Plate I.—Comparative Altitudes.
The development of meteorological knowledge to the commencement of the present century has now been traced, but before beginning the consideration of the methods of exploring the atmosphere that form the subject of the book, let us, in order to understand this work better, review the general knowledge which we possess of our atmosphere as regards its origin, composition, extent, and conditions of heat and moisture. First, then, regarding the Origin of the Atmosphere, or vapour envelope which the name means. According to the nebular hypothesis of Laplace, our earth, like all existing suns and planets, was condensed from clouds of nebulous matter and became a highly-heated globular mass rotating, like every celestial body, from west to east. As the earth cooled, a crust was formed, and many of the substances that now exist in the earth were suspended as clouds in the cooler atmosphere surrounding it. Eventually, these substances were condensed upon the crust; the oxygen, especially, must have been diminished by combining with the rocks, while the lighter gases, such as hydrogen, may have escaped from the earth's atmosphere. No doubt, when vegetable and animal life began, the earth's atmosphere was denser than now and much richer in carbonic acid, which, during the carboniferous period, was absorbed by plants, and is now imprisoned in coal and limestone. Within historic times, however, there is no evidence of any change in the composition of our atmosphere, or the climatic conditions as a whole.
M. Jourdanet, a distinguished French physiologist, maintained that man appeared on the earth at the close of the tertiary period, when the barometric pressure at sea-level was, he supposed, about forty-three inches, or nearly a half more than it is to-day, and owing to the greater density of the air its temperature was also considerably higher. Under these circumstances he believed that man first occupied the high regions of Central Asia, and only emigrated to lower levels when the climatic conditions became ameliorated. In other words, M. Jourdanet believed in a literal "descent of man," but if this be true, many of the 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.
