[Gk. baros, weight; and metron, measure.] An instrument for measuring the pressure of the atmosphere. Barometers are divided into two classes, cistern and siphon; and in each of these classes there are several forms. The simplest form of the cistern barometer consists of the Torricellian tube, with the. addition of a graduated scale to show the height of the mercury column. In ordinary weather-glasses, the tube is bent round at the bottom, and the cistern is an expansion of the tube.' At the upper end of the tube is a fixed scale of inches, and tenths of inches, beginning with 27 and ending at 31 inches. A vernier slides along the fixed scale for measuring fractional divisions. In the siphon barometer the tube, which is generally of uniform bore, is bent into the form of a siphon, the longer leg of which is closed and' the shorter opened. Of the various forms of this class, that known as the wheel barometer is the most common for household use. In this the rise and fall of the mercury turns a small wheel, which moves a pointer and dial. The wide sweep of the pointer makes small changes in the height of the mercury very evident. One of the important uses of the barometer is that of measuring the heights of mountains. The pressure of the air on the top of a mountain is less than that at the bottom by the weight of the column of air intervening between the top and the bottom. The difference between the readings of the barometer at the foot of the mountain and the readings on the top gives the means of calculating its height. The most important of all the uses of the barometer is its employment for meteorological observations.- Aneroid Barometer. (See Aneroid.)

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(See Barometer.) [Gk. a, not; neros, wet; and eidos, form.] A barometer whose action depends on the varying pressure of the atmosphere upon the elastic top of a metallic box from which the air has been exhausted. By a system of levers connected with the box, and an internal spring, motion is given to an index, which registers the variation of atmospheric pressure.