(also torr), a subsidiary unit of pressure used in measurements of atmospheric pressure, partial pressure of water vapor, and high vacuums. The notations are as follows: Russian, mm rt. St.; international, mm Hg. A millimeter of mercury is equal to the hydrostatic pressure of a column of mercury 1 mm high at a density of 13.5951 × 10³ kg/m³ and a gravitational accelerationg = 9.80665 m/sec². The relationships between the millimeter of mercury and other units of pressure are as follows: 1 mm Hg = 133.322 newtons per sq m = 1.35951 × 10⁻³ kilogram-force per sq cm = 13.5951 mm H₂O.