BLUENESS OF THE AIR. 195 



Although loosely stated, the account of this 

 mountain is considered by many to be accurate, 

 and if so, it undoubtedly forms one of the most 

 remarkable exceptions to the general rule, with 

 regard to the temperature of high air in the 

 tropics. It is but right to add, that it has been 

 much disputed whether the white cloud on the 

 mountain really was snow. The white and 

 glistening appearance has been supposed to 

 have been caused by something else, because by 

 ordinary calculations a mountain in East Africa 

 to be covered with perpetual snow must approach 

 16,000 feet in height; and it is by no means 

 clear that Kilima dja aro approaches this degree 

 of altitude. The mean height of the line of 

 perpetual snow is at the equator 15,200 feet* 



We say that the air is transparent,! and 

 without colour ; why then, it may be asked, as 

 we gaze upwards, do our eyea rest in every 

 direction upon a vault of so intense and beau- 



* Since the above was written, intelligence of a more de- 

 finite character as to the nature of this mountain has reached 

 England. It now appears to be without a doubt that its 

 summit is capped with perpetual snow. 



t Mr. Darwin says that while ascending the Bell mountain 

 in Central Chile " the evening was fine, and the atmosphere 

 so clear that the masts of the vessels at anchor in the bay of 

 Valparaiso, although no less than twenty-six geographical 

 miles distant, could be distinguished clearly as little black 

 streaks." 



