barometer (barometer)

[ IMG:content-1686.png:[blocks in formation] Cistern of Fortin's Barom etera, extremity of ivory pointer marking the zero of the scale. a wooden button the mer 456 phon barometer having a float resting on the surface of the mercury in the open branch, and a thread attached to the float pa ing over a pulley, and having a weight at its extremity as a counterpoise to the float. As the mercury rises and falls the thread turns the pulley which moves the index of the dial. The barometer is used in many physical and chemical determinations, but its most ordinary applications are (1) to the prediction of changes in the weather, and (2) to the determination of the elevation of stations above the sea-level. - Aneroid barometer, a portable instrument, invented by M. Vidi of Paris, for in dicating the pre ure of the atmosphere without the use of mercury or other fluid. It consists of a circular metallic box which is exhausted of air, and of which the corrugated diaphragms are held in a state of tension by powerful springs. The varying pre ure of the atmosphere causes a variation of the surface of the diaphragm, which variation, being multiplied by delicate levers and a fine chain wound around a pinion, actuates an index-pointer which moves over a graduated scale. Bourdon's metallic barometer is an aneroid barometer consisting of a flattened, curved tube, ex IMG:content-1687.png:[blocks in formation] A, front view, showing hand or indicator, a , and the scale; b, c, mercurial thermometers. B, back view: d, d, tube secured at its middle, e, and having its ends connected by links, f, f, to two short levers, g, g, on the same axis as the hand, a, h, open plate. hausted of air and having one end fixed and the other geared to an index-pointer which traverses a graduated arc. The curvature of the tube is affected by variations in the atmospheric pre ure, and the pointer is moved correspondingly on the dial.- Marine barometer, a cistern barometer adapted to the conditions of a ship's motion, being suspended by gimbals, and having a stricture in the tube to le en the oscillations of the mercury.Pumping of the barometer, an unsteadine in the barometric column, due to a gusty wind. - Self-registering barometer, a barograph (which see). - True height of the barometer, the height of the barometer corrected to the standard density of mercury (that is, its density at the freezing-point of water), for variations of gravity, for the effect of capillarity, index-error, expansion of the scale, etc. The United States Signal Office also corrects for the elevation of the station above the sea-level. See atmosphere and sympiesometer.