y associating
two engines capable of working in a reversible cycle, that the
principle is founded on the impossibility of perpetual motion.

His memoir, now celebrated, did not produce any great sensation, and
it had almost fallen into deep oblivion, which, in consequence of
the discovery of the principle of equivalence, might have seemed
perfectly justified. Written, in fact, on the hypothesis of the
indestructibility of caloric, it was to be expected that this memoir
should be condemned in the name of the new doctrine, that is, of the
principle recently brought to light.

It was really making a new discovery to establish that Carnot's
fundamental idea survived the destruction of the hypothesis on the
nature of heat, on which he seemed to rely. As he no doubt himself
perceived, his idea was quite independent of this hypothesis, since,
as we have seen, he was led to surmise that heat could disappear; but
his demonstrations needed to be recast and, in some points, modified.

It is to Clausius that was reserved the credit of rediscovering the
principle, and of enunciating it in language conformable to the new
doctrines, while giving it a much greater generality. The postulate
arrived at by experimental induction, and which must be admitted
without demonstration, is, according to Clausius, that in a series of
transformations in which the final is identical with the initial
stage, it is impossible for heat to pass from a colder to a warmer
body unless some other accessory phenomenon occurs at the same time.

Still more correctly, perhaps, an enunciation can be given of the
postulate which, in the main, is analogous, by saying: A heat motor,
which after a series of transformations returns to its initial state,
can only furnish work if there exist at least two sources of heat, and
if a certain quantity of heat is given to one of the sources, which
can never be the hotter of the two. By the expression "source of
heat," we mean a body exterior to the system and capable of furnishing
or withdrawing heat from it.

Starting with this principle, we arrive, as does Clausius, at the
demonstration that the output of a reversible machine working between
two given temperatures is greater than that of any non-reversible
engine, and that it is the same for all reversible machines working
between these two temperatures.

This is the very proposition of Carnot; but the proposition thus
stated, while very useful for the theory of engi