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