Another and very different use to which pumps are put is seen in the compression of gases.  Air is forced into the tires of bicycles and automobiles until they become sufficiently inflated to insure comfort in riding.  Some present-day systems of artificial refrigeration (Section 93) could not exist without the aid of compressed gases.

Compressed air has played a very important role in mining, being sent into poorly ventilated mines to improve the condition of the air, and to supply to the miners the oxygen necessary for respiration.  Divers and men who work under water carry on their backs a tank of compressed air, and take from it, at will, the amount required.

There are many forms of pumps, and they serve widely different purposes, being essential to the operation of many industrial undertakings.  In the following Sections some of these forms will be studied.

[Illustration:  FIG. 131.—­Carrying water home from the spring.]

182.  The Air as Man’s Servant.  Long before man harnessed water for turbines, or steam for engines, he made the air serve his purpose, and by means of it raised water from hidden underground depths to the surface of the earth; likewise, by means of it, he raised to his dwelling on the hillside water from the stream in the valley below.  Those who live in cities where running water is always present in the home cannot realize the hardship of the days when this “ready-made” supply did not exist, but when man laboriously carried to his dwelling, from distant spring and stream, the water necessary for the daily need.

What are the characteristics of the air which have enabled man to accomplish these feats?  They are well known to us and may be briefly stated as follows:—­

(1) Air has weight, and 1 cubic foot of air, at atmospheric pressure, weighs 1-1/4 ounces.

(2) The air around us presses with a force of about 15 pounds upon every square inch of surface that it touches.

(3) Air is elastic; it can be compressed, as in the balloon or bicycle tire, but it expands immediately when pressure is reduced.  As it expands and occupies more space, its pressure falls and it exerts less force against the matter with which it comes in contact.  If, for example, 1 cubic foot of air is allowed to expand and occupy 2 cubic feet of space, the pressure which it exerts is reduced one half.  When air is compressed, its pressure increases, and it exerts a greater force against the matter with which it comes in contact.  If 2 cubic feet of air are compressed to 1 cubic foot, the pressure of the compressed air is doubled. (See Section 89.)

[Illustration:  FIG. 132.—­The atmosphere pressing downward on a pushes water after the rising piston b.]