for protection; the principle, however, can be understood by reference
to Figure 46.

[Illustration: FIG. 46.--Principle of the aneroid barometer.]

80. The Weight of the Air. We have seen that the pressure of the
atmosphere at any point is due to the weight of the air column which
stretches from that point far up into the sky above. This weight
varies slightly from time to time and from place to place, but it is
equal to about 15 pounds to the square inch as shown by actual
measurement. It comes to us as a surprise sometimes that air actually
has weight; for example, a mass of 12 cubic feet of air at average
pressure weighs 1 pound, and the air in a large assembly hall weighs
more than 1 ton.

We are practically never conscious of this really enormous pressure of
the atmosphere, which is exerted over every inch of our bodies,
because the pressure is exerted equally over the outside and the
inside of our bodies; the cells and tissues of our bodies containing
gases under atmospheric pressure. If, however, the finger is placed
over the open end of a tube and the air is sucked out of the tube by
the mouth, the flesh of the finger bulges into the tube because the
pressure within the finger is no longer equalized by the usual
atmospheric pressure (Fig. 47).

[Illustration: FIG. 47.--The flesh bulges out.]

Aëronauts have never ascended much higher than 7 miles; at that height
the barometer stands at 7 inches instead of at 30 inches, and the
internal pressure in cells and tissues is not balanced by an equal