Instrument that measures atmospheric 
pressure as an indication of weather.
Most often used are the mercury barometer
and the aneroid barometer. In a mercury
barometer a column of mercury in a glass
tube roughly 0.75 m/2.5 ft high (closed
at one end, curved upwards at the other) is
balanced by the pressure of the atmosphere on
the open end; any change in the height of the
column reflects a change in pressure. An
aneroid barometer achieves a similar result
by changes in the distance between the faces
of a shallow cylindrical metal box which is
partly exhausted of air.