ing. 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.

[Illustration: FIG. 136.--Centrifugal pump with part of the casing]
cut away to show the wheel.

[Illustration: FIG. 137.--Agriculture made possible by irrigation.]

The draining of land is also a matter of considerable importance;
swamps and marshes which were at one time considered useless have been
drained and then reclaimed and converted into good farming land. The
surplus water is best removed by centrifugal pumps, since sand and
sticks which would clog the valves of an ordinary pump are passed
along without difficulty by the rotating wheel.

[Illustration: FIG. 138.--Rice for its growth needs periodical
flooding, and irrigation often supplies the necessary water.]

186. Camping.--Its Pleasures and its Dangers. The allurement of a
vacation camp in the heart of the woods is so great as to make many
campers ignore the vital importance of securing a safe water supply. A
river bank may be beautiful and teeming with diversions, but if the
river is used as a source of drinking water, the results will almost
always be fatal to some. The water can be boiled, it is true, but few
campers are willing to forage for the additional wood needed for this
apparently unnecessary requirement; then, too, boiled water does not
cool readily in summer, and hence is disagreeable for drinking
purposes.

The only safe course is to abandon the river as a source of drinking
water, and if a spring cannot be found, to drive a well. In many
regions, especially in the neighborhood of streams, water can be
found ten or fifteen feet below the surface. Water taken from such a
depth has filtered through a bed of soil, and is fairly safe for any
purpose. Of course the deeper the well, the safer will be the water.
With the use of such a pump as will b