As the atmosphere is a fluid, subject at any place to the pressure of all the air above it. it is not only compressed by this weight, but by its elastic reaction it presses outward in all directions with an equal force; consequently, every substance that is immersed in the atmosphere is buoyed up to an extent equal to the weight of the air that it displaces. This buoyancy must be allowed for in all delicate weighings. If the object is a bag or a balloon full of hydrogen, hot air, or other very light gas, the upper pressure or buoyancy may be greater than the weiggt of the balloon; consequently, the latter rises. (See Aeronautics. ) This principle gave rise, in the hands of Montgolfier, to the art of aerial naviga- tion, the future of which now depends almost en- tirely upon our knowledge of the currents of air and our power to steer or guide the balloon.
The air is not a perfect gas, but has an inter- nal friction called viscosity, by reason of which one layer of .air sliding past another, experiences a drag, or retard, or resistance. Therefore, any movement of one portion of the atmosphere over or through another portion soon ceases, unless an external force is continuously applied suffi- cient to overcome the viscosity. This viscous re- sistance increases with temperature, but is apparently independent of pressure ; it is, there- fore, greatest in the warmer portions of the globe and less at the poles, and may be zero at the outer boundary of the atmosphere, where the air loses the ordinary characteristics of a gas. The elastic pressure pervading a mass of quiet air is measured by the barometer (q.v. ) ; but if the air is in motion the barometer must move with the air, or else some device must be used in order to separate the elastic pressure from the pressure produced by the action of the wind on the barometer itself, considered as an obstacle to the wind. This matter will be found fully explained in Abbe, Meteorological Apparatus and Methods (Washington, 1887). The observed pressure at any point on the earth's surface is subject to great changes from day to day, which are associated with the movements of the areas of high and low pressure attending fair weather and storms respectively; therefore, the barom- eter can serve a useful pur))ose as an indicator of approaching changes of weather. The local pressure also goes through diurnal and annual periodic changes ; formerly, these were spoken of as waves of pressure, moving as such over the surface of the globe, but it is not necessary to commit ourselves to such an hypothesis before the true explanation of these variations has been discovered.
Another important physical property is the power of the air to absorb and radiate heat. The specific heat of the air wlien it is kept at a constant pressure is 0.2.370, that of water being unity, and the specific heat, when kept at a constant volume, is O.IOSU; the ratio of the two is 1.40G. This latter ratio aft'ects the velocity of sound, and, in general, it determines the rate at which air will cool when it is al- lowed to expand without the addition of lieat from extraneous sources. This so-called adia- batic expansion and cooling is the principle that is utilized in cooling the air to the liquid, or even solid state; it is also (he princi])le that controls the cooling of ascending masses of air in the free atmosphere, by which the moisture cont.ained therein condenses into ch)ud, rain, snow, or hail ; this is, therefore, a property of great importance in meteorology.
The absorption of radiant heat by the atmos- phere, especially the radiation from the sun and the earth, has been studied minutely, both with the spectroscope and the bolometer. Fraunhofer showed that the dark lines seen in the spectrum of a narrow beam of sunlight proceed mostly from absorption taking place in the sun's atmos- phere, but that additional lines and bands seen when the sun is near the horizon nuist be due to absorption by the earth's atmosphere. The amount of absorption was assumed to be about 2-5 per cent, when the sun is in the zenith, until Professor Langley showed that it must be at least 40 per cent, of all the energy originally present in the sunbeam. The absorption is greater among the short waves than among the long waves, although there are specific wave- lengths in all parts of the spectrum that are almost wholly absorbed, and others that are very little aflfeeted. By virtue of this absorption the air retains a considerable body of heat, which it can only lose by the slow process of radiation; it, therefore, acts as a moderator of our ex- tremes of temperature both by day and by night, both in summer and in winter. Without the atmosphere and especially without its moisture and its carbonic acid gas, we should be subject to much greater vicissitudes of cold and lieat than we are at present.
Bibliography. With reference to the chemical constitution of the atmosphere, general sum- maries of our knowledge will be found in Schmid, hehrhuch der Meteorologie (.Jena, 1861) ; Hann, Lehrbuch der Meteorologie (Leipzig, 1901) ; Eb- ermayer. Die Beschaffenheit der Waldluft (Munich, 1885) ; Spring and Roland, Recherches sur I'acide carbonique de I'air (Brussels, 1886) ; Letts and Blake, "On the Carbonic Acid Gas in the Atmosphere," Memoirs of the Royal liocieti/ of Dublin, 1S99. With reference to the absorption and radiation of heat by the atmos- jihere, see Langley, "Researches on Solar Heat : A Report of the Expedition to Mount Whitney," in Professional Papers United States Signal Service, No. 15 (Washington, 1884) : Langley, Annals of the Astro-Physical Observatory, Vol. I. (Wash- ington, 1901) ; Very, "Radiation of the Earth's .atmosphere," in Vnited States Weather Bureau, Bulletin 16 (Washington, 1001).
AT'MOSPHERIC E'LECTRIC'ITY. Observation shows that the lower atmosphere, with its clouds and rain, or snow and dust, is generally in an electrified state. In November, 1749, Benjamin Franklin argued that lightning is but an immense electric flash, similar to the sparks obtained from electrical machines. On May 10, 1752, BufTon and d'.VIibard experimentally confirmed this hypothesis by the use of a lightning-rod near Paris: and in .June, Franklin himself, by means of a kite raised during a thunderstorm at Philadelphia, was able to draw electric sparks from the clouds and the atmosphere.
Since that day an immense number of observations have been made upon the various electric phenomena of the air. Regular observations of atmospheric electricity were originally made, either by means of kites carrying insulated wire from the ground up toward the clouds, or more conveniently by means of metal rods insulated from the ground by resin, or glass, or silk; the upper end of the rod was sometimes pointed, but
