518 



During the short space of time that the sky 

 was serene at the zenith, I found the blue of 

 the atmosphere sensibly deeper than on the 

 coasts. It was 26*5° of Saussure's cyanometer. 

 At Caraccas, the same instrument generally 

 indicated only 18° in fine and dry weather. It 

 is probable, that, in the months of July and 

 August, the difference of the colour of the sky 

 on the coasts and the summit of the Silla is 

 still more considerable # , but the meteorologi- 

 cal phenomenon, with which Mr. Bonpland and 

 myself were most struck during the hour we 

 passed on the mountain, was that of the ap- 

 parent dryness of the air, which seemed to 

 increase as the fog augmented. When I took 

 the whalebone hygrometer out of it's case, it 

 pointed to 52° (87° Sauss.). The sky was clear ; 

 yet streams of vapours with distinct outlines 

 passed from time to time in the midst of us, 

 sweeping the ground. Deluc's hygrometer went 

 back to 49° (85° Sauss.). Half an hour later, 

 a thick cloud surrounded us; we could no 

 longer perceive the objects that were the near- 

 est to us, and we saw with surprise, that the 

 instrument continued retrograding toward dry- 

 ness as low as 47*7° (84° Sauss.). The tempe- 

 rature of the air in the mean time was from 1 2° to 

 13°. Though in the whalebone hygrometer the 



* See chap, ii, vol. i, p. 138; and chap, iii, vol. ii, p. 95. 



