ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 229 



of dust, it must arise from some effect of the mixture of different cur- 

 rents. A wind from the Atlantic on the west coast of Great Britain 

 generally has a west wind above it, and is fairly homogeneous, but 

 an east wind generally has to encounter and drive back a westerly or 

 southerly wind, and has an opposing current within 3 to 7 miles above. 

 There must in these cases be a great deal of mixture of portions of air 

 of different humidity, temperature, and electrical tension. The contig- 

 uous parcels of air produce at a number of points momentary deposi- 

 tion of vapor on dust particles, and the resulting effect is haze. The 

 dew point is attained in the molecular environment by momentary 

 contact of cold, dry, dust-bearing with moist, warmer, less dusty air. 



It is well to bear in mind the large extent and small depth of the 

 whole of the lower region of winds. Currents of air, say within 25,000 

 feet of the surface, extended over a territory 400 miles square, would 

 be represented by a layer of water an inch deep in a basin 80 inches 

 square. 



On the east coast of Scotland an east wind often brings a thick haze 

 which may last two or three days, and is followed by rainy weather. 

 But a much less thick blue haze prevails during fine weather, with light 

 or variable easterly breezes, both in Scotland and England. The 

 deusity of the haze in these conditions depends less on the number 

 of dust particles than on the mixture of differing currents and on the 

 moisture and warmth of the one current, the coldness and moisture of 

 the other. There is do reason for supposing that a wind blowing from 

 the polar regions and over the breadth of the North Sea is heavily 

 charged with dust, yet the haziness is as great looking seaward as over 

 the land of Berwickshire or Fife. 



The clear air of continental climates, such as the European and North 

 American, is partially explained by the moderate amount of dust, the 

 infrequency of a condition approaching saturation in the lower air, and 

 the absence generally of local winds such as are produced by a varied 

 distribution of land and sea. Haze is very often the result of the pas- 

 sage of air over water of a lower tenrperature, and the difference of 

 the temperatures may decide whether the obfuscation shall be haze, 

 fog, mist, or fine rain. No amount of dust is in general competent in 

 a dry, uniform air to produce apppreciable haze beyond what is due 

 to its own particles. Thus in Colorado there is often a great deal of 

 dust in the air, but the air is clearer at such times than it commonly is 

 in England; in the Punjaub dust winds obscure the air for a long dis- 

 tance; in the Sahara Desert there is often thick dust, but the hazing is 

 not great except with strong wind; when, however, this dust is blown 

 far out over the Atlantic, the haze becomes very considerable, and is a 

 common phenomenon about the Cape de Verde Islands. Towns, again, 

 such as Paris and Pittsburg, which produce a great deal of dust, by 

 the test of the dust counter, are not affected by haze in clear, dry 

 weather, and even London, in some states of the air and very often at 



