The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

302 THE STEAM ENGINE.
considerable pressure, being in a condensed state, the same
weight of it will occupy a less space; and consequently the
cylinders of high-pressure engines are smaller than those of
the same power in low-pressure engines: the magnitude of
the cylinder and the piston therefore, as well as the velocity
of the latter, will depend first upon the pressure of the
steam.
But with steam of a given pressure, the velocity of the
piston will be different: with a given capacity of cylinder,
and a given pressure of steam, the power of the engine will
determine the number of strokes per minute. But the
actual velocity of the piston will depend, in that case, on the
proportion which the diameter of the cylinder bears to its
length; the greater the diameter of the piston is with respect
to its length, the less will be its velocity. In case of stationary engines used on land, that proportion of the diameter
of the cylinder to its length is selected which is thought
to contribute to the most efficient performance of the machine. According to some engineers, the length of the
cylinder should be twice its diameter;* others make the length
equal to two diameters and a half; but there are circumstances in which considerations of practical convenience render it necessary to depart from these proportions. In marine
engines, where great length of cylinder would be inadmissible, and where, on the other hand, considerable power is
required, cylinders of short stroke and great diameter are
used. In these engines the length of the stroke is often not
greater than the diameter of the piston, and sometimes even
less.
The actual velocity which has been found to have the best
practical effect, for the piston in low-pressure engines, is
about 200 feet per minute. This, however, is subject to
some variation.
(140.) A given weight or measure of fuel burnt tinder the
* This is the proportion under which the cylinder with a given capacity will
present the least possible surface to the cooling effect of the atmosphere.