ON THE CONSERVATION OP FOECE. 347 



tion 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, 

 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. 



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 effected. 



If we review the result of all these instances, which 

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

 perience, 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 

 mentioned, lead us a step further. He has measured in 

 foot pounds the amount of work which is destroyed by the 

 friction 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 between the two. His experiments show tliat 

 when heat is produced by the consumption of work, a 

 definite quantity of work is required to produce that 

 amount of heat which is known to physicists as the unit 

 of heat ; the heat, that is to say, which is necessary to 

 raise one gramme of water through one degree centigrade. 

 The quantity of wcrk necessary for this is, according to 

 Joule's best experiments, equal to the work which a 

 16 



