2 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



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 ascer- 

 tained 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 ( = 2G,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, Weiglit of the atmosphere. — But the air is elastic, and very 

 unlike water. That at the bottom is pressed down by the super- 

 incumbent 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 atmo- 

 sphere : 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 

 cany 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 helow 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, imtil 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. 



