Barometer n. : An instrument for determining the weight or pressure of the atmosphere, and hence for judging of the probable changes of weather, or for ascertaining the height of any ascent.

Example:
What is the pressure in atmosphere in the cabin if the Barometer reading is 688mm Hg?

Taxonomy
barometer, aneroid barometer, mercury, atmospheric pressure, mercury barometer, pressure, air pressure, water level, reservoir, body, altitude, low pressure, Patent, measuring instrument, measure, cell, capsule, 2007-05-05.

Defination from the web:
barometer has a glass tube of at least 33 inches (about 84 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. ... mechanism is made deliberately 'stiff' so that tapping the barometer reveals whether the pressure is rising or falling as the ...