A glass manometer, employing mercury in its vertical column, that is used to measure atmospheric pressure. The basic construction, unchanged since Torricelli's experiment in 1643, is a glass tube about three feet long, closed at one end, filled with mercury, and inverted with the open end immersed in a cistern of mercury. With the cistern surface exposed to atmospheric pressure, the height of the mercury column varies with that pressure. Mercury barometers may be classified into three groups according to their construction: cistern barometers, siphon barometers, and weight barometers. See also aneroid barometer, inch of mercury.