sea level
there is a high column of air, and a heavy air pressure. As one passes
from sea level to mountain top a gradual but steady decrease in the
height of the air column occurs, and hence a gradual but definite
lessening of the air pressure.

[Illustration: FIG. 41.--The water in the tube is at the same level as
that in the glass.]

77. Air Pressure. If an empty tube (Fig. 41) is placed upright in
water, the water will not rise in the tube, but if the tube is put in
water and the air is then drawn out of the tube by the mouth, the
water will rise in the tube (Fig. 42). This is what happens when we
take lemonade through a straw. When the air is withdrawn from the
straw by the mouth, the pressure within the straw is reduced, and the
liquid is forced up the straw by the air pressure on the surface of
the liquid in the glass. Even the ancient Greeks and Romans knew that
water would rise in a tube when the pressure within the tube was
reduced, and hence they tried to obtain water from wells in this
fashion, but the water could never be raised higher than 34 feet. Let
us see why water could rise 34 feet and no more. If an empty pipe is
placed in a cistern of water, the water in the pipe does not rise
above the level of the water in the cistern. If, however, the pressure
in the tube is removed, the water in the tube will rise to a height of
34 feet approximately. If now the air pressure in the tube is
restored, the water in the tube sinks again to the level of that in
the cistern. The air pressing on the liquid in the cistern tends to
push some liquid up the tube, but the air pressing on the water in the
tube pushes downwards, and tends to keep the liquid from rising, and
these two pressures balance each other. When, however, the pressure
within the tube is reduced, the liquid rises because of the unbalanced
pressure which acts on the water in the cistern.

[Illustration: FIG. 42.--Water rises in the tube when the air is
withdrawn.]

[Illustration: FIG. 43.--The air supports a column of mercury 30
inches high.]

The column of water which can be raised this way is approximately 34
feet, sometimes a trifle more, sometimes a trifle less. If water were
twice as heavy, just half as high a column could be supported by the
atmosphere. Mercury is about thirteen times as heavy as water and,
therefore, the column of mercury supported by the atmosphere is about
one thirteenth as high as the column of water supported by the
at