ce left vacant by
the retreating piston. The common pump works in a similar manner. It
consists of a piston or plunger which moves back and forth in an
air-tight cylinder, and contains an outward opening valve through
which water and air can pass. From the bottom of the cylinder a tube
runs down into the well or reservoir, and water from the well has
access to the cylinder through another outward-moving valve. In
practice the tube is known as the suction pipe, and its valve as the
suction valve.

In order to understand the action of a pump, we will suppose that no
water is in the pump, and we will pump until a stream issues from the
spout. The various stages are represented diagrammatically by Figure
133. In (1) the entire pump is empty of water but full of air at
atmospheric pressure, and both valves are closed. In (2) the plunger
is being raised and is lifting the column of air that rests on it. The
air and water in the inlet pipe, being thus partially relieved of
downward pressure, are pushed up by the atmospheric pressure on the
surface of the water in the well. When the piston moves downward as in
(3), the valve in the pipe closes by its own weight, and the air in
the cylinder escapes through the valve in the plunger. In (4) the
piston is again rising, repeating the process of (2). In (5) the
process of (3) is being repeated, but water instead of air is escaping
through the valve in the plunger. In (6) the process of (2) is being
repeated, but the water has reached the spout and is flowing out.

[Illustration: FIG. 133. Diagram of the process of pumping.]

After the pump is in condition (6), motion of the plunger is followed
by a more or less regular discharge of water through the spout, and
the quantity of water which gushes forth depends upon the speed with
which the piston is moved. A strong man giving quick strokes can
produce a large flow; a child, on the other hand, is able to produce
only a thin stream. Whoever pumps must exert sufficient force to lift
the water from the surface of the well to the spout exit. For this
reason the pump has received the name of _lifting pump_.

[Illustration: FIG. 134.--Force pump.]

184. The Force Pump. In the common pump, water cannot not be raised
higher than the spout. In many cases it is desirable to force water
considerably above the pump itself, as, for instance, in the fire
hose; under such circumstances a type of pump is employed which has
received the name of _f