certainly the first to solve them.

It is known how, in 1824, in his _Réflexions sur la puissance motrice
du feu_, he endeavoured to prove that "the motive power of heat is
independent of the agents brought into play for its realization," and
that "its quantity is fixed solely by the temperature of the bodies
between which, in the last resort, the transport of caloric is
effected"--at least in all engines in which "the method of developing
the motive power attains the perfection of which it is capable"; and
this is, almost textually, one of the enunciations of the principle at
the present day. Carnot perceived very clearly the great fact that, to
produce work by heat, it is necessary to have at one's disposal a fall
of temperature. On this point he expresses himself with perfect
clearness: "The motive power of a fall of water depends on its height
and on the quantity of liquid; the motive power of heat depends also
on the quantity of caloric employed, and on what might be called--in
fact, what we shall call--the height of fall, that is to say, the
difference in temperature of the bodies between which the exchange of
caloric takes place."

Starting with this idea, he endeavours to demonstrate, by 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