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 boiler is reversed, and the steam is introduced on the other side of the piston, while the steam which has just urged the piston forwards is permitted to escape into the atmosphere. It is evident that the only resistance to the motion of the piston here is the pressure of that portion of steam which does not escape into the air; which pressure will be equal to that of the air itself, inasmuch as the steam will continue to escape from the cylinder as long as its elastic force exceeds that of the atmosphere. In this manner the alternate motion of the piston in the cylinder will be continued; the efficient force which urges it being estimated by the excess of the actual pressure of the steam from the boiler above the atmospheric pressure. The superior simplicity and lightness of the high-pressure engine must now be apparent, and these qualities recommend it strongly for all purposes in which the engine itself must be moved from place to place.