FRICTION. 239 



same argument ; and this we shall endeavour to 

 illustrate. 



The rule that a body naturally moves for ever 

 with an undiminished speed, is so far from being 

 obviously true, that it appears on a first exami- 

 nation to be manifestly false. The hoop of the 

 school-boy, left to itself, runs on a short distance, 

 and then stops ; his top spins a little while, but 

 finally flags and falls ; all motion on the earth 

 appears to decay by its own nature ; all matter 

 which we move appears to have a perpetual 

 tendency to divest itself of the velocity which we 

 communicate to it. How is this reconcileable 

 with the first law of motion on which we have 

 been insisting? 



It is reconciled principally by considering the 

 effect of Friction. Among terrestrial objects 

 friction exerts an agency almost as universal and 

 constant as the laws of motion themselves ; an 

 agency which completely changes and disguises 

 the results of those laws. We shall consider some 

 of these effects. 



It is probably not necessary to explain at any 

 length the nature and operation of friction. 

 When a body cannot move without causing two 

 surfaces to rub together, this rubbing has a 

 tendency to diminish the body's motion or to 

 prevent it entirely. If the body of a carriage be 

 placed on the earth without the wheels, a consi- 

 derable force will be requisite in order to move 

 it at all : it is here the friction against the 



