The variations of the B. are found to be both periodical and irregular. Periodical variations are those taking place at stated and regular intervals, and irregular. such as have no regular period of recurrence. The only truly periodical variation is the daily one. which varies from 0.150 to 0.001 inch. In most regions of the globe there is also a well-mi;rked annual variation, widely different for different regions. Accidental varia tions have a range of about 3 inches. See ATMOSPHERE.
The uses of the B. may be classified into physical, hypsometrical, and meteorologi cal. It is of essential use in all physical researches where the mechanical, optical, acous tical, and chemical properties of air or other gases are dependent on the pressure of the atmosphere. Its use in hypsotnetry, or the art of measuring the heights of mountains, is very valuable. When a B. is at the foot of a mountain, the pressure it sustains is greater than that which it experiences at the top by the weight of the column of air intervening between the top and bottom. A formula of considerable complexity is given by mathematicians for finding very nearly the true height of a mountain from barometri cal and thermometrical observations made at its base and summit, the interpretation of which does not come within the compass of this work. The following rules give very nearly the same result: 1. Reduce the mercurial heights at both stations to 32° F. 2. Take the logarithms of the corrected heights, subtract them, and multiply the result by 10,000, to give the approximate height in fathoms of the upper above the lower station. 3. Take the mean of the temperature at both stations, take the difference between this mean and 32, multiply the difference by the approximate height, and divide the product by 433. This last result is to be added to the approximate height, if the mean temperature is above 32, and subtracted, if below, to find the true height in fathoms. A Fortin's or Gay-Lussac's B. is employed in measuring heights.
The best known use of the B. is as a meteorological instrument, or as a weather-glass. Opticians have attached to certain heights of the B. certain states of weather, and at cer tain points of the scale the words "rain," "changeable," "fair," etc., are marked; but the connection thus instituted is very misleading. Those who have observed most care fully the connection of barometric heights with changes of the weather, discard entirely the use of these terms; and state that it is not the actual height of the B. at any place, but this height as compared with that of surrounding regions which indicates the corn ing weather. Several elaborate codes of rules have been drawn up to serve as a key to the variations, but as these are more or less of a local character, they would be out of place here. Generally speaking, a falling B. indicates rain, a rising B. fair weather. A steady B. fore tells a continuance of the weather at the time; when low, this is generally broken or bad, and when high, fair. A sudden fall usually precedes a storm, the violence of which is in
proportion to the baroinetric gradient. An unsteady B. shows an unsettled state of weather; gradual changes, the approach of some permanent condition of it. The vari ations must also be interpreted with reference to the prevailing winds, each different wind having some peculiar rules. The connection between changes of weather and the pressure of the atmosphere is by no means well understood. One reason is given, which may to some extent account for the B. being lower in wet than in dry weather— viz., since, as has been shown by Dalton, moist air is lighter than dry air, wherever a large amount of aqueous vapor has displaced a part of the drier air, the barometric column will read relatively low. Hence much depends on the nature of the winds. The s. and s.w. winds, which are, in western Europe, more than any other, the rain-bring ing winds, are warm and moist winds. Now, a column of such air, to be of the same weight as one of cold dry air, must be higher; but this it cannot well be in the atmos phere, for no sooner does the warm moist column, by its lightness, rise above the sur rounding level of the upper surface of the aerial ocean, than it flows over, and becomes nearly of the same height as the cold air around it. The interchange taking place less interruptedly, and consequently less slowly, in the higher strata than in those near the ground, it is some time before the equilibrium thus disturbed is restored, and meanwhile the B. keeps low under the pressure of a rarer atmospheric column. On the other hand, the northerly and easterly winds, being comparatively cold and dry, are accompanied with fair weather and a high barometer. It is thus to the warmth, as well as to the moisture of these winds that the low pressure is to be ascribed. Hence, then, the rain attendant on a low B., as well as the fine weather accompanying a. high B., are the neces sary concomitants of our geographical position—of our having the land to the e., and the ocean to the W. of us. In Great Britain a high and rising B. frequently accompa nies e. winds with a drenching drizzle; and on the La Plata river, things are the reverse of what they are with us; there the cold s.e. wind, coming from the ocean, brings rain with a high barometer, and the land winds, warmed by the plains of South America, maintain fine weather with a low barometer. That the temperature, as well as the mois ture of the air, is at least an important cause of the changes of the B., is also shown by the fact, that, in the tropics, where the variations of the temperature are slight compared with the temperate zones, the B. experiences almost no change. In central Asia, the summer pressure is nearly an inch less than that of winter, and at Deniliquin, towards the interior of Australia, it amounts to 0.250 inch.