346 ON THE CONSERVATION OF FORCE. 



a cooled space he made two pieces of ice rub against each 

 other, and thereby caused them to melt. The latent heat 

 which the newly formed water must have here assimilated 

 could not have been conducted to it by the cold ice, or 

 Lave 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 wliich 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 mo- 

 tion of the terrestrial bodies, for they are always in con- 

 tact with other bodies which are at rest, and rub against 

 them. We can, indeed, very much diminish their fric- 

 tion, 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 seems 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 fric- 



