A device to measure gas pressure is called a barometer. There are two major types of barometers, the mercury barometer and the aneroid barometer. Galileo’s secretary, Evangelista Torricelli (1606-1647; the unit of pressure, the torr, was named after him), developed the mercury barometer in 1643. It consists of a thin glass tube about 80 centimeters (31 inches) long, which is closed at the top, filled with liquid mercury and placed upside down in another mercury-filled dish. Depending upon the atmospheric pressure pushing on the mercury in the dish, the level of mercury in the tube will rise or fall because there is no air above it. By measuring the height of the mercury, which would usually be between 737 and 775 millimeters (29 to 30.5 inches) high, atmospheric pressure of the atmosphere can be measured.

The most common household barometer is the aneroid barometer, in which atmospheric pressure bends the elastic top of an extremely low-pressure drum; by measuring the amount the top bends, a measurement of atmospheric pressure can be determined. The aneroid barometer is often used in airplane altimeters to measure altitude. Since atmospheric pressure decreases as altitude increases, the aneroid barometer is an ideal instrument to use. It is much safer than the mercury barometer, because mercury vapor is poisonous and the mercury must be exposed to the atmosphere.