From the descriptions and experiments preceding, which illustrate the
development of storms, reference was made only to the winds blowing in
toward the storm center. Naturally the question comes to your mind: What
happens to them after the cold air has taken the place of the warm air?
They change to other directions when the storm has passed away. It is
because of this fact that we look for a change in weather conditions
when the wind changes—a very important sign that you will be interested
in later on.
It is well to mention here a thing that is going to be very important to
us when we study the barometer, that is, the pressure of the atmosphere.
Should the pressure of the air, which is normally at sea level 14.7
pounds to the square inch, change, that is, become lighter, it would not
exert so much pressure on the column of mercury in the tube of the
barometer and the mercury would drop in the tube. (See Fig. 7.) On the
other hand, if the weight of the air was increased, that is, if it
became heavier, it would force the mercury to rise in the tube. This
should be quite clear to you, because it is the lightness and heaviness
of the air that is going to interest us more particularly than any other
part of the subject when we get into the study of the atmospheric
changes, what causes them, and the indications that lead up to our
conclusions. In order that this principle is absolutely clear to you,
you should perform Experiment 4, or if you have not facilities for doing
it, it is well to see it performed in any physics laboratory.
Immediately you ask yourself: If air has such a tremendous pressure as
14.7 pounds to the square inch, why is it that a weight of air amounting
to thirty-five thousand pounds bearing down on the average individual
does not cave the body in? Simply because air penetrates the body so
easily that it exerts as much pressure on the inside as on the outside,
and thereby equalizes itself. For instance, if you go down into a subway
or a caisson (a water-tight box or chamber within which submarine
construction is carried on under great air pressure to keep out the
water), where the pressure is sometimes greater than it is outside, have
you noticed the effect this pressure exerts on the ear drums? As it
becomes greater, you may equalize it by swallowing, which allows the air
to get back of the ear drums through the Eustachian tubes, which lead
from the mouth to the inner ear.