One atmosphere--standard pressure
The arrows in Figure L indicate that atmospheric pressure exerted on a body at
sea level is in a downward direction (toward the center of gravity of the earth).
This downward force (atmospheric pressure) is a measure of the weight of the air at
a given point (relative to sea level) at a given temperature.
atmospheric pressure exerted on a body at sea level is approximately 14.7 lbs/in2
and is called one atmosphere. This principle, illustrated in Figure 4, is based on
the fact that, in fluids, pressure is exerted in all directions by equal amounts.
Atmospheric pressure also exerts force in all directions.
The air over a
horizontal surface exerts a force equal to the weight of all the air over the
surface. If the surface is not horizontal, the air still presses perpendicularly
against it with a force equal to the weight of all the air that would press on it
if it were in a horizontal position.