Real 



58 COLOUR OF THE SKY. 



banks of the Lake of Geneva the mean Immidity of the 

 same month is only 80*^, the average heat being G6"2°. 



iiuinidity On reducing these observations to a uniform temperature, 

 we lind that the real humidity in the equinoctial basin 

 of the Atlantic Ocean is to that of the summer months 

 at Geneva as 12 to 7. This astonishing degree of mois- 

 ture in the air accounts to a great extent for the 

 vigorous vegetation which presents itself on the coast 

 of South America, where almost no rain falls during 

 many years. 



Intensity Intensity of the Colour of the Sky and Ocean. — From 



?i ^'."'"" the coasts of Spain and Africa to those of South America, 

 the azure colour of the sky increased from 13^ to 23° of 

 Saussure's cyanometer. From the 8th to the 12th of July, 

 in lat. 12.',° and 14° N., the sky, although no vapour 

 could be observed, was of an extraordinary paleness, the 

 instrument indicating only 16° or 17°, although on the 

 preceding days it had been at 22°. The tint of the sky 

 is generally deeper in the torrid zone than in high lati- 

 tudes, and in the same parallel it is fainter at sea than 

 on land. The latter circumstance may be attributed to 

 the quantity of aqueous vapour which is continually 

 rising towards tlie higher regions of the air from the 

 surface of the sea. From the zenitli to the horizon, there 

 is in all latitudes a diminution of intensity, which follows 

 nearly an arithmetical progression, and depends upon the 

 moisture suspended in the atmosphere. If the cyano- 

 meter indicate this accumulation of vapour in the more 

 elevated portion of the air, the seaman possesses a simpler 

 method of judging of the state of its lower regions, by 



So'ioj dib'i observing the colour and figure of the solar disk at its 

 rising and setting. In the torrid zone, where meteoro- 

 logical ])henoniena followeach other witli great regularity, 

 the prognostics are more to be depended upon than in 

 nortliern regions. Great paleness of the setting sun, 

 and an extraordinary disfiguration of its disk, almost 

 certainly pre&ige a storm ; and yet one can hardly con- 

 ceive iiow tlic condition of the lower strata of the air, 

 which is announced in this manner, can be so intimately 



