Mr. J. Gill on the Dynamical Theory of Heat. 87 



the heat consumed, or which permanently disappears, is supposed 

 to be converted into work in the proportion of Joule's equivalent 

 of 772 foot-pounds for every unit of heat, neither more nor less 

 By means of a regenerator (as used in Stirling's and Ericsson's 

 air-engines) of theoretically perfect action, all the heat with- 

 drawn from the air as above described could be stored up in 

 such a way as to be transferred back again to the same air under 

 the same circumstances as before, and the operation could be 

 repeated over and over again indefinitely, with the production 

 of available work, provided that at each stroke of the piston a 

 fresh quantity of heat represented by 42 be thrown into the 

 apparatus. The heat consumed by 1000 strokes of the piston 

 would be 1000 times 42, and the work produced would be 1000 

 quarters of an atmosphere on the area of piston raised 1 foot. 



In the former case 1000 repetitions of the operation, with 

 fresh air and fresh heat each time, would equally consume 1000 

 times 42 of heat, the 100 of heat remaining unchanged at the 

 end of each operation ; but if it is assumed that in each cycle of 

 heating and cooling, expanding and contracting the confined air, 

 work is actually done equal to raising the atmospheric column 

 on the piston through 1 foot (which is the apparent effect), we 

 must allow that the work done by 1000 repetitions of the ope- 

 ration would be 1000 atmospheres on the piston raised 1 foot. 

 How is this apparent discrepancy to be accounted for ? 



Another serious difficulty in the dynamical theory is the ques- 

 tion of the specific heat of air. Regnault's experiments appear 

 to prove that the specific heat of air is constant at different den- 

 sities. This result is in accordance with the dynamical theory, 

 and in fact the theory could not stand unless this assumption were 

 admitted. My own experiments have given very different results; 

 but as it might be thought presumptuous to place these experi- 

 ments in opposition to those of such a high authority as Reg- 

 nault, I beg to submit the following reasoning to candid scru- 

 tiny. 



Imagine a vacuum to be made in a cylinder by drawing up 

 its piston by a descending weight, in which operation the work 

 done by the descent of the weight gives an equivalent amount of 

 potential energy in the tendency possessed by the atmospheric 

 column raised by the piston to fall back into the vacuum ; and 

 so far things are in a state of equilibrium. Imagine further a 

 second cylinder, twice the size of the former one, full of atmo- 

 spheric-pressure air, which should be compressed into half its 

 original volume by some exterior force applied, allowing the 

 developed heat to pass out during the compression. In this 

 operation work has been expended, and heat has been produced, 

 which heat, according to the dynamical theory, would be consi- 



