the fuel. Strictly speaking, marsh gas should be separately
determined. It often happens that available energy is not in a form in
which it can be applied directly to our needs. The water flowing down
from the mountains in the neighborhood of the Alpine tunnels was
competent to provide the power necessary for boring through them, but
it was not in a form in which it could be directly applied. The
kinetic energy of the water had first to be changed into the potential
energy of air under pressure, then, in that form, by suitable
mechanism, it was used with signal success to disintegrate and
excavate the hard rock of the tunnels. The energy resulting from
combustion is also incapable of being directly transformed into useful
motive power; it must first be converted into potential force of steam
or air at high temperature and pressure, and then applied by means of
suitable heat engines to produce the motions we require. It is
probably to this circumstance that we must attribute the slowness of
the human race to take advantage of the energy of combustion. 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,