orce pump_. This differs but little from the
ordinary lift pump, as a reference to Figure 134 will show. Here both
valves are placed in the cylinder, and the piston is solid, but the
principle is the same as in the lifting pump.

An upward motion of the plunger allows water to enter the cylinder,
and the downward motion of the plunger drives water through _E_. (Is
this true for the lift pump as well?) Since only the downward motion
of the plunger forces water through _E_, the discharge is intermittent
and is therefore not practical for commercial purposes. In order to
convert this intermittent discharge into a steady stream, an air
chamber is installed near the discharge tube, as in Figure 135. The
water forced into the air chamber by the downward-moving piston
compresses the air and increases its pressure. The pressure of the
confined air reacts against the water and tends to drive it out of the
chamber. Hence, even when the plunger is moving upward, water is
forced through the pipe because of the pressure of the compressed
air. In this way a continuous flow is secured.

[Illustration: FIG 135.--The air chamber _A_ insures a continuous flow
of water.]

The height to which the water can be forced in the pipe depends upon
the size and construction of the pump and upon the force with which
the plunger can be moved. The larger the stream desired and the
greater the height to be reached, the stronger the force needed and
the more powerful the construction necessary.

The force pump gets its name from the fact that the moving piston
drives or forces the water through the discharge tube.

185. Irrigation and Drainage. History shows that the lifting pump
has been used by man since the fourth century before Christ; for many
present-day enterprises this ancient form of pump is inconvenient and
impracticable, and hence it has been replaced in many cases by more
modern types, such as rotary and centrifugal pumps (Fig. 136). In
these forms, rapidly rotating wheels lift the water and drive it
onward into a discharge pipe, from which it issues with great force.
There is neither piston nor valve in these pumps, and the quantity of
water raised and the force with which it is driven through the pipes
depends solely upon the size of the wheels and the speed with which
they rotate.

Irrigation, or the artificial watering of land, is of the greatest
importance in those parts of the world where the land is naturally too
dry for farm