ON THE CONSERVATION OF FORCE. 



295 



Elastic forces are produced in gases by the aid of heat, on a 

 far greater scale. 



Let us take, as the most simple instance, atmospheric air. 

 In Fig. 44 an apparatus is represented such as Regnault used 

 for measuring the expansive force of heated gases. If no great 

 accuracy is required in the measurement, the apparatus may be 

 arranged more simply. At C is a glass globe filled with dry 



FIG. 44. 



air, which is placed in a metal vessel, in which it can be heated 

 by steam. It is connected with the U-shaped tube, S s, which 

 contains a liquid, and the limbs of which communicate with 

 each other when the stop-cock B, is closed. If the liquid is in 

 equilibrium in the tube S s when the globe is cold, it rises in the 

 leg s, and ultimately overflows when the globe is heated. If, 

 on the contrary, when the globe is heated, equilibrium be re- 

 stored by allowing some of the liquid to flow out at K, as the 



