A mercury barometer in which the lower mercury surface is larger in area than the upper surface. The basic construction of a cistern barometer is as follows: A glass tube 1 m in length, sealed at one end, is filled with mercury, and then inverted. The tube is mounted so that its mouth penetrates the upper surface of a reservoir of mercury called the cistern of the barometer. Cistern barometers are classified according to whether the cistern is fixed in volume (see Kew barometer) or variable in volume (see Fortin barometer).