232 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



either clears at once or remains for some time as a light blue haze. 1 

 The strata below it, submitted to the same influence, successively rise 

 and take its place, and the evaporated moisture mingles with the gen- 

 eral air. 



Fogs of this kind locate themselves in low-lying valleys, basins, and 

 plains, for the air, chilled by contact with the radiating ground, sinks 

 by gravitation into such situations and in them is least likely to be 

 disturbed. Sometimes a white fog may be seen pouring down an open 

 and rather steep ravine like water. 2 Slopes of hills, especially their 

 southern sides, some hundreds of feet above the plain, are comparatively 

 free from these fogs, and are much drier and warmer during their 

 prevalence than lower places in the neighborhood. Such an elevation 

 is more favorable on this account to the human constitution ; both the 

 daily and yearly thermometric range is much smaller. Dense fog and 

 frost often remain throughout the day on the northern side of hills 

 when the southern slope is bathed in sunshine. This has been observed 

 on several occasions on Hindhead, Surrey, the air in the fog keeping 

 much colder than the air above it and on the southern slope. 



In the still air which precedes and accompanies radiation fogs the 

 number of dust particles is high above the average, owing partly to 

 their becoming gathered by undisturbed precipitation into the lowest 

 strata. On several occasions when the dust particles were counted 

 they amounted to between 45,000 and 80,000 per cubic centimeter. 

 Each of these is a nucleus for the deposition of vapor. The water 

 particles are so small that they evaporate before touching solid objects 

 during the daytime, the objects being warmer than themselves. For 

 this reason these fogs have no wetting effect. In a fog, when objects 

 were invisible at 100 yards distance, 19,350 droplets sometimes fell on a 

 square inch per minute, but the average was much less than this, and 

 the smallest number about 1,900 per minute. 3 The large number of 

 particles favors the formation of fog. Considerable numbers of living 

 organisms no doubt exist among the water particles of the fog, but are 

 not known to be a cause of ill-health in the country remote from towns. 

 Nor is great cold combined with fog productive of much illness in the 

 country. In smoky towns the case is far different. Thus, in London 

 the death rate was raised in a single fortnight, from January 21 to 

 February 7, 1880, from 27.1 to 48.1 per thousand. The fatality and 

 prevalence of respiratory diseases were enormously increased. The 

 excess of deaths over the average in the three weeks ending February 

 14 was 2,994, and in the week ending February 7 the deaths from 

 whooping cough were unprecedentedly numerous — 248 — and from bron- 

 chitis numbered 1,223. At least 30,000 persons must have been ill 



'This haze may be taken to be caused by the aggregated nuclei of dust left after 

 evaporation of the water -which condensed upon them. 



£ This was seen by the author with remarkable distinctness near Alum Bay, in the 

 Isle of Wight. 



s Aitken . 



