barometer An instrument used for measuring atmospheric pressure. The mercury barometer consists of a glass tube 80 cm long, one end of which is closed. The tube is filled with mercury, and placed vertically with open end downwards, in a reservoir of mercury. The air pressure supports the mercury column in the glass tube. The upper portion of the glass tube beyond the mercury level is called `Torricelly vacuum'. The atmospheric pressure is stated directly as the height of mercury column. See also aneroid barometer.