[Illustration: FIG. 40.--To illustrate the decrease in pressure with
height.]
Sea level is a low portion of the earth's surface, hence at 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.]