DUST-HAZE. 209 



of the atmosphere, but which while the sun shines high 

 in the heavens, remains most generally invisible. 



An illuminated ray of light thrown across a dark 

 room in our own country will demonstrate the exist- 

 ence of this dust, which has been treated of elsewhere 

 in these pages. When however it becomes irradiated 

 upon the plains, by means of cross rays from the 

 evening sun, the dust thus brought into view so far 

 impairs that apparently perfect clearness of the atmo- 

 sphere of which we have spoken, as to destroy the 

 intense sharpness of detail with which the mountains 

 were previously exhibited, which caused them to appear 

 so near at hand in the morning: they therefore then 

 are seen as if they had been set back to a greater 

 distance from the spectator. Such at least is the 

 inference which we have ventured to draw from this 

 remarkable phenomenon. 



The blue haze which fills the air in dry tropical 

 regions, during the hot season, has been made the 

 frequent subject of remark by travellers. Its density 

 is such that at times it has all the effects of a regular 

 fog, and restricts the range of vision to a compar- 

 atively short distance. In the intensely dry state of the 

 atmosphere, when even dews are often conspicuously 

 absent, we can have no doubt that this misty appear- 

 ance is due to dust clouds. Dr. Livingstone seems to 

 have been of the same opinion, for he mentions the 

 existence of this haze in several passages in his travels 

 on the Zambesi, and also refers to the extreme clearness 

 of the atmosphere, and the great distance at which 

 objects became visible after the advent of the first 

 rains,* which would have the effect of carrying down 



* See The Expedition to the Zambesi, by David Livingstone, 1865, p, 64. 

 VOL. III. 14 



