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 ...

WATT S SINGLE-ACTING STEAM ENGINE. 85
iteam piston falls, and vice versa. In the single engine
there are some advantages in the latter arrangement. As the
Iteam piston descends, the steam rushes into the condenser,
mnd the jet is playing; and this, therefore, is the most favourtble time for drawing out the water and condensed steam
from the condenser by the ascent of the pump piston, since
by this means the descent of the steam piston is assisted; an
-effect which would not be produced if the steam piston and
pump piston descended together.
Writh respect to the method of opening and closing the
valves, it is evident that the three valves which are simultaneously opened and closed may be so connected as to be
worked by the same lever. This lever may be struck by a
pin fixed upon the rod q of the air-pump, so that, when the
pistons have arrived at the top of the cylinders, the pin strikes
the lever, and opens the three valves. A catch or detent is
provided for keeping them open during the descent of the
piston, from which they are disengaged in a similar manner
on the arrival of the piston at the bottom of the cylinder,
and they close by their own weight.
In exactly the same way, the lower steam valve is opened
on the arrival of the piston at the bottom of the cylinder,
and closed on its arrival at the top, by the action of a pin
placed on the piston-rod of the air-pump.
(51.) Soon after the invention of these engines, Watt
found that in some instances inconvenience arose from the
too rapid motion of the steam piston at the end of its stroke,
owing to its being moved with an accelerated motion.
This was owing to the uniform action of the steam pressure
upon it: for upon first putting it in motion at the top of the
cylinder, the motion was comparatively slow; but from the
continuance of the same pressure, the velocity with which the
piston descended was continually increasing, until it reached
the bottom of the cylinder, where it acquired its greatest
velocity. To prevent this, and to render the descent as
nearly as possible uniform, it was proposed to cut off the
H