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ATMOSPHERE ; commonly, the air in which our earth appears to swim; but, in the widest sense, it is that mass of thin, elastic fluid, with which any body is completely surrounded. Hence we speak of an atmosphere of the sun, of the moon, of the planets, of electric and magnetic bodies, &c, the existence of which may not be fully proved, but is more or less probable. It is certain that our earth has an atmosphere, by which, according to the preceding definition, we understand the surrounding body of air and vapor. By means of its weight, the air is inseparably connected with the earth, and presses on it according to the laws of heavy, elastic fluids. Its whole pressure is equal to its weight, and, like that of all other heavy, elastic fluids, is exerted equally on all sides. If, now, by any circumstance, a stronger pressure is exerted on one side, certain phenomena are observed, which continue till the equilibrium is restored. Thus, for instance, water ascends, in the bore of a pump, above its general level, as soon as a vacuum is made between it and the piston, which is drawn up. The cause of this is the disturbance of the equilibrium, since the air without the bore presses on the water without, while no air is present within. By means of this pressure, if the bore is long enough, the water may be raised to the height of 32£ feet. This is the weight with which the atmosphere presses on the earth, and which is equal to the pressure of an ocean 32£ feet deep, spread over the whole earth. Hence it follows, that, at 28 inches barometrical height, the atmosphere presses with a weight of 32,440 pounds on the human body, estimated at 15 square feet. The man does not perceive this pressure, because the air entirely surrounds him, and is, besides, within him. On account of its elasticity, it presses in every direction, even from within the man outwards, and consequently counterbalances the air spread over the body. That the atmosphere has not a uniform density, may be inferred from this, that the lower strata of the air have to support the weight of the upper oneg, on which account they must become more compressed and denser. Ac cording to the law of Mariotte, the density of the atmosphere diminishes in geometrical, while the height increases in arith metical progression. This law may not hold at the extreme limits of the atmosphere, because the air at that height, free from all pressure, must be completely in its natural state. The height of the atmosphere has been estimated, by natural philosophers, at from 30 to 40 miles partly from the pressure which it exerts, partly from the twilight; since it is to be supposed, that the air, as far as it reflects light or receives illumination, belongs to our planet. Delambre, however (Astronomie, vol. 3, p. 337), considers this height to be almost 46 miles, which, remarkably enough, Kepler has mentioned in the Cap. Astr., p. 73. In respect to its form, the atmosphere may be considered as a spheroid, elevated at the equator, on account of the diurnal motion of the earth, and also on account of the great rarefaction of the air by the sun's rays, which there exert a powerful influence. The constituent parts of the earth's atmosphere are nitrogen and oxygen, which are found every where, and at all times, nearly in the proportion 76 : 23. Beside these, there is a small portion of carbonic acid, a variable portion of aqueous vapor, and a very small, indefinite quantity of hydrogen. (See Gas.) It also contains, in the form of vapor, a multitude of adventitious substances, in those injurious mixtures known under the name miasmata, the nature of which can hardly be investigated. As to the manner in which these different ingredients are united, various hypotheses have been formed, of which that of Dalton, which denies a chemical mixture, is one of the most celebrated, but also the most opposed. (For what has been written upon the atmosphere, see the article Atmosphere, in the new edition of Gehler's Dictionary of JVatural Philosophy, 1 vol., Leipsic, 1825. De Luc's Recherches sur les Modifications de VAtmosphhe, 2 vols. 4to., Geneva, 1772 (in German, Leipsic, 1776-78), still continues to be held in high esteem. See the section d?Atmosphere, in Biot's TraiU d'Astronomie Physique, 2d ed.j Paris, 1810, 3 vols. On the atmosphere of the sun, moon and the other planets, see the respective articles. See, also, Air.)