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 farming.  In the United States, approximately two fifths of the land area is so dry as to be worthless for agricultural purposes unless artificially watered.  In the West, several large irrigating systems have been built by the federal government, and at present about ten million acres of land have been converted from worthless farms into fields rich in crops.  Many irrigating systems use centrifugal pumps to force water over long distances and to supply it in quantities sufficient for vast agricultural needs.  In many regions, the success of a farm or ranch depends upon the irrigation furnished in dry seasons, or upon man’s ability to drive water from a region of abundance to a remote region of scarcity.