The height of the A. has not yet been determined. That it must have a certain limit, is evident from the consideration that there must be a point at which gravity on the one hand, and centrifugal force and the repulsive action of the particles on the other, are poised,and beyond which, the latter forces overbalancing the former force, the aerial par ticles would be borne away from the earth. As, however, the law of the diminution of temperature, which materially affects the repulsive action, is unknown for the upper regions of the air, it is impossible to calculate the height of the atmosphere from the relations of these forces. From the observation of luminous meteors, it is inferred that it is at least 100 m. high, and that, in an extremely attenuated form, it may even reach 200 miles.
The of the A. is one of its most important properties. Its effect is exhibited in the action of the ordinary water-pump. The piston is fitted air-tight in its cylinder; and on being drawn up, creates a vacuum. The water within the pump being thus freed from pressure, while that outside of it is exposed to the pressure of a column of air reaching to the surface of the A., is at once forced up by reason of the weight of air which it must rise to balance. The ascent of the water takes place till the piston has reached the height of nearly 34 ft., from which we conclude that a column of air is equal in weight to a column of water of the same horizontal section. and of the height of nearly 34 feet. As mercury is 13.6 times heavier than water, a mercurial column freed from atmospheric pressure at the one extremity, and subjected to it at the other, is 13.6 times less in height than the column of water, or about 30 in. From the more convenient size of this column, mercury has been adopted as the standard for atmospheric pressure, and is employed in our ordinary barometers (q.v.) A mercurial column of 30 in. in height, and 1 sq.in. in section, weighs 15 lbs. (more accurately,14.73), which gives us the equivalent weight of a column of atmospheric air of the same section. The word A. is often employed to express this weight or pressure on a sq.in. of surface, so that when we speak, in mechanics, of the pressure of steam on a boiler as amounting to three atmospheres, we mean a pressure of 45 lbs. on the sq.in. Th.1 pressure on a sq.in. being thus ascertained, we have merely to multiply it by the number of sq.in. on the earth's sur face to obtain the total weight of the A. It amounts to 11.67085 trillions of lbs., or about
of the earth's mass. It must be observed that the height of the barometric column is not a constant quantity, as it varies with the latitude, the season of the year, and the hour of the day. At London, its mean height is 29.88 in.; at Paris, 20.92 in. The pressure of the A. in the northern hemisphere increases as we recede from the equator, reaching a maximum at 30° n. hit., and decreasing from 30° to 65°, where it again begins to rise. The greater height at 30° is said to be due to the accumulation of air at that latitude by the action of the trade winds. As the heat of the earth's surface increases the rarity of the air above it, and causes the air at the top of the heated column to overflow, we would expect that, during the year, the barometer would stand at a minimum in summer, and a maximum in winter. In reality, however, although the barometer is highest in midwinter, there is another maximum in midsummer, making thus two minima—one in spring, the other in autumn. This arises from the part which watery vapor plays in the pressure of the atmosphere. The heat of midsummer intro duces into the air a large quantity of moisture, in the form of elastic vapor, which, adding its pressure to that of the dry air, raises what would otherwise be the minimum barometric column to a higher point than that at which it stands in spring and autumn. Similar causes affect the pressure of the A. during the 24 hours of the day. There are two maxima—one at 10 A.M., the other between 10 and 11 P.M. ; and two minima—at 4 A.M. and 4 P.M. Very slight variations indicate the existence of atmospheric tidal waves; but this subject is still involved in some obscurity. The pressure of the A. exercises a most important influence on the organism of the human frame. A man of ordinary' stature is exposed to a pressure of about 14 tons; but as the air permeates the whole body, and presses equally in all directione, no inconvenience is found to result front it. From experiments instituted by the brothers Weber in Germany, it has been ascertained that the heads of the thigh and arm bones are kept in their sockets by the pressure of the A.; and in balloon ascents the aaronaut often suffers from bleeding•at the nose, lips, and even eyes—a fact that would seem to indicate that the strength of the blood-vessels has been adjusted with reference to atmospheric pressure.