HEAT. 59 



paratively, 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. 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 approached 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, pro- 

 duced by a certain loss of heat, the number of degrees lost 

 from the original temperature may be restored to the con- 

 denser, 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 conveyed 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, however its expansive force 

 varied. Clement Desormes was supposed to have experi- 

 mentally 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 



