BAROMETER, is the name applied to those instruments in which a column of air is weighed against a column of mercury.
Galileo, Tos-ricelli, and Pascal, in the 17th century, successively made those experiments and observations on the pressure of the air, which led to the invention of the barometer. It was first found that the pressure of a column of the entire atmosphere is equal to that of a column of water (of the same diameter) about 33 feet high; it was next found that the pressure is equal to that of a column of mercury about 30 inches high; and it was afterwards ascertained that on a mountain the pressure will sustain a less height of mercury than at the earth's surface. These facts proved the existence of atmo spheric pressure, and also the law of diminu tion in this pressure at different altitudes.
If a tube, closed at one end, be deprived of air, and the open end be immersed iu mer cury, the mercury will rise in the tube to the height of 28 or 30 inches, by the pressure of the atmosphere on the mercury in the vessel. Descartes, Huyghens and Dr. Hooke, de vised barometers in which the use of one or more fluids of different specific gravity in con nection with mercury was tried as a means of obtaining more distinct indications of very small changes of level ; and many other forms of simple mercurial barometers have been constructed. One contrived by Amon tons, consisting of a conical tube of glass closed at the smaller end, partially filled with mercury, and then inverted, is more simple and elegant in principle than any other ; but the obtaining of a tube of the requisite accu racy is almost an ideal supposition. One contrived by Gay-Lussac for portable purposes permits the access of air to the mercury only by a hole too minute to allow the escape of mercury. Fortin's is a Torricellian barometer with a contrivance for raising or lowering the bottom of the cistern by a screw, so as to ad. just the lower level of the mercury exactly to the zero point before commencing an obser vation. Hooke's wheel barometer, though too
inaccurate for scientific use, is very much used as a weather-glass ; for which it may answer well enough, if it be remembered that it is not the state of the barometer which fur nishes any probable test of the weather, but the change which is taking place for the time being. This change is indicated by it pretty distinctly, though it cannot be trusted for showing either the exact amount of the change, or the exact height of the column of mercury. In this contrivance,aweight is placed on the mercury of a siphon barometer, and nearly counterpoised by another weight con nected with it by a string, which passes over a pulley. The movement of the mercury causes this floating weight to rise and fall, and con sequently the pulley, which carries an index, to turn more or less on its axis. The form of barometer invented by Gay-Lussac has been recently much employed in determining the heights of the mountains along the Anglo American frontier in New Brunswick.
To observe the temperature of the mer cury, which, by altering its bulk, affects the indications of the barometer, a thermometer is attached to the best instruments, the bulb of which is in the cistern.
In using the barometer as a weather-glass, it must be remembered that no rule which can be given will always hold true. The rising of the mercury usually presages fair weather, and its falling foul weather, as rain, snow, high winds, and storms, the lowest fall being found in great winds, though unaccom panied by rain. In very hot weather the falling of the mercury usually foreshows thunder ; in winter, the rising presages frost; in frosty weather a continued fall foretells a I thaw, and, in a continued frost, a rise indicates the approach of snow. If a change of weather follows very close upon a change in the baro meter, it may be expected to last but for a short time, and vice versa; and where the mo tion of the mercury is unsettled, changeable weather may be anticipated.