In due time—some hundreds of millions of years—the cooling of the sun will leave the earth to freeze and all life to become extinct, unless, perchance, the oxygen of the air is so far absorbed by its rocks, or filtered away into space, as to destroy life before that time. No matter what may be the achievements of the human mind, what wonderful civilizations may be developed, what powerful empires created, or what wonderful secrets of creation discovered, it seems certain that these all will pass away, and finally the surface of the earth be as if man never lived. The dust of ages will wipe out and obliterate every trace and vestige of the operations of life. Silence, cold, and darkness will then reign supreme. But the time of this is indescribably far off in the future, and man will have ample opportunity to develop to the highest mental and spiritual estates of which he has inherent possibilities.
The moon already is dead. If it is formed of matter abandoned by the earth, as we believe, it once must have had an atmosphere, a portion of which was absorbed by its rocks as it cooled, and the remainder lost as the result of the low power of attraction of so small a body, which is insufficient to prevent the darting molecules of the gases of its air from shooting off into space. The absence of an atmospheric covering allows the heat from the sun to escape almost as rapidly as it is received; and the long nights of the moon (each as long as fourteen of our days) during which the sun’s rays are entirely cut off, permit the temperature of the dark side to fall to something like -400° F.
How Atmospheres Are Maintained and How Lost. The processes of nature are always adding to the various gases of the atmosphere in some ways, and transforming or taking from them in other ways. On the earth the loss and the gain are so nearly equal as to maintain at present a nearly constant condition. Marked changes have taken place, however, in long geologic periods. Our early atmosphere probably contained large quantities of carbon dioxide which were absorbed by the rank vegetable growth that now forms the coal beds of the earth, and the slowly cooling rocks that constitute the crust took in large quantities of oxygen; in fact, nearly one half of the weight of the crust of the earth is composed of the latter element.
In consequence it may be said that our present atmosphere is what remained after the earth had absorbed its gases nearly to depletion, and after the lighter gases, like hydrogen and helium, which seem to have too great molecular velocity to be imprisoned by the earth’s attraction of gravitation, had been lost in space. Gases that cannot be held by the moon may be imprisoned by the earth and those that can escape from the earth may be held by the larger planets.
Height of the Earth’s Atmosphere. Exact computation has shown that if the air were the same density at all elevations, which it is not, it would extend upward a distance of only five miles. From laws that are well understood it is known that at a height of thirty miles the atmosphere is only about one hundredth as dense as it is at the surface of the earth, and that at fifty miles it is too light to manifest a measurable pressure. The oxygen ceases at about thirty miles and the nitrogen at about fifty miles, the water vapor being restricted below the five-mile level. The appearance of meteors, which are rendered luminous by rushing into the earth’s atmosphere, and whose altitudes have been determined by simultaneous observations at several stations, reveals the presence of hydrogen and helium at a height of nearly two hundred miles.
CHAPTER II
A SYNOPTIC PICTURE OF THE AIR
How much do you know of the great aërial ocean on the bottom of which you live and in which human beings are just beginning to fly? Its variations of heat, cold, sunshine, cloud, and tempest materially affect not only the health and happiness of man but his commercial and industrial welfare, and yet few know more than little of the wonders of the life-giving medium that so intimately concerns them.
At the Height of Two Hundred Miles. Here is only the invisible, the intangible ether which, while too tenuous to be detected or measured by any appliances of man, is supposed to transmit the rays of the sun. These rays, coming in the form of many different wave lengths, and with widely differing velocities of vibration, produce a multitude of phenomena as they are absorbed by or pass through the air, or as they reach the surface of the earth. The longer and slower waves are converted into heat, the shorter and more rapid ones into light, and the minutest movements probably into electricity.
Oxygen and nitrogen, which form the greater part of the atmospheric gases, absorb comparatively little of the solar rays, while water vapor, which constitutes a little more than one per cent. of the atmosphere and which remains close to the earth, absorbs large quantities. From the fact that one half of the atmosphere, including nearly all of its water vapor, lies below an elevation of three and one half miles, it becomes evident that the greater part of the absorption of the sun’s rays must take place in the lower strata. On clear days the atmosphere absorbs nearly one half of the sun’s heat rays; the remainder reaches the surface of the earth, warms it and in turn is radiated back into the air,—with this difference: that as earth radiation the wave motion of the rays is longer and slower than it was when the rays entered our atmosphere as solar radiation. In this slower form the rays are the more readily absorbed. The atmosphere is thus warmed largely from the bottom upwards, which accounts for the perpetual freezing temperatures of high mountain peaks, although they are nearer the sun than are the bases from which they rise.