HYPSOMETER, an instrument for measuring heights (Gr. i height, /2 Tpov, a measure), which employs the principles that the boiling-point of a liquid is lowered by diminishing the pressure (see HEAT) and that the barometric pressure varies with the height of the point of observation. (See METEOROLOGY.) The instrument consists of a cylindrical vessel in which the liquid, usually water, is boiled, surmounted by a jacketed column, in the outer partitions of which the vapour circulates, while in the central one a thermometer is placed.