HEAT. 75 



without an extra force being employed equivalent to, or 

 rather a fraction more than the force of the descending 

 water ; but though its power is spent with reference to the 

 first wheel, the same water may, by falling over a new 

 precipice upon a second wheel, again reproduce the same 

 mechanical effect (strictly speaking, rather more, for it has 

 approximated the centre of gravity), and so on, until no 

 lower fall can be attained. So with heat: it involves no 

 necessity of assuming perpetual motion to suppose that, after 

 a given mechanical effect, produced by a certain loss of heat, 

 the number of degrees lost from the original temperature 

 may be restored to the condenser, but at a lower point of the 

 thermometric scale. 



If work has been done, i. e. if force has been parted 

 with, the original temperature itself cannot be restored, but 

 there is no a priori impossibility in the same number of 

 degrees of heat as have been converted into work being con- 

 veyed to a condensing body so cold that, when it receives this 

 heat, it will still be below the original temperature to which 

 the work-producing heat was added. 



In the theory of the steam-engine, this subject possesses 

 a great practical interest. Watt supposed that a given 

 weight of water required the same quantity of what is 

 termed total heat (that is, the sensible added to the latent 

 heat) to keep it in the state of vapour, whatever was the 

 pressure to which it was subjected, and, consequently, how- 

 ever its expansive force varied. Clement Desormes was 

 also supposed to have experimentally verified this law. If 

 this were so, vapour raising a piston with a weight attached 

 would produce mechanical power ; and yet, the same heat 

 existing as at first, there would be no expenditure of the 

 initial force ; and if we suppose that the heat in the condens- 

 er was the real representative of the original heat, we 

 should get perpetual motion. Southern supposed that the 

 latent heat was constant, and that the heat of vapour under 



