Manometer

Manometer (Gr. manos, 'thin,' 'rare') is properly an instrument for measuring the rarity of the air or of other gases; but the name is most frequently applied to instruments for indicating the elastic pressure of gases, which is always, for each kind of gas, inversely proportional to its rarity, or directly proportional to its density. The several kinds of Barometers (q.v.) are really manometers, and so is the steam-gauge of a Steam-engine (q.v.).