348 ON THE CONSERVATION OF FORCE. 



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 vertical 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 vessel was substituted for water in a brass one, gave 

 425 and 426-3 metres. 



3. Two series of experiments, in which a conical ring 

 rubbed against another, both surrounded by mercury, 

 gave 426-7 and 425-6 metres. 



Exactly the same relations between heat and work 

 were also found in the reverse process that is, when 

 work was produced by heat. In order to execute this 

 process under physical conditions that could be controlled 

 as perfectly as possible, permanent gases and not vapours 

 were used, although the latter are, in practice, more con- 

 venient for producing large quantities of work, as in the 

 case of the steam-engine. A gas which is allowed to 

 expand with moderate velocity becomes cooled. Joule 

 was the first to show the reason of this cooling. For the 

 gas has, in expanding, to overcome the resistance, which 

 the pressure of the atmosphere and the slowly yielding 

 side of the vessel oppose to it ; or, if it cannot of itself 

 overcome this resistance, it supports the arm of the 

 observer which does it. Gas thus performs work, and 

 this work is produced at the cost of its heat. Hence the 



