31 8 Line of perpetual Froft. [Book VI. 



fome of the mountains in that planet being calculated 

 to exceed nine miles in height. 



The line of congelation, or perpetual froft, on moun- 

 tains, is calculated at 15,400 feet, at or near the equa- 

 tor ; at the entrance of the temperate zone, at 13,428 j 

 onTenerifF, at 1,000; in Auvergne (lat. 45.) 6,74Oj 

 with us (lat. 52.) 5,740. On the Andes, vegetation 

 ceafes at 14,697 feet; and on the Alps, at 9,585. 

 The air is fo dry in thefe elevated fituations, that M. 

 D'Arcel obferved, that on the Pic de Midi, one of the 

 Pyrenees, fait of tartar remained dry for an hour and a 

 half, though it immediately moiftened in the fame tem- 

 perature at the bottom of the mountain. 



