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



Suppose the air to be again at 32°, and the piston free to move 

 when any disturbing cause should be applied. If heat be gra- 

 dually communicated to the confined air it will expand, raising 

 the piston and with it the superincumbent column of the atmo- 

 sphere, which may be taken in round numbers at 15 lbs. per 

 square inch. When 490 degrees of temperature have been given 

 to the confined air, its volume will be doubled, and if the piston 

 be 1 square foot area, it will have moved upwards through 1 foot ; 

 and it would be said that in its ascent it had performed the work 

 of 144 square inches x 15 lbs. =2160 lbs. raised 1 foot. In 

 this process 1 cubic foot of air has been heated 490 degrees, as in 

 the former case, but the circumstances are different. In the 

 former case the air was confined to constant volume, and tension 

 was induced, but no work was done. In the present case the 

 air was free to expand, no tension was induced, and work was 

 performed ; and as heat was the only energy employed in the 

 operation, we should naturally expect an excess of heat to be 

 used equivalent to the work done. In fact we find that 142 

 of heat has been thrown into the air in this case, while to heat 

 the air to the same degree under constant volume 100 sufficed; 

 it is therefore said that the difference, represented by 42, has 

 done the work. 



The work assumed to be done in the case under consideration, 

 by raising the column of the atmosphere resting on the piston, 

 is not available, it cannot be detached from the apparatus — a 

 circumstance which might induce doubt as to its reality. If it 

 possesses the entity which it appears to have, it should not be 

 impossible to detach it from the matrix where it exists, and to 

 exhibit it in some other form, thus removing the suspicion of 

 imperceived coefficients acting in the phenomena. By altering 

 the circumstances, the work which exists as potential energy in 

 the raised atmospheric column ought, if all real, to be convert- 

 ible into an equivalent of actual energy, or work done in another 

 shape. By withdrawing the heat from the air in the cylinder, 

 the piston will descend with a power corresponding to the dif- 

 ference of pressure between the total atmospheric pressure out- 

 side and the diminished pressure inside; and this power will be 

 equal to half the atmospheric pressure at the beginning of the 

 piston's stroke, diminishing to nothing at the end, when the 

 confined air will, as at first, occupy 1 cubic foot at atmospheric 

 pressure, and 32° temperature. The amount of power obtained 

 in this operation will be less than a quarter of an atmosphere on 

 the area of the piston moved through 1 foot. For simplicity 

 of reasoning we may suppose it to be one quarter of an atmo- 

 sphere. 



In the working of a theoretically perfect thermic engine, all 



