230 THE ATMOSPHERE AND METEOEOLOGY. 



fact, the boiling point, or the temperature at which the tension of 

 the vapour of water exactly balances the atmospheric pressure, must 

 necessarily sink in proportion as the pressure diminishes. It has been 

 calculated that the average fall of boiling point is 18" Fahrenheit for 

 every 1062 feet of vertical height. But experiments may give 

 for the heights of mountains differences of many hundred feet. 

 Thus, Tyndall found in August, 1859, that the temperature of boiling 

 water on the summit of Mont Blanc was 84*97°, while in the preced- 

 ing year he had observed a slightly lower boiling point on Mont Rosa, 

 though this latter peak is 558 feet lower than the giant of the Alps. 



To what height is the air dense enough for a man to be able to find 

 the oxygen necessary for his lungs, and to live there for a few seconds 

 at least? The climbers of mountains have never reached this ex- 

 treme limit, because of the fatigues of the ascent, which add to their 

 difficulty of finding a sufficient quantity of air. Thus the highest 

 peaks of the Himalayas and the Andes have remained to this day 

 untrodden by human foot.* At the summit of Ibi-Gamin, the highest 

 point yet scaled in an ascent, Robert Schlagintweit found himself at 

 an elevation of 4^ miles. The barometer was only 13*3 inches, so that 

 the traveller had beneath him nearly three-fifths of the mass of air. 



Nevertheless, thanks to the balloon, aeronauts have been able 

 to ascend to heights, which even the condor does not reach, and 

 from whence the highest mountains would appear as if they rose 

 from the depth of an abyss. In 1804, Gay Lussac ascended to 

 41 miles; in 1851, Barral and Bixio ascended a little higher; 

 in 1858, Rush and Green rose to 5 miles. But these are all alti- 

 tudes inferior to the highest summits of the continents. Finally, 

 on September 5, 1862, Glaisher and Coxwell undertook an aero- 

 nautic expedition, in which they resolved to ascend as long as they 

 could preserve the sense of their own existence. The air becoming 

 too rare for their lungs, hardly allowed them to pant, they had palpi- 

 tations of the heart, singing in the ears, the blood swelled the arteries 

 of their temples, their fingers froze and refused to move ; but their 

 will sustained them, they threw more sand from the car, and thus 

 gave themselves a new impetus into the atmosphere. Glaisher fainted 

 away, but his companion did nothing to arrest the ascent; his eyes 

 fixed on the instruments, he noted with a glance the gradual sinking 

 of the column of mercury in the barometer and thermometer, as if 

 he were in the observatory at Xew. Gradually taken possession of 

 by torpor, the aeronaut lost the use of his hands ; but he still held 

 * See The Earth, the section entitled, Mountains. 



