100 



perfectly transparent ; and the existence of the 

 opake vapor is announced to navigators only by 

 the little intensity of the azure color of the sky. 

 We shall hereafter have occasion to return to 

 these phenomena, which modify the extinction 

 of light ; and which, like the fogs popularly 

 called dry, remain so confined to the high regions 

 of the atmosphere, that our hygrometers undergo 

 no sensible change. 



I have often repeated, in the equinoctial part 

 of the Atlantic Ocean, the experiments of Mr. de 

 Saussure on the decrement of the intensity of 

 color observed from the zenith to the horizon. 

 On the 14th of July, in latitude 16° 19', the sky 

 being of the purest blue, the thermometer keep- 

 ing at twenty-two degrees, and the hygrometer 

 at eighty-eight degrees, I found, toward noon, 



at 1° of height, 3° of the cyanometer 



10 6 



20 10 



30 16-5 



40 18 



60 22 



between 70 & 90 235 



The 30th of June, in latitude 18° 58', the ther- 

 mometer being at 21*2°, and the hygrometer at 

 81*5°, the cyanometric decrement had been 

 somewhat less regular : 



at 1° of height, 2*5° of the cyanometer 

 10 4 



