58 CORRELATION OF PHYSICAL FORCES. 



piston will restore that portion of the heat which has been 

 lost by the expansion, and by the mechanical effect consequent 

 thereon ; but if the weight be removed when at its maximum 

 of elevation, and the piston be brought back to its starting 

 point by a necessarily cooler body than could restore it if the 

 weight were not removed, would the return of the piston now 

 yield to the cooling body the heat which had been lost by the 

 dilatation, or, in other words, should we in pulling the piston 

 down by cold reacquire the same quantity of heat as would 

 be restored by pressing it down by mechanical force ? The 

 argument from the impossibility of perpetual motion would 

 say no, for if all the heat were restored, the mechanical effect 

 produced by the fail of the weight, or the heating effect 

 which might be made to result from this mechanical power, 

 would be got from nothing. 



Then follows another question, viz. whether, where an 

 external or derived mechanical effect has been obtained, 

 would the return of the piston, effected without the weight or 

 external force to assist it, but solely by the colder body, give 

 to this latter the same number of thermometric degrees as had 

 been lost by the hot body in the first instance ? Suppose, for 

 instance, the cold body in our experiment to be at 20 instead 

 of 30, would this body gain 20, and then reach the tempera- 

 ture of 40 when the piston is brought back, or would its tem- 

 perature be higher or lower than 40 ? The argument from 

 the impossibility of perpetual motion does not apply here, for 

 it does not necessarily follow that 20, on the thermometric 

 scale from 20 to 40, represents an equal amount of force to 

 20 on the scale from 70 to 90, and therefore it is quite con- 

 ceivable that we may lose 20 from the furnace, and gain 20 

 in the condenser, and yet have obtained a certain amount of 

 derived mechanical power. It will also follow, upon a con- 

 sideration of the above imaginary experiments, that the greater 

 the mechanical power required, the greater should be the dif- 

 ference between the temperature of the furnace and that of 

 the condenser ; but the exact relation in temperature between 

 these for a given mechanical effect, has not, as far as I am 

 aware, been satisfactorily established by experiment, though 

 it has been shown that steam at high pressure produces, com- 



