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.