2 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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 ; consequent^, 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. But the air is elastic, and very unlike water. That at the 

 Wei ht of the at- bottom is wcighcd down by the superincumbent 

 mosphere. ^[j. y^{i\i thc forcc of about 15 pounds to the square 

 inch, while that at the top is inconceivably light. If 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 imagine 

 that the bottom layer, though it might have been ten feet thick 

 when it first fell, yet with the weight of the accumulated and su^ 

 perincumbent 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 un- 

 til he reached the top. So it is with the barometer and the at- 

 mosphere; 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. More than three fourths of the entire atmosphere is below 

 Three fourths beio^^ thc Icvcl of tlic highcst mouutaius ; the other fourth 

 the mountain tops, ^g rarcficd and expanded in consequence of the di- 

 minished 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. 



6. At the height of 26,000 miles from the earth, the centrifugal 

 Its height. 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 at- 

 mosphere can not extend beyond that. This limit, however, has 

 been greatly reduced, for Sir John Herschel has shown, by bal- 



