bulk takes place. In this change, a solid inch
of water enlarges its size about 1,700 times, and
forms 1,700 solid inches of steam. This expansion
takes place accompanied with a certain force or pressure,
by which the vapour has a tendency to burst the bounds
of any vessel which contains it. The steam which
fills 1,700 solid inches at the temperature of 212
deg., will, if cooled below that temperature, return
to the liquid form, and occupy only one solid inch,
leaving 1,699 solid inches vacant; and, if it be included
in a close vessel, leaving the surfaces of that vessel
free from the internal pressure to which they were
subject before the return of the water to the liquid
form. If it be possible, therefore, alternately
to convert water into vapour by heat, and to reconvert
the vapour into water by cold, we shall be enabled
alternately to submit any surface to a pressure equal
to the elastic force of the steam, and to relieve
it from that pressure, so as to permit it to move
in obedience to any other force which may act upon
it. Or again, suppose that we are enabled to expose
one side of a movable body to the action of water
converted into steam, at the moment that we relieve
the other side from the like pressure by reconverting
the steam which acts upon it into water, the movable
body will be impelled by the unresisted pressure of
the steam on one side. When it has moved a certain
distance in obedience to this force, let us suppose
that the effects are reversed. Let the steam which
pressed it forwards be now reconverted into water,
so as to have its action suspended; and at the same
moment, let steam raised from water by heat be caused
to act on the other side of the movable body; the
consequence will obviously be, that it will now change
the direction of its motion, and return in obedience
to the pressure excited on the opposite side.
Such is, in fact, the operation of an ordinary low-pressure
steam-engine. The piston or plug which plays in
the cylinder is the movable to which we have referred.
The vapour of water is introduced upon one side of
that piston at the moment that a similar vapour is
converted into water on the other side, and the piston
moves by the unresisted action of the steam. When
it has arrived at the extremity of the cylinder, the
steam which just urged it forwards is reconverted
into water, and the piston is relieved from its action.
At the same moment, a fresh supply of steam is introduced
upon the other side of the piston, and its pressure
causes the piston to be moved in a direction contrary
to its former motion. Thus the piston is moved
in the cylinder alternately in the one direction and
in the other, with a force equivalent to the pressure
of the steam which acts upon it. A strong metal
rod proceeds from this piston, and communicates with
proper machinery, by which the alternate motion of
the piston backwards and forwards, or upwards and downwards,
in the cylinder, may be communicated to whatever body
is intended to be moved.