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.

The re-conversion of steam into water, or, as it is technically called, the condensation of steam, is however by no means necessary to the effective operation of a steam-engine.  From what has been above said, it will be understood that this effect relieves the piston of a part of the resistance which is opposed to its motion.  If that part of the resistance were not removed, the pressure of steam acting upon the other side would be affected in no other way than by having a greater load or resistance to overcome; and if that pressure were proportionately increased, the effective power of the machine would remain the same.  It follows, therefore, that if the steam upon that side of the piston towards which the motion is made were not condensed, the steam urging the piston forwards on the other side would require to have a degree of intensity greater than the steam in a low-pressure engine, by the amount of the pressure of the uncondensed steam on the other side of the piston.  An engine working on this principle has therefore been called a high-pressure engine.  Such an engine is relieved from the incumbrance of all the condensing apparatus and of the large supply of cold water necessary for the reduction of steam to the liquid form; for instead of being so reduced, the steam is in this case simply allowed to escape into the atmosphere.  The operation, therefore, of high-pressure engines will be readily understood.  The boiler producing steam of a very powerful pressure, is placed in communication with a cylinder furnished in the usual manner with a piston; the steam is allowed to act upon one side of the piston so as to impel it from the one end of the cylinder to the other.  When it has arrived there, the communication with the