206 



have "been conducted to it by the cold ice, or have been pro- 

 duced 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 



f 10 9$ 



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 proc- 

 esses 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, 



