tly in the pocket, but they get out of order
easily and must be frequently readjusted. The aneroid barometer is an
air-tight box whose top is made of a thin metallic disk which bends
inward or outward according to the pressure of the atmosphere. If the
atmospheric pressure increases, the thin disk is pushed slightly
inward; if, on the other hand, the atmospheric pressure decreases, the
pressure on the metallic disk decreases and the disk is not pressed so
far inward. The motion of the disk is small, and it would be
impossible to calculate changes in atmospheric pressure from the
motion of the disk, without some mechanical device to make the slight
changes in motion perceptible.

[Illustration: FIG. 45.--Aneroid barometer.]

In order to magnify the slight changes in the position of the disk,
the thin face is connected with a system of levers, or wheels, which
multiplies the changes in motion and communicates them to a pointer
which moves around a graduated circular face. In Figure 45 the real
barometer is scarcely visible, being securely inclosed in a metal case
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.]

Aeronauts have never ascended much higher than 7 mile