The whole earth is surrounded by this marvelous air-ocean; an ocean of gaseous matter, at least one hundred times as deep as the water-ocean. At the bottom of the gaseous ocean we small human creatures crawl about, commonly on flat lower levels--the ocean bottom, in fact. Sometimes, with much toil and trouble, we climb the little ridges and mounds called “mountains”; little compared with the depth of the atmosphere, though not little compared with ourselves. The highest mountain-peaks of even the vast Himalayas lie low down near the bottom of the ocean of air. But the very extent of the ocean of air adds to our difficulty in studying its nature. All observations that we can make must be limited by the state of the atmosphere just around ourselves. We can never get out of and beyond the atmosphere, so as to see it as a whole. At any time a slight local fog is enough to put a stop altogether to such observations, beyond the unpleasant experience of the fog itself. It used to be supposed that the atmosphere reached only to a height of about fifty miles above earth’s surface. Of late years the opinion has gained ground that the atmosphere reaches to a height certainly of two or three hundred miles, probably of four or five hundred, possibly a good deal more. But the condition of the air far above is different from that of the air in lower levels, where we live and breathe. The higher we ascend, the more thin or “rare” becomes the air. A less quantity fills a certain space up there than down here. The particles float further apart one from another. This difference in the density of the air is chiefly due to attraction. Each separate air-particle is drawn steadily earthward by the force of gravitation, and that force is stronger on the surface of earth than at a distance. The closer to earth, the heavier the pull; the further from earth, the less the pull. Besides the actual attraction of the earth drawing the air-particles downward, there is the great weight of the whole atmosphere above, caused by the same attraction. Miles and miles of air overhead press mightily downward, packing tightly together the lower layers of air near to earth’s surface. Without this pressure of the overlying atmosphere, the air down here would not be nearly so dense as it is; and, indeed, would not be fitted to support life. A man ascending a mountain or rising in a balloon leaves heavy layers of air below, and has an ever-lightening weight above, so that the atmosphere around him becomes constantly more thin, more difficult to breathe. In the beginning of the last century Humboldt made a vigorous attempt to scale Chimborazo, one of the loftiest of the Andes. He and his party suffered severely from sickness, giddiness, and difficulty in breathing, and the attempt proved a failure. Not till over seventy years later was the ascent actually accomplished by Mr. Whymper.