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 ...

t 10 THE STEAM ENGINE.
Dupper exhausting valve, which, when open, draws off the;steam from the piston to the condenser. c' is the lower
steam valve, which admits steam -below the piston; and -D,
The lower exhausting valve, which draws off the steam from
below the piston to the condenser.
Now, suppose the piston to be at the top of the cylinder,
the cylinder below it being filled with steam, which has just
pressed it up. Let the upper steam:valve A', and the lower
exhausting valve D', be opened, and the other two valves
closed. The steam which fills the cylinder below the
piston will immediately pass through the valve D' into the
condenser, and a vacuum will be produced below the piston.
At the same time, steam is admitted from the steam pipe
through the valve A' above the piston, and its pressure will
force the piston to the bottom of the cylinder. On the
arrival of the piston at the bottom of the cylinder, the upper
steam valve A', and lower exhausting valve D', are closed;
and the lower steam valve c', and upper exhausting valve','are opened. The steam which fills the cylinder above
the piston now passes off through B' into the condenser, and
leaves a vacuum above the piston. At the same time, steam
from the boiler is admitted through the lower steam valve
c',-below the piston, so that it will press the piston to the top
of the cylinder; and so the process is continued.
It appears, therefore, that the upper steam valve, and the
lower exhausting valve, must be opened together, on the
arrival of the piston at the top of the cylinder. To effect
this, one lever, E', is made to communicate by jointed rods
with both these valves, and this lever is moved by a pin
placed on the piston-rod of the air-pump; and such a position
may be given to this pin as to produce the desired effect
exactly at the proper moment of time. In like manner, another lever, F', communicates by jointed rods with the upper
exhausting valve and lower steam valve, so as to open them
and close them together; and this lever, in like manner, is
worked by a-pin on the piston-rod of the air-pump.