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 ...

DOUBLE-CYLINDER ENGINES. 143
above the surface of the liquid in H forces it through i into
the boiler. When the air accumulates in too great a degree
in H, the surface of the liquid is pressed so low that the ball
falls and opens the valve, and allows it to escape. The air
in H is that which is pumped from the condenser with the
liquid, and which was disengaged from it.
Let us suppose the piston at the top of the cylinder; it
strikes the tail of the valve T, and raises it, while the stem
of the piston valve R strikes the top of the cylinder, and is
pressed into its seat. A free communication is at the same
time open between the cylinder, below the piston and the
condenser, through the tube D. The pressure of the steam
thus admitted above the piston, acting against the vacuum
below it, will cause its descent. On arriving at the bottom
of the cylinder, the tail of the piston valve R will strike the
bottom, and it will be lifted from its seat, so that a communication will be opened through it with the condenser. At
the same moment, a projecting spring i, attached to the
piston-rod, strikes the stem of the steani valve T, and presses
it into its seat. Thus, while the further admission of steam
is cut off, the steam above the piston flows into the condenser,
and the piston, being relieved from all pressure, is drawn up
by the momentum of the fly wheel, which continues the
motion it received from the descending force. On the
arrival of the piston again at the top of the cylinder, the
valve T is opened and R closed, and the piston descends as
before, and so the process is continued.
The mechanism by which motion is communicated from
the piston to the fly wheel is peculiarly elegant. On the
axis of the fly wheel is a small wheel with teeth, which work
in the teeth of another large wheel L. This wheel is turned
by a crank, which is workied by a cross piece attached to the
end of the piston-rod. Another equal-toothed wheel M is
turned by a crank, which is worked by the other end of the
cross arm attached to the piston-rod.