Z PHYSICAL GEOGEAPHY OF THE SEA, AXD ITS METEOROLOGY. 



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 

 hmidredth part of the wdiole; consequently, by going up 300 

 + 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. But the air is elastic, and very unlike water. That at the 

 Weight of the atmo- bottom is prosscd down by the superincumbent 

 ^P^^^^- 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 do-^Ti, 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 fi^om 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 eveiy successive 87 feet 

 upward through which he may carry it. To get it to fall a tenth 

 of an inch, he must cany it higher and higher for every successive 

 layer. 



5. More than three foinths of the entire atmosphere is below 

 Three fourths below the Icvcl of the highost mouutains ; the other fom-th 

 the moimtaia tops, jg rarefied and expanded in consequence of the di- 

 minished pressm-e, 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. 



6. At the height of 26,000 miles fi'om the earth, the centrifugal 

 Its height. force would counteract gravity ; consequently, all 



ponderable matter that the earth carries with it in its dim'nal 



