race have returned to their birthplace, for to-day millions of people dwell on the great Asiatic plateau, and on the South American Cordillera, at an average altitude of 10,000 feet, while a few live throughout the year at extreme heights of 15,000 feet.

Composition of the Atmosphere.—Dry air is a mixture of about one-fifth of a volume of oxygen to four-fifths of a volume of nitrogen, besides a very small quantity (³⁄_10,000) of carbonic acid, traces of ammonia, ozone, argon, and other recently discovered gases. The oxygen consumed, and the carbonic acid given off by animal life and by combustion, are maintained in this fixed proportion in the free air by the absorption of the carbonic acid, and the setting free of oxygen by vegetation. By diffusion and the mobility of the air, a thorough mixture is effected, with the result that the fundamental composition of our atmosphere is everywhere nearly the same. In the lower atmosphere the vapour of water is present in a varying quantity, in the average about one per cent. in weight, with a volume depending on the temperature. Dust is always suspended in the atmosphere; the coarser particles settle, but the finer ones, that come from volcanoes, may float for a long time in the high atmosphere. Dust is an important factor in the production of clouds and rain, and occasions many optical phenomena.

Plate II.—Optical Phenomena showing the Height of the Atmosphere.

Extent of the Atmosphere.—If the atmosphere were incompressible and had throughout the density that it has at the earth, its height would be about five miles only, but actually it is composed of gases that follow Boyle's law and vary in volume inversely as the pressure upon them. Since the pressure decreases with height in a geometrical progression, it would be halved for each three and a half miles of ascent were the temperature constant, but as the temperature also decreases with height, the successive intervals, beginning with three and a half miles, become shorter because the volume of a gas depends on its temperature as well as on the pressure upon it. The decrease of pressure with increasing height above the earth is shown by the left-hand scale of Plate I., already described, and the subsequent diminution of density to the limits of our measurable atmosphere is indicated on the right of Plate II., Optical Phenomena showing the Height of the Atmosphere. The gases composing the atmosphere probably extend to heights proportional to their density; viz. oxygen to about thirty miles and nitrogen to thirty-five miles, although water-vapour nearly disappears at twelve miles. From these considerations it is supposed that the atmosphere, as measurable by the barometer, vanishes at about thirty-eight miles, and this is about the height indicated by twilight, which is the reflected light of the sun when 18° below the horizon. After the great eruption of the volcano Krakatoa in the South Seas in 1883, the brilliant sunset glows and the longer twilight showed that the dust emitted by the eruption remained for more than a year suspended at a height of at least sixty miles. The so-called "luminous clouds" seen at night during the same period, and which were probably these same dust particles still illumined by the sun, were found by trigonometrical measurements to have about the same altitude. Although it is computed that at a height of seventy miles the air has less than one-millionth of its density at sea-level—which is about the density of the air remaining in the exhausted bulb of an incandescent electric lamp—it is there sufficiently dense to render meteors luminous by friction after they with great velocity enter our atmosphere. The height of these meteors has been found, from simultaneous trigonometrical measures at two stations, sometimes to exceed one hundred miles, and if we suppose the aurora borealis to be an electrical discharge in highly rarefied air, measures made in the same way indicate as great a height for our atmosphere. The height of the aurora varies enormously, but the average altitude of it and of the other phenomena described, with the corresponding computed density of the air, are shown in the preceding diagram, in which the depth of the ocean of air may be compared with the deepest seas and the highest mountains. While, as Professor Young says, it cannot be asserted that the atmosphere has any defined upper limit, yet the kinetic theory of gases seems to afford evidence that the molecules of oxygen and nitrogen do not escape from the earth's attraction, and therefore the hypothesis of Professor Förster is unwarranted, that interplanetary space is filled with Himmelsluft, or very thin air.

Temperature of the Atmosphere.—The warmth of the atmosphere is derived chiefly from the sun's rays which, arrested by the earth's surface, are partly reflected and partly radiated back through the atmosphere. Not more than seventy-five per cent.—Professor Langley says only sixty per cent.—of the heat of the sun, which is received vertically on the upper surface of the atmosphere, penetrates to the earth, and very much less than this when the angle of the sun is low. The reason why temperature diminishes as we ascend, is partly owing to the greater loss of heat by radiation through the thinner envelope of the upper strata, and partly owing to the greater absorption of the heat given off from the earth by the lower and denser strata. In general, it may be said that there is a diminution of 1° Fahrenheit for each three hundred and thirty feet that we rise vertically, but, this rate varies greatly at different heights, places, and times. For instance, the decrease is not the same on mountains as it is in the free air, and in the northern hemisphere it is greater on the south than on the north sides of mountains; it is usually greatest near the ground, and is faster in summer than in winter. But in the average, the temperature falls as much for three hundred and thirty feet of elevation as it does for a change of seventy miles on the earth's surface north or south of the equator. When dry air rises, because it is heated and thereby is made lighter, the laws of thermo-dynamics show that, by reason of its expansion, its temperature is decreased 1° Fahrenheit for each one hundred and eighty-three feet that it ascends, and, by compression, its temperature is increased as much if it is made to descend the same distance. This is called the "adiabatic rate of change of temperature," because it is produced by an alteration in the density of the air, due to variation in pressure, without the addition or loss of heat. In the course of this book there will be occasion frequently to refer to this law of heating and cooling. The adiabatic rate of change is seldom observed on mountains because of their influence upon the currents of air in contact with their flanks, or even in balloons, on account of imperfect measurements, but, as will be explained in the closing chapter, the adiabatic change of temperature is confirmed by the observations with kites, which furnish the best method of