CARNOT’S THEORY OF THE MOTIVE POWER OF HEAT. 545 
II. On the measurement of Thermal Agency, considered with reference to its 
equivalent of mechanical affect. 
12. A perfect thermo-dynamic engine of any kind, is a machine by means of 
which the greatest possible amount of mechanical effect can be obtained from a 
given thermal agency ; and, therefore, if in any manner we can construct or ima- 
gine a perfect engine which may be applied for the transference of a given quan- 
tity of heat from a body at any given temperature, to another body, at a lower 
given temperature, and if we can evaluate the mechanical effect thus obtained, 
we shall be able to answer the question at present under consideration, and so to 
complete the theory of the motive power of heat. But whatever kind of engine 
we may consider with this view, it will be necessary for us to prove that it is a 
perfect engine; since the transference of the heat from one body to the other may 
be wholly, or partially, effected by conduction through a solid,* without the de- 
velopment of mechanical effect; and, consequently, engines may be constructed 
in which the whole, or any portion of the thermal agency is wasted. Hence it is 
of primary importance to discover the criterion of a perfect engine. This has 
been done by Carnot, who proves the following proposition :— 
13. A perfect thermo-dynamic engine is such that, whatever amount of mecha- 
nical effect it can derive from a certain thermal agency ; if an equal amount be spent 
in working it backwards, an equal reverse thermal effect will be produced.+ 
14. This proposition will be made clearer by the applications of it which 
are given below (§ 29), in the cases of the air-engine and the steam-engine, than it 
could be by any general explanation; and it will also appear, from the nature 
of the operations described in those cases, and the principles of CarNnot’s reason- 
ing, that a perfect engine may be constructed with any substance of an in- 
destructible texture as the alternately expanding and contracting medium. 
Thus we might conceive thermo-dynamic engines founded upon the expansions 
* When “ thermal agency” is thus spent in conducting heat through a solid, what becomes of 
the mechanical effect which it might produce? Nothing can be lost in the operations of nature— 
no energy can be destroyed. What effect then is produced in place of the mechanical effect which is 
lost? A perfect theory of heat imperatively demands an answer to this question; yet no answer can 
be given in the present state of science. A few years ago, a similar confession must have been made 
with reference to the mechanical effect lost in a fluid set in motion in the interior of a rigid closed 
vessel, and allowed to come to rest by its own internal friction; but in this case, the foundation of a 
solution of the difficulty has been actually found, in Mr Jovtz’s discovery of the generation of heat, 
by the internal friction of a fluid in motion. Encouraged by this example, we may hope that the 
very perplexing question in the theory of heat, by which we are at present arrested, will, before long, 
be cleared up. 
It might appear, that the difficulty would be entirely avoided, by abandoning Carnot’s funda- 
mental axiom ; a view which is strongly urged by Mr Jouxe (at the conclusion of his paper “ On 
the Changes of Temperature produced by the Rarefaction and Condensation of Air.” Phil. Mag., 
May 1845, vol. xxvi.) If we do so, however, we meet with innumerable other difficulties—insuper- 
able without farther experimental investigation, and an entire reconstruction of the theory of heat, 
from its foundation. It is in reality to experiment that we must look—either for a verification of 
Carnot’s axiom, and an explanation of the difficulty we havé been considering ; or for an entirely new 
basis of the Theory of Heat. 
+ For a demonstration, see § 29, below. 
VOL. XVI. PART Y. 
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