Instrument for measuring atmospheric Pressure. The mercurial barometer consists of a Mercury-filled glass tube that is sealed at one end and inverted in a cup of mercury.
Pressure on the surface of the mercury in the cup supports the mercury in the
tube, which varies in height depending on variations in atmospheric pressure.
At 32 degrees Fahrenheit (0 degrees centigrade, see Temperature), standard sea-level pressure (1 standard atmosphere) is 14.7 lb/in.2 (1,030
g/cm2), which is equivalent to a column of mercury 29.92 in. (76 cm) in height.
The aneroid barometer contains a sealed, partially evacuated Metallic box. As the Air pressure on it varies, one of its surfaces expands or contracts; this motion
is transmitted by a train of levers (see Machine) to a pointer, which shows the pressure on a graduated scale. In weather
forecasting, a rising barometer usually indicates fair weather; a rapidly falling
barometer, stormy weather.