of all the condensing
apparatus and of the large supply of cold water necessary for the
reduction of steam to the liquid form; for instead of being so
reduced, the steam is in this case simply allowed to escape into the
atmosphere. The operation, therefore, of high-pressure engines will be
readily understood. The boiler producing steam of a very powerful
pressure, is placed in communication with a cylinder furnished in the
usual manner with a piston; the steam is allowed to act upon one side
of the piston so as to impel it from the one end of the cylinder to
the other. When it has arrived there, the communication with the
boiler is reversed, and the steam is introduced on the other side of
the piston, while the steam which has just urged the piston forwards
is permitted to escape into the atmosphere. It is evident that the
only resistance to the motion of the piston here is the pressure of
that portion of steam which does not escape into the air; which
pressure will be equal to that of the air itself, inasmuch as the
steam will continue to escape from the cylinder as long as its elastic
force exceeds that of the atmosphere. In this manner the alternate
motion of the piston in the cylinder will be continued; the efficient
force which urges it being estimated by the excess of the actual
pressure of the steam from the boiler above the atmospheric pressure.
The superior simplicity and lightness of the high-pressure engine must
now be apparent, and these qualities recommend it strongly for all
purposes in which the engine itself must be moved from place to place.

The steam-engine therefore consists of two distinct parts,--the
boiler, which is at once the generator and magazine of steam, and the
cylinder with its piston, which is the instrument by which this power
is brought into operation and rendered effective. The amount of the
load or resistance which such a machine is capable of moving, depends
upon the intensity or pressure of the steam produced by the boiler,
and on the magnitude of the surface of the piston in the cylinder,
upon which that steam acts. The rate or velocity of the motion
depends, not on the power or pressure of the steam, but on the rate at
which the boiler is capable of generating it. Every stroke of the
piston consumes a cylinder full of steam; and of course the rate of
the motion depends upon the number of cylinders of steam which the
boiler is capable of generating in a given time. These are two p