Page:Physical Geography of the Sea and its Meteorology.djvu/29

Rh 6. Its height.—At the height of 26,000 miles from the earth, the centrifugal force would counteract gravity; consequently, all ponderable matter that the earth carries with it in its diurnal revolution must be within that distance, and consequently the atmosphere cannot extend beyond that. This limit, however, has been greatly reduced, for Sir John Herschel has shown, by balloon observations, that at the height of 80 or 90 miles there is a vacuum far more complete than any which we can produce by any air-pump. In 1783 a large meteor, computed to be half a mile in diameter and fifty miles from the earth, was heard to explode. As sound cannot travel through vacuum, it was inferred that the explosion took place within the limits of the atmosphere. Herschel concludes that the aerial ocean is at least 50 miles deep.

7. Data conjectural.—The data from which we deduce our estimate, both as to the mean height of the atmosphere and average depth of the ocean, are, to some extent, conjectural; consequently, the estimates themselves must be regarded as approximations, but sufficiently close, nevertheless, for the present purposes of this work.

8. Analysis of air.—Chemists who have made the analysis, tell us that, out of 100 parts of atmospheric air, 99.5 consist of oxygen and nitrogen, mixed in the proportion of 21 of oxygen to 79 of nitrogen by volume, and of 23 to 77 by weight. The remaining half of a part consists of .05 of carbonic acid and .45 of aqueous vapour.

9. Information respecting the depth of the ocean.—The average depth of the ocean has been variously computed by astronomers, from such arguments as the science affords, to be from 26 to 11 miles. About ten years ago I was permitted to organize and set on foot in the American navy a plan for "sounding out" the ocean with the plummet. Other navies, especially the English, have done not a little in furtherance of that object. Suffice it to say that, within this brief period, though the undertaking has