umps, 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_.]

183. The Common Pump or Lifting Pump. Place a tube containing a
close-fitting piston in a vessel of water, as shown in Figure 132.
Then raise the piston with the hand and notice that the water rises in
the piston tube. The rise of water in the piston tube is similar to
the raising of lemonade through a straw (Section 77). The atmosphere
presses with a force of 15 pounds upon every square inch of water in
the large vessel, and forces some of it into the spa