]34 EXTENT OF THE ATMOSPHEEE. [SECT. xv. 



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 at- 

 tenuated 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 pene- 

 trating 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 tropics 

 or in the higher latitudes, in consequence of the ascent of the 

 heated air from the surface of the earth ; it is less also on the 

 shores of the Baltic Sea than it is in France, probably from 

 some permanent eddy in the air, arising from the conforma- 

 tion of the surrounding land ; and to similar local causes 

 those barometric depressions may be attributed which have 

 been observed by M. Erman near the sea of Ochotzk in 

 Eastern Siberia, and by Captain Foster near Cape Horn. 



There are various periodic oscillations in the atmosphere 

 which, rising and falling like waves in the sea, occasion cor- 

 responding changes in the height of the barometer, but they 

 differ as much from the trade-winds, monsoons, and other 

 currents, as the tides of the sea do from the Gulf-stream and 



