304 ON THE CONSERVATION OF FORCE. 



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 me- 

 chanical work which this velocity could have effected. 



If we review the results of all these instances, which each of 

 you could easily add to from your own daily experience, we shall 

 see that friction and inelastic impact are processes in which me- 

 chanical 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 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 experi- 

 ments show that 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 work 

 necessary for this is, according to Joule's best experiments, 

 equal to the work which a gramme would perform in falling 

 through a height of 425 metres. 



In order to show how closely concordant are his numbers, 

 I will adduce the results of a few series of experiments which 

 he obtained after introducing the latest improvements in his 

 methods. 



1. A series of experiments in which water was heated by 

 friction in a brass vessel. In the interior of this vessel a ver- 

 tical axle provided with sixteen paddles was rotated, the eddies 

 thus produced being broken by a series of projecting barriers, 

 in which parts were cut out large enough for the paddles to pass 

 through. The value of the equivalent was 424'9 metres. 



2. Two similar experiments, in which mercury in an iron 



