Barometer, (of Baro Metrum 1370 weight, and measure ). An instrument to determine the weight of the air, or observe the changes of weather; it is commonly called a weather glass, and frequently the Torricellian tube, from Torricelli its inventor.

The bore of the common tubes is too small. The glass tube should be one third, or at least one fourth of an inch in diameter, hermetically sealed at one end, and open at the other; the length should be thirty-four inches: the mercury with which it is filled must be pure. Fill the tube quite full with this mercury; and having in readiness a basin with a flat bottom, and about two inches high, in which is also some mercury, invert the tube, and put it in the basin, still holding your finger underneath it till it is in the mercury of the basin, then place it in a frame. On taking away your finger, the mercury in the tube will immediately subside to about twenty-nine or thirty inches, according to the state of the air, it being very rarely lower than twenty-eight, or higher than thirty inches, when the air is heavy. If a scale of four inches be divided into tenths, and placed against the upper end of the tube, the instrument is complete, and equal to every change. The mercury, however, still contains some air entangled with it, which will in time rise to the upper part of the tube, and not only by its pressure prevent the rising of the mercury, but by its expansion, from a change of temperature, produce many irregularities in the motion of the quicksilver. The tube may be previously cleaned by a wire, to the end of which a bit of leather is attached, and this will lessen the quantity of entangled air; but, to render the instrument perfect, the mercury must be boiled in the tube. We once separated in this manner more than an inch of air.

The barometer measures the weight of the air with sufficient exactness for the general purposes of life, yet it is affected with many irregularities, that no exactness in the instrument can remedy, and no theory explain. Many of these irregularities can be however explained, by considering the mercury as suspended, not only by the weight of the air, but in part by its elasticity. The barometer is, in reality, a monometer. Moist air is very inelastic, the mercury of course falls; and the human body, from a diminution of the tone which pressure gives, feels a languor. Dry air produces a contrary effect; and Saussure found, that water evaporating, produced a gas of very considerable elasticity. When high winds blow, the mercury is generally low; it rises higher in cold weather than in warm; and is usually higher in the morning and evening than at mid day: it generally descends lower after rain than it was before it. On advancing up high mountains, the air is less and less dense, and usually the same inconveniences are felt that are complained of when the air is moist. But M. De Luc and his companions observed, when at the summit o,f the Buet, 3315 English yards above the level of the Mediterranean, no difference in the effects of the diminished density of the air, which was one third less than that of the plains below them. In this place, M. De I uc observes, ' how much naturalists are deceived in attributing the alterations that many persons experience, upon the falling of the barometer, to a difference either in the weight or density of the air. For if these changes, he says, could so sensibly affect our organs, what would become of those chamois hunters, who pass every day from the bottom of the vallies to the highest mountains? These people perceive no inconvenience; even asthmatic people find little, notwithstanding the barometer varies in these several places, as is usually observed in other similar ones.' If, however, our feelings are connected not only with the actual weight but the elasticity of the air, these difficulties will vanish; for the highly elastic gas which is produced by evaporating water, rises to the upper regions of the atmosphere, and compensates for the want of density. M. De Luc's reasoning) however, is very vague and inconclusive. Those accustomed to considerable changes of any kind, experience but little inconvenience from them; and other travellers have really found considerable languor on these very elevated spots.

The barometer may be applied to several uses, as measuring the heights of mountains: for twelve thousand and forty inches of air being equal to one inch of mercury near the surface of the earth, twelve hundred and four inches, or one hundred feet, must be equal to one tenth of an inch of mercury. Consequently, if a barometer be carried up any great eminence, the mercury will descend one tenth of an inch for every hundred feet that the barometer ascends. This we do not mean as a correct view of the subject, for many circumstances must be attended to in the actual measurement of heights, particularly the temperature. To consider all the necessary corrections is not a part of our subject. See De Luc, Saussure, Trembley, and Sir G. Shuck-burgh's papers in the Philosophical Transactions.

But the great use of the barometer is to predict the future state of the weather for severed hours, and some-limes days preceding, in many instances to a degree of probability. 1st, The rising of the mercury presages fair weather, and its falling, wet. 2d, In very hot weather, the sudden falling of the mercury foretels thunder. 3d, In winter, its rising portends frost; and in a continued frost, foretels snow. 4th, When foul weather happens soon after the falling of the mercury, it will soon again change, and so on the contrary. 5th, When the mercury continues to rise for some time before the foul weather is over, expect a continuance of fair weather to follow. 6th, In fair weather, when the mercury continues to fall before rains come, then expect a great deal of it, and probably high winds. 7th, The unsettled motion of the mercury denotes changeable weather.

It is not so much the height of the mercury that indicates the weather, as its motion up and down; therefore, to know whether the mercury is actually rising or falling, observe whether the surface of the mercury be convex, for it is then rising: if the surface be concave, it is falling: if the surface be plain, or a little convex, it may be considered as stationary.

There are different forms of this instrument which have each their advantages and disadvantages: but the common sort is perhaps better than any other if carefully constructed.