230 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



night, is only covered by a barely perceptible light haze. But coal 

 smoke, commonly has the effect of causing a very persistent haze, and 

 this, in the case of London, spreads consjjicuously with the wiud to 

 places distant 100 miles or more. Coal smoke, we must remember, is 

 accompanied by a good deal of water vapor and sulphurous acid. Gas 

 and wood, when burned in large towns, produce no fog and very little 

 haze, though the dust counter might show as many particles as where 

 coal is burned. Dust in geueral may therefore be acquitted of taking 

 an important part in producing any but a light, thin haze, except where 

 there is a mixture of currents at different temperatures, and then some 

 haze would in most instances be produced in any case by the normal 

 average amount of very fine dust which exists everywhere in the atmos- 

 phere. In clear, homogeneous air, even near saturation, much dust or 

 smoke may be added to the air without causing haze; in dry, hazy air 

 much dust may be added without much intensifying the haze. In cer- 

 tain conditions of wind and weather much haze may exist without an 

 abnormal quantity of dust, and, except on rare occasions, there is 

 always enough dust, maybe of almost molecular dimensions, in the 

 lower strata of the air to admit of precipitation of moisture where con- 

 ditions are otherwise favorable. 1 A great deal of this dust probably 

 consists of chloride of sodium, or sea salt. 



The following instances may serve to show how haze and cloud are 

 successively formed by a conflict of differing currents of air. St. Fil- 

 lans Hill is a small, steep, isolated, conical hill about 300 feet in height, 

 standing in the middle of the valley of the upper Earn, in Perthshire, 

 about 2 miles from the lower end of Loch Earn, and flanked by moun- 

 tains about 2,000 feet high on each side of the valley. The author was 

 on the summit about 5 o'clock one evening in August, 2 when the breeze, 

 which had been blowing freshly from the west, with a clear air, sud- 

 denly began to slacken, and in about five minutes dropped altogether. 

 Then down the valley, eastward, a blue haze began swiftly to climb the 

 glens tributary to Strathearn, and the whole air eastward grew obscure. 

 The calm only lasted a little more than two minutes, and then suddenly 

 a strong wind from the east set in, and soon the air westward as well 

 as eastward had turned thick. The east wind continued, and in a few 

 minutes the tops of the hills rising precipitously from Strathearn to a 

 height of about 2,000 feet were obscured with cloud banners which 

 grew continuously, and descended till in about two hours not only the 

 hills above alevel of about 1,000 feet, but the whole sky, was covered with 

 gray clouds. The duration of the neutral calm corresponded with tbe 

 time usually occupied, according to my observations in the neighbor- 

 hood of London, by a moderate east wind in driving back the opposing 

 current. At Eichmond, and between Eichmond and London, such a 



•These observations are derived from many years' attention to tbe conditions of 

 prevalence of baze and fog in and near London. 

 2 About 1877 or 1878. 



