103 



are a great number of circumstances, in which 

 the intensity of the aerial light is very small 

 while the cyanometer indicates deeper tints. 

 Mr. Leslie f has observed, for instance, by his 

 photometer, that the light diffused is weaker, 

 when the sky is of a very deep and pure blue, 

 than when it is slightly covered by transparent 

 vapor. So, on the mountains where the inten- 

 sity of the direct light is the greatest the 

 aerial light is very weak, because the rays are 

 reflected by air of less density. A very deep tint 

 corresponds there to the feebleness of the dif- 

 fused light ; and the aspect of the sky on the 

 mountains would resemble that offered by the 

 heavenly vault on the plains, when it is illumin- 

 ed by the feeble light of the Moon, if the state 

 of the aqueous vapor did not produce a sensible 

 difference in the quantity of white rays reflected 

 toward the lower regions of the Earth. In these 

 regions, the vapors are condensed after sunset, 

 and the descending currents disturb that equi- 

 librium of temperature, which has been estab- 

 lished during the day. On the ridge of the Cor- 

 dilleras, the azure of the sky is less mingled with 

 white, because the air there is always extremely 

 dry. The rarer atmosphere of the mountains, 

 illumined by the vivid light of the Sun, reflects 



* On the Propagation of Heat, p. 441. 

 f Laplace, Mecan. Celeste, t. iv,p. 282. Expos. duSyst. 

 du Monde, p. 96. 



