228 THE ATMOSPIIEEE AND METEOEOLOGT. 



CHAPTER II. 



■WEIGHT OF THE AIR. — HEIGHT OF THE UPPEE STRATA.— BAEOMETRIC 

 MEASURES. 



The weight of the aerial particles, which makes itself felt in so 

 terrible a manner in hurricanes, is relatively very small, since a litre 

 of air (nine-tenths of a quart) taken at the surface of the ground, and 

 at the temperature of zero, weighs 770 times less than a litre of water. 

 But the atmospheric mass surrounding the globe is such that if it 

 were to be entirely agglomerated in a single ball, it would weigh as 

 much as a sphere of copper nearly C3 miles in diameter ; that is, the 

 twelve hundred thousandth part of the mass of the earth.* The 

 pressure exercised by the atmosphere on a man of middle size is not 

 less than 14 or 15 tons ; it is true, however, that this pressure making 

 itself felt at the same time in all directions on our frame, is by that 

 very fact neutralized. AYe know that a column of air on any point 

 whatever of the earth is equivalent on an average to that of a column 

 of water of 32 feet, or to 30 inches of mercury ; it is the knowledge 

 of this fact that has enabled us to construct the barometer. 



Still if we know the weight of the atmosphere, we cannot yet say 

 in a positive manner to what distance it rises in space. If the higher 

 aerial strata had the same density as those on the surface of the sea, 

 the total thickness of the air would not exceed 5 miles, and con- 

 sequently, the highest mountains of the earth, the Gaourisankar, the 

 Kinchinjunga, the Dapsang, and many others, would raise their 

 peaks into empty space above the atmosphere. But it is not so ; 

 above the lower strata, compressed by the weight of all the super- 

 incumbent aerial mass, the particles separate in proportion as the 

 pressure diminishes, the air becomes rarer and rarer in the heights of 

 space, and ends by being completely lost like the thin fluid which 

 composes the tail of comets. According to the calculations of Laplace, 

 it is at more than 26,000 miles above the surface of the earth that, 

 in consequence of the increase of centrifugal force, and the diminution 

 * Sir John Herschel, Meteorology, p. 16. 



