By far the best way to avoid contamination is to see to it that the sewage discharges into the ground below the well; that is, to dig the well in such a location that the sewage drainage will be away from the well.

In cities and towns and large summer communities, the sewage of individual buildings drains into common tanks erected 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 of air into the tire, which thus in time becomes inflated.  In most cheap bicycle pumps, the piston valve is replaced by a soft piece of leather so attached to the piston that it allows air to slip around it and into the cylinder, but prevents its escape from the cylinder (Fig. 144).