The physical properties of the atmosphere are as important as its chemical. The air becomes thinner as we ascend above sea-level, owing to the fact that it is a compressible gas. The loga rithm of the pressure due to the weight of the air above us diminishes in inverse proportion to our altitude above sea-level; this logarithmic law (erroneously called the law of Boyle and Alariotte), which obtains only so long as the air is dense enough to be considered as a gas that obeys the law of Boyle, gives us for the actual density of the atmosphere at an altitude of 50 kilometres, or 31 miles, something less than the 1-7000 part of the density at the earth's surface; that is to say, if there is a pressure of 760 millimeters at sea-level, there will be a pressure of only 0.1 millimeter at 50 kilometers. At some elevation not much greater than this it is necessary to consider each particle of air as a separate satellite, moving rather freely with other particles around the earth in orbits con trolled by the general law of gravity, but modi fied by an occasional impact of one particle on the other. At the earth's surface, these impacts occur with great frequency and control the phe nomena or determine the properties of the gas; but at great altitudes the impacts have less im portance, and the gas behaves so differently, that it is sometimes spoken of as the ultra-gaseous state, or a fourth condition of matter, or a Crookes's vacuum, since the phenomena peculiar to this condition were first illustrated by Crookes in his vacuum tube. That particles of matter in some form permanently attend the earth at an hundred miles above sea-level, seems to be dem onstrated by the behavior of shooting- stars or aerolites, which enter the atmosphere at a veloc ity of 20 miles per second, and are at once heated up to the burning point by striking the air in front of them; it is only when they descend to the lower atmosphere that they can be said to be heated and burned up by the heat evolved by the compression of the gas immediately in front of them.
The density of the atmosphere at the earth's surface is about the 1-800 part of that of water, varying from the 750th to the 850th with the ordinary variations in pressure and tempera ture. This slight density offers no appreciable resistance to the ordinary motion of men and quadrupeds, but makes it possible for birds and insects to fly. On the other hand, when the air itself is in motion, the breeze produces a pressure that has been utilized by mankind Iron) the ear liest ages to propel sail-boats and ships and drive windmills, so that the atmosphere must be rec ognized as affording a motive power that has played a most important part in the development of the human race. In fact, the winds set in motion by the solar heat constitute a great 'hot air' engine. The fact that the atmosphere has weight was first maintained by Galileo and demonstrated by the experiments of Otto von Guericke in his air-pump, and Torrieelli with his mercurial barometer, but more especially by Pascal, who, at the request of Descartes. carried his barometer to the summit of Puy-de-Mime, a mountain in Central France. It by virtue of
this pressure that water ascends the bore of a pump when the piston is raised, allowing the atmosphere to push the water in the well up to the bore as fast as the piston rises. The total mass of the earth's atmosphere has been calcu lated to be about 1-1,000,000 or 1-1,125.000 of the mass of the earth. (See the Monthly Wenth• Reriew for January, 1899, page 58.) As the atmosphere is a Ouid, 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 direc tions 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 weight 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 most 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 purpose 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, it is not necessary to commit ourselves to such an hypothesis before the true explanation of these variations has been discovered.