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_). The greatest range of
temperature possible or conceivable is from the absolute temperature
of the substance at the commencement of the operation down to absolute
zero of temperature, and the fraction of this which can be utilized is
the ratio which the range of temperature through which the substance
is working bears to the absolute temperature at the commencement of
the action. If W = the greatest amount of effect to be expected, T and
_t_ the absolute temperatures, and H the total quantity of heat
(expressed in foot pounds or in water evaporated, as the case may be)
potential in the substance at the higher temperature, T, at the
beginning of the operation, then Carnot's law is expressed by the
equation:

/ T - t \