ON THE CONSERVATION OF FORCE 207 



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 ins 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 de- 

 stroyed 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 con- 

 tinue to oscillate for hours if the suspension is good, with- 

 out 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. 



A stone which has fallen from a height has acquired a 

 certain velocity on reaching the earth; this we know is the 

 equivalent of a mechanical work; so long as this velocity 

 continues as such, we can direct it upwards by means of 

 suitable arrangements, and thus utilise it to raise the stone 

 again. Ultimately the stone strikes against the earth and 

 comes to rest; the impact has destroyed its velocity, and 

 therewith apparently also the mechanical work which this 

 velocity could have affected. 



If we review the results of all these instances, which 

 each of you could easily add to from your own daily experi- 

 ence, we shall see that friction and inelastic impact are 

 processes in which mechanical work is destroyed, and heat 

 produced in its place. 



The experiments of Joule, which have been already men- 

 tioned, lead us a step further. He has measured in foot 

 pounds the amount of work which is destroyed by the fric- 

 tion of solids and by the friction of liquids; and, on the 

 other hand, he has determined the quantity of heat which 

 is thereby produced, and has established a definite relation 



