The
history of the steam engine hardly dates back 200 years, a very small
fraction of the centuries during which man has existed, even since
historic times.
The apparatus by means of which the potential energy of fuel with
respect to oxygen is converted into the potential energy of steam, we
call a steam boiler; and although it has neither cylinder nor piston,
crank nor fly wheel, I claim for it that it is a veritable heat
engine, because it transmits the undulations and vibrations caused by
the energy of chemical combination in the fuel to the water in the
boiler; these motions expend themselves in overcoming the liquid
cohesion of the water and imparting to its molecules that vigor of
motion which converts them into the molecules of a gas which,
impinging on the surfaces which confine it and form the steam space,
declare their presence and energy in the shape of pressure and
temperature. A steam pumping engine, which furnishes water under high
pressure to raise loads by means of hydraulic cranes, is not more
truly a heat engine than a simple boiler, for the latter converts the
latent energy of fuel into the latent energy of steam, just as the
pumping engine converts the latent energy of steam into the latent
energy of the pumped-up accumulator or the hoisted weight.
If I am justified in taking this view, then I am justified in applying
to my heat engine the general principles laid down in 1824 by Sadi
Carnot, namely, that the proportion of work which can be obtained out
of any substance working between two temperatures depends entirely and
solely upon the difference between the temperatures at the beginning
and end of the operation; that is to say, if T be the higher
temperature at the beginning, and _t_ the lower temperature at the end
of the action, then the maximum possible work to be got out of the
substance will be a function of (T-_t_).
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