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 ...

'92 THE STEAM ENGINE.
taking place only in one direction would be inadmissible.
To drive the machinery generally employed in manufactures
a constant and uniform force is required; and to render the
steam engine available for this purpose, it would be necessary
that the beam should be driven by the moving power as well
in its ascent as in its descent.
When WVatt first conceived the notion of extending the
application of the engine to manufactures generally, he proposed to accomplish this double action upon the beam by
placing a steam cylinder under each end of it, so that while
each piston would be ascending, and not impelled by the
steam, the other would be descending, being urged downwards by the steam above it acting against the vacuum
below. Thus, the power acting on each during the time
when its action on the other would be suspended, a constant
force would be exerted upon the beam, and the uniformity
of the motion would be produced by making both cylinders
communicate with the same boiler, so that both pistons
would be driven by steam of the same pressure. One condenser might also be used for both cylinders, so that a similar
vacuum would be produced under each.
This arrangement, however, was soon laid aside for one
much more simple and obvious. This consisted in the production of exactly the same effect by a single cylinder in
which steam was introduced alternately above and below
the piston, being at the same time withdrawn by the condenser at the opposite side. Thus the piston being at the
top of the cylinder, steam is introduced from the boiler
above it, while the steam in the cylinder below it is drawn
off by the condenser. The piston, therefore, is pressed firom
above into the vacuum below, and descends to the bottom of
the cylinder. Having arrived there, the top of the cylinder
is cut off from all communication with the boiler; and, on
the other hand, a communication is opened between it and
the condenser. The steam which has pressed the piston
down is therefore drawn off by the condenser, while a ccm.