An instrument called a barometer measures the
atmospheric pressure. The barometer was invented using principles
developed by Evangelista Torricelli in 1643. It uses a long glass tube
with one end closed and the other open. Air pressure is measured by
observing the height of the column of mercury in the tube. At sea level,
air pressure will push on the mercury at the open end and support a
column of mercury about 30 inches high. Barometers are used to measure
the air pressure in "inches of mercury" or in millibars (mb).
The typical pressure at sea level is 1013.25 millibars or 14.7 pounds
per square inch.
As atmospheric pressure increases, the mercury is
forced from the reservoir by the increasing air pressure and the column
of mercury rises; when the atmospheric pressure decreases, the mercury
flows back into the reservoir and the column of mercury is lowered.
There are other types of barometers that record the changing atmospheric
pressure in the air.