ON THE CONSERVATION OF FORCE. 303 



have here assimilated could not have been conducted to it by 

 the cold ice, or have been produced by a change of structure ; it 

 could have come from no other cause than from friction, and 

 must have been created by friction. 



Heat can also be produced by the impact of imperfectly 

 elastic bodies as well as by friction. This is the case, for 

 instance, when we produce fire by striking flint against steel, or 

 when an iron bar is worked for some time by powerful blows 

 of the hammer. 



If we inquire into the mechanical effects of friction and of 

 inelastic impact, we find at once that these are the processes 

 by which all terrestrial movements are brought to rest. A 

 moving body whose motion was not retarded by any resisting 

 force would continue to move to all eternity. The motions of 

 the planets are an instance of this. This is apparently never 

 the case with the motion of the terrestrial bodies, for they are 

 always in contact with other bodies which are at rest, and 

 rub against them. We can, indeed, very much diminish their 

 friction, but never completely annul it. A wheel which turns 

 about a well-worked axle, once set in motion continues it for a 

 long time ; and the longer, the more truly and smoother the 

 axle is made to turn, the better it is greased, and the less the 

 pressure it has to support. Yet the vis viva of the motion 

 which we have imparted to such a wheel when we started it, 

 is gradually lost in consequence of friction. It disappears, and 

 if we do not carefully consider the matter, it sterns as if the vis 

 viva which the wheel had possessed had been simply destroyed 

 without any substitute. 



A bullet which is rolled on a smooth horizontal surface 

 continues to roll until its velocity is destroyed by friction on 

 the path, caused by the very minute impacts on its little 

 roughnesses. 



A pendulum which has been put in vibration can continue 

 to oscillate for hours if the suspension is good, without being 

 driven by a weight ; but by the friction against the surrounding 

 air, and by that at its place of suspension, it ultimately comes 

 to rest. 



