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. The power of such a machine will obviously depend partly on the magnitude of the piston or the movable surface which is exposed to the action of the steam, and partly on the pressure 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 place, is to relieve the piston from all resistance to the moving power. This renders it unnecessary to use steam of a very high pressure, inasmuch as it will have no resistance to overcome, except the friction of the piston with the cylinder, and the ordinary resistance of the load which it may have to move. Engines constructed upon this principle, not requiring, therefore, steam of a great pressure, have been generally called "low-pressure engines." The re-conversion of the steam into water requires a constant and abundant supply of cold water, and a fit apparatus for carrying away the water which becomes heated, by cooling the steam, and for supplying its place by a fresh quantity of cold water. It is obvious that such an apparatus is incompatible with great simplicity and lightness, nor can it be applied to cases where the engine is worked under circumstances in which a fresh supply of water cannot be had.