like water, non-elastic, and not more compressible than this non-elastic fluid, we could sound out the atmospherical ocean with the barometer, and gauge it by its pressure. The mean height of the barometer at the level of the sea in the torrid and temperate zones, is about 30 inches. Now, it has been ascertained that, if we place a barometer 87 feet above the level of the sea, its average height will be reduced from 30.00 in. to 29.90 in.; that is, it will be diminished one-tenth of an inch, or the three hundredth part of the whole; consequently, by going up 300 X 87 ( = 26,100) feet, the barometer, were the air non-elastic, would stand at 0. It would then be at the top of the atmosphere. The height of 26,100 feet is just five miles lacking 300 feet.
4. Weight of the atmosphere.—But the air is elastic, and very unlike water. That at the bottom is pressed down by the superincumbent air with the force of about 15 pounds to the square inch, while that at the top is inconceivably light. If, for the sake of explanation, we imagine the lightest down, in layers of equal weight and ten feet thick, to be carded into a pit several miles deep, we can readily perceive how that the bottom layer, though it might have been ten feet thick when it first fell, yet with the weight of the accumulated and superincumbent mass, it might now, the pit being full, be compressed into a layer of only a few inches in thickness, while the top layer of all, being uncompressed, would be exceedingly light, and still ten feet thick; so that a person ascending from the bottom of the pit would find the layers of equal weight thicker and thicker until he reached the top. So it is with the barometer and the atmosphere: when it is carried up in the air through several strata of 87 feet, the observer does not find that it falls a tenth of an inch for every successive 87 feet upward through which he may carry it. To get it to fall a tenth of an inch, he must carry it higher and higher for every successive layer.
5. Three-fourths below the mountain tops.—More than three-fourths of the entire atmosphere is below the level of the highest mountains; the other fourth is rarefied and expanded in consequence of the diminished pressure, until the height of many miles be attained. From the reflection of the sun's rays after he has set, or before he rises above the horizon, it is calculated that this upper fourth part must extend at least forty or forty-five miles higher.
