GAY LUSSAC S BALLOON ASCENT. 173 



a year. The natives of the Himalayas ascribe 

 the difficulty of breathing experienced in the 

 higher alpine passes, 15,000 to 16,900 feet 

 above the sea, to the exhalations of poisonous 

 plants. It is, in reality, due simply to the 

 rarefaction of the air. Gay Lussac ascended 

 in a balloon to the enormous altitude 'of 

 nearly 22,000 feet above the earth. In these 

 lofty regions the gas of his balloon expanded 

 so much as to require the relief of the safety 

 valve ; and he himself underwent the most 

 acute sufferings from the intensity of the cold, 

 and extreme rarity of the air. Even birds 

 flying over the summits of lofty mountains, 

 are said to labour incredibly when they reach 

 the highest points ; and pigeons dropped from 

 balloons at great heights, fall like stones to 

 the earth. All these facts show us, that as 

 far as man is concerned, the limits of the atmo- 

 sphere are about three miles; for beyond this 

 his respiratory system would cease to act. And 

 if such is the tenuity of the air at the distance 

 of a few miles, what extreme thinness must be 

 attained just at its verge ; such perhaps as is 

 attained in the imperfect vacuum of the air- 

 pump ; which is known to be almost imme- 

 diately fatal to animal life. 



In the study of this fact we are forcibly im- 

 pressed with the truth that an impassable 

 barrier exists, which shuts in the world from 



