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.

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