Barometer

Part of speech: noun (n.)

1. An instrument for determining the weight or pressure of the atmosphere, and hence for judging of the probable changes of weather, or for ascertaining the height of any ascent.

2. It is made in its simplest form by filling a graduated glass tube about 34 inches long with mercury and inverting it in a cup containing mercury. The column of mercury in the tube descends until balanced by the weight of the atmosphere, and its rise or fall under varying conditions is a measure of the change in the atmospheric pressure. At the sea level its ordinary height is about 30 inches (760 millimeters). See Sympiesometer. Nichol. Aneroid barometer. See Aneroid barometer, under Aneroid. -- Marine barometer, a barometer with tube contracted at bottom to prevent rapid oscillations of the mercury, and suspended in gimbals from an arm or support on shipboard. -- Mountain barometer, a portable mercurial barometer with tripod support, and long scale, for measuring heights. -- Siphon barometer, a barometer having a tube bent like a hook with the longer leg closed at the top. The height of the mercury in the longer leg shows the pressure of the atmosphere. -- Wheel barometer, a barometer with recurved tube, and a float, from which a cord passes over a pulley and moves an index.