, when 

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