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 mover 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 forward is reconverted into water, the piston is relieved from its action, and returns again to the bottom of the cylinder, by which a partial motion is continued. 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.
The power of such a machine will obviously depend on the dimensions of the boiler, and on the magnitude of the piston or the movable surface which is exposed to the action of the steam, and partly on the pressure or temperature of the steam itself. The object of converting the steam into water by cold, upon that side of the piston towards which the motion takes
