114 EXTENT OF THE ATMOSPHERE. SECT. XV. 



known by experience to be of extreme tenuity at very 

 small heights. The barometer rises in proportion to 

 the super-incumbent pressure. At the level of the sea 

 in the latitude of 45 and at the temperature of melting 

 ice, the mean height of the barometer being 29-922 

 inches, the density of the air is to the density of a simi- 

 lar volume of mercury as 1 to 10477-9. Consequently 

 the height of the atmosphere supposed to be of uniform 

 density would be about 4-95 miles. But as the density 

 decreases upward in geometrical progression it is consid- 

 erably higher, probably about fifty miles ; at that height 

 it must be of extreme tenuity, for the decrease in density 

 is so rapid that three fourths of all the air contained in 

 the atmosphere is within four miles of the earth ; and, 

 as its superficial extent is 200 millions of square miles, 

 its relative thickness is less than that of a sheet of paper 

 when compared with its breadth. The air even on 

 mountain tops is sufficiently rare to diminish the intensity 

 of sound, to affect respiration, and to occasion a loss of 

 muscular strength. The blood burst from the lips and 

 ears of M. de Humboldt^as he ascended the Andes; 

 and he experienced the same difficulty in kindling and 

 maintaining a fire at great heights which Marco Polo 

 the Venetian felt on the mountains of Central Asia. M. 

 Gay-Lussac and M. Biot ascended in a balloon to the 

 height of 4-36 miles, which is the greatest elevation that 

 man has attained, and they suffered greatly from the 

 rarity of the air. It is true that at the height of thirty- 

 seven miles, the atmosphere is still dense enough to 

 reflect the rays of the sun when 18 below the horizon ; 

 but the tails of comets show that extremely attenuated 

 matter is capable of reflecting light. And although, at 

 the height of fifty miles, the bursting of the meteor of 

 1783 was heard on earth like the report of a cannon, it 

 only proves the immensity of the explosion of a mass 

 half a mile in diameter, which could produce a sound 

 capable of penetrating air three thousand times more 

 rare than that we breathe. But even these heights are 

 extremely small when compared with the radius of the 

 earth. 



The mean pressure of the atmosphere is not the same 

 all over the globe. It is less at the equator than at the 



