COLOUR OF THE SKY/ 57 



coasts of South America, where so little rain falls 

 throughout the year. 



Intensity of the Colour of the Sky and Ocean. 

 From 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 free of vapour, was of an extra- 

 ordinary 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 latitudes, 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 

 the higher regions of the air from the surface of the 

 sea. From the zenith 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 

 cyanometer 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 observing the colour and 

 figure of the solar disk at its rising and setting. In 

 the torrid zone, where meteorological phenomena 

 follow each other with great regularity, the prog- 

 nostics are more to be depended upon than in north- 

 ern regions. Great paleness of the setting sun, and 

 an extraordinary disfiguration of its disk, almost 

 certainly presage a storm ; and yet one can hardly 

 conceive how the condition of the lower strata of 

 the air, which is announced in this manrler, can be 

 so intimately connected with those atmospherical 

 changes that take place within the space of a few 

 hours. 



Mariners are accustomed to observe the appear- 

 ances of the sky more carefully than landsmen, and 

 among the numerous meteorological rules which 



