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How is atmospheric pressure measured?
An instrument that measures air pressure is called a barometer. One of the first barometers was developed in the 1600s. The original instrument had mercury in the small basin, with an upside-down glass tube placed in the mercury. As air pressure increased, the pressure would force more mercury in the tube.
In aircraft, an aneroid barometer measures the atmospheric pressure from a static port outside the aircraft. Air pressure decreases with an increase of altitude. For more information on air pressure and the instruments that use it Check out https://howthingsfly.si.edu/flight-dynamics/instruments.