74 COEEELATION OF PHYSICAL FOKCES. 



Then follows another question, viz. whether, where an ex- 

 ternal or derived mechanical effect has been obtained, would 

 the return of the piston, effected without the weight or exter- 

 nal 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 

 temperature 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 ther- 

 mometric 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 conceivable that we may lose 20 from the furnace, 

 and gain 20 in the condenser, and yet have obtained a cer- 

 tain amount of derived mechanical power. It will also follow, 

 upon a consideration of the above imaginary experiments, 

 that the greater the mechanical power required, the greater 

 should be the difference 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, comparatively, a greater mechanical 

 effect for the same number of degrees than steam at low 

 pressure. 



Carnot, assuming the number of degrees of temperature 

 to be restored, but at a lower point of the thermometric scale, 

 termed this the fall (chute) of caloric. The mechanical effect 

 of heat, on this view, may be likened to that of a series of 

 cascades on water-wheels. The highest cascade turns a 

 wheel, and produces a given mechanical effect ; the water 

 which has produced this cannot again effect it at the same 

 level without being carried back to its original elevation, i. e. 



