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. [Illustration: PLATE II.--OPTICAL PHENOMENA SHOWING THE HEIGHT OF THE ATMOSPHERE. ]