cted at public
expense; the contents of these are discharged in turn into harbors and
streams, or are otherwise disposed of at great expense, although they
contain valuable substances. It has been estimated that the drainage
or sewage of England alone would be worth $ 80,000,000 a year if used
as fertilizer.

A few cities, such as Columbus and Cleveland, Ohio, realize the need
of utilizing this source of wealth, and by chemical means deodorize
their sewage and change it into substances useful for agricultural and
industrial purposes. There is still a great deal to be learned on this
subject, and it is possible that chemically treated sewage may be made
a source of income to a community rather than an expense.

189. Pumps which Compress Air. The pumps considered in the preceding
Sections have their widest application in agricultural districts,
where by means of them water is raised to the surface of the earth or
is pumped into elevated tanks. From a commercial and industrial
standpoint a most important class of pump is that known as the
compression type; in these, air or any other gas is compressed rather
than rarefied.

Air brakes and self-opening and self-closing doors on cars are
operated by means of compression pumps. The laying of bridge and pier
foundations, in fact all work which must be done under water, is
possible only through the agency of compression pumps. Those who have
visited mines, and have gone into the heart of the underground
labyrinth, know how difficult it is for fresh air to make its way to
the miners. Compression pumps have eliminated this difficulty, and
to-day fresh air is constantly pumped into the mines to supply the
laborers there. Agricultural methods also have been modified by the
compression pump. The spraying of trees (Fig. 143), formerly done
slowly and laboriously, is now a relatively simple matter.

[Illustration: FIG. 143.--Spraying trees by means of a compression
pump.]

190. The Bicycle Pump. The bicycle pump is the best known of all
compression pumps. Here, as in other pumps of its type, the valves
open inward rather than outward. When the piston is lowered,
compressed air is driven through the rubber tubing, pushes open an
inward-opening valve in the tire, and thus enters the tire. When the
piston is raised, the lower valve closes, the upper valve is opened
by atmospheric pressure, and air from outside enters the cylinder; the
next stroke of the piston drives a fresh supply