In 1831, the steam-engine for these purposes was commonly the
condensing beam engine, and was supplied with steam from boilers,
known, from their shape, as wagon boilers; this shape appears to have
been chosen rather for the convenience of the sweeps, who periodically
went through the flues to remove the soot consequent on the imperfect
combustion, than for the purpose of withstanding any internal pressure
of steam. The necessary consequence was, that the manufacturing
engines of those days were compelled to work with steam of from only
3½ lb. to 5 lb. per square inch of pressure above atmosphere. The
piston speed rarely exceeded 250 feet per minute, and as a result of
the feeble pressure, and of the low rate of speed, very large
cylinders indeed were needed relatively to the power obtained. The
consumption of fuel was heavy, being commonly from 7 lb. to 10 lb. per
gross indicated horsepower per hour. The governing of the engine was
done by pendulum governors, revolving slowly, and not calculated to
exert any greater effort than that of raising the balls at the end of
the pendulum arms, thus being, as will be readily seen, very
inefficient regulators. The connection of the parts of the engine
between themselves was derived from the foundation upon which the
engine was supported. Incident to the low piston speed was slowness of
revolution, rendering necessary heavy fly wheels, to obtain even an
approach to practical uniformity of rotation, and frequently rendering
necessary also heavy trains of toothed gearing, to bring up the speed
from that of the revolutions of the engine to that of the machinery it
was intended to drive.
In 1881, the boilers are almost invariably cylindrical, and are very
commonly internally fired, either by one flue or by two; we owe it to
the late Sir William Fairbairn, President of the British Association
in 1861, that the danger, which at one time existed, of the collapse
of these fire flues, has been entirely removed by his application of
circumferential bands. Nowadays there are, as we know, modifications
of Sir William Fairbairn's bands, but by means of his bands, or by
modifications thereof, all internally flued boilers are so
strengthened that the risk of a collapse of the flue is at an end.
Boilers of this kind are well calculated to furnish--and commonly do
furnish--steam of from 40 lb. to 80 lb. pressure above atmosphere.
The piston speed is now very generally 400 feet or more, so that,
notwithstanding that there is usually a liberal expansion, the mean
pressure upon the piston is increased, and this, coupled with its
increased speed, enables much more power to be obtained from a given
size of cylinder than was formerly obtainable. The revolutions of the
engine now are as many as from 60 to 200 per minute, and thus, with
far lighter fly-wheels, uniformity of rotation is much more nearly
attained.