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SECTION 2. ATMOSPHERIC PRESSURE

1. Pressure

Atmospheric pressure is the amount of weight that a column of air has on an object.

Atmospheric pressure is measured with a barometer.

2. Mercurial Barometer

A mercurial barometer uses a column of mercury in a glass tube to measure the weight of the
atmosphere. The height of the column of mercury in the tube changes as the weight of the
atmosphere changes. If the atmospheric pressure increases, the mercury will rise higher in
the tube; if the atmospheric pressure decreases, the mercury will move downward in the
tube. The height of the mercury in the tube is measured in inches. This height, measured in
inches, is used as a measurement of atmospheric pressures.

At the left: A mercurial barometer showing air pressure

supporting a column of mercury 29.92 inches high in the
barometer tube.

At the right: A scale showing that the weight of a 29.92
inch high column of mercury, one inch square, is the same
weight as a column of air one inch square that extends to

the top of the atmosphere.
Figure 2