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 ...

136 THE STEAMI ENGINE.
continually increasing in its bullk, and therefore decreasing
in its pressure, from whence it follows, that the force which
resists the lesser piston is continually decreasing, while that
which presses it down remains the same, and, therefore,
the effective force which impels it must be continually
increasing.
On the other hand, the force which urges the greater piston
is continually decreasing, since there is a vacuum below it,
and the steam which presses it is continually expanding into
an increased bulk.
Impelled in this way, let us suppose the pistons to have
arrived at the bottoms of the cylinders, as in fig. 42, and let
the valves G, L, and o be closed, and the valves I and N
opened. No steam is allowed to flow from the boiler, G being
closed, nor any allowed to pass into the condenser, since o
is closed, and all communications between the cylinders is
stopped by closing L. By opening the valve I, a firee comnication is made between the top and bottom of the lesser
piston through the tube H, so that the steam which presses
above the lesser piston will exert the same pressure below it,
and the piston is in a state of indifference. In the same
manner the valve N being open, a free communication is
made between the top and bottom of the greater piston, and
the steam circulates above and below the piston, and leaves
it free to rise. A counterpoise attached to the pump rods in
this case draws up the piston, as in Watt's single engine; and
when they arrive at the top, the valves I and N are closed,
and G, L, and o opened, and the next descent of the piston is
produced in the manner already described, and so the process
is continued.
The valves are worked by the engine itself, by means
similar to some of those already described. By computation, we find the power of this engine tc be nearly the same
as a similar engine on Watt's expansive principle. It does
not, however, appeal that any adequate advantage was gained