THERMODYNAMICS OF ELASTIC BODIES. 163 



between the sensible temperature of a body and the actual 

 heat transferred. If the heat energy is considered under two 

 heads, namely, that required to change the sensible tempera- 

 ture of the systems, and secondly, that concerned in the 

 changes of potential energy, it is easy to show that the actions 

 are, as of course the}' must be, in accordance with the law of 

 the conservation of energy. 



In the first place it is to be noted that the amount of heat 

 required to produce the sensible thermometric change (together 

 with any heat absorbed or released in other molecular work) 

 is greater than the heat equivalent due to raising or lowering 

 the weights. When the systems are heated, less heat in compar- 

 ison is absorbed by system S, the spiral spring, than by system 

 R, the rubber, bj^ an amount equivalent to the difference of 

 level of the weights at the higher temperature. Upon cooling 

 the two systems, the heat radiated by system S is less, com- 

 paratively, than that" given off by system R, because in the 

 first case energy is required to raise the -weight to the original 

 level, while in the second case heat has resulted from the fall- 

 ing of the weight. Thus each system in returning to the 

 original position gives out just the amount of heat it absorbed 

 in the first change. The difference between the two cases is 

 that the actual transfer of heat energy, other things being 

 equal, is greater in sj^stem R than in s^^stem S. 



These considerations show that the actions are easily 

 brought into consonance with thermodynamic theory. 



If we limit our consideration to the second or cooling-off 

 phase of the operation of s^^stem S, with the spiral spring, it 

 is almost startling to see actual mechanical work done by tak- 

 ing away heat energy from the system. And, indeed, this is 

 just what is taking place, although as a matter of fact we are 

 only reaping the benefits of the previous heated condition of 

 the body. That is, mechanical work is obtained from the 

 heat energy of system S when by the proximity of a cold 

 bod\' a flow of heat is caused to take place from the warm 

 system to the cold bodv. The direction of flow, whether into 



