cock I sufficiently open. It will thus be immediately condensed or reduced
to water, and the cylinder below the piston will have a vacuum in it. The
steam therefore admitted from the steam pipe through the open valve V to
the top of the cylinder, not being resisted by pressure below, will press
the piston to the bottom of the cylinder. As it approaches that position,
the handle H will be struck down by the upper pin or tappet on the air pump
rod, and the valves V and E', previously open, will be closed, while the
valves V' and E, previously closed, will be opened. The steam which has
just pressed down the piston, and which now fills the cylinder above the
piston, will then flow off, through the open valve E, to the condenser,
where it will be immediately condensed by the jet of cold water; and steam
from the boiler, admitted through the open valve V', will fill the cylinder
below the piston, and press the piston upwards. When the piston has reached
the top of the cylinder, the lower pin on the air pump rod will have struck
the handle upwards, and will thereby have closed the valves V' and E, and
opened the valves V and E'. The piston will then be in the same situation
as in the commencement, and will again descend, and so will continue to be
driven up and down by the steam.

101. Q.--But what becomes of the cold water which is let into the condenser
to condense the steam?

A.--It is pumped out by the air pump in the shape of hot water, its
temperature having been raised considerably by the admixture of the steam
in it. When the air pump piston ascends it leaves behind it a vacuum; and
the foot valve being relieved from all pressure, the weight of the water in
the condenser forces it open, and the warm water flows from the condenser
into the lower part of the air pump, from which its return to the condenser
is prevented by the intervening valve. When the air pump piston descends,
its pressure on the liquid under it will force open the valve in it,