<;HAP. ix. THE IMPORTANCE OF DUST. 73 



while from the still greater heights attained in balloons 

 the sky appears of a blue-black color, the blue reflected 

 from the comparatively small amount of dust particles 

 being seen against the intense black of stellar space. It 

 is for the same reason that the " Italian skies " are of so 

 rich a blue, because the Mediterranean Sea on one side 

 and the snowy Alps on the other do not furnish so large 

 a quantity of atmospheric dust in the lower strata of air 

 as in less favorably situated countries, thus leaving the 

 blue reflected by the more uniformly distributed fine 

 dust of the higher strata undiluted. But these Mediter- 

 ranean skies are surpassed by those of the central Pacific 

 ocean, where, owing to the small area of land, the lower 

 atmosphere is more free from coarse dust than any other 

 part of the world. 



If we look at the sky on a perfectly fine summer's day, 

 we shall find that the blue color is the most pure and 

 intense overhead, and when looking high up in a direc- 

 tion opposite to the sun. Near the horizon it is always 

 less bright, while in the region immediately round the 

 sun it is more or less yellow. The reason of this is that 

 near the horizon we look through a very great thickness 

 of the lower atmosphere, which is full of the larger dust 

 particles reflecting white light, and this dilutes the pure 

 blue of the higher atmosphere seen beyond. And in 

 the vicinity of the sun a good deal of the blue light is 

 reflected back into space by the finer dust, thus giving a 

 yellowish tinge to that which reaches us reflected chiefly 

 from the coarse dust of the lower atmosphere. At sun- 

 set and sunrise, however, this last effect is greatly inten- 

 sified, owing to the great thickness of the strata of air 

 through which the light reaches us. The enormous 



