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mixture is proceeding extends down to the houses and the 

 ground. In this way there may be brought about a condition 

 in which the lowest air will be the coldest, and in which there 

 will be a gradual increase of temperature in ascending for 

 a considerable height from the earth. If now the newly- 

 arrived warm air maintains a brisk flow there certainly will be 

 no smoky fog, because the smoke will be carried away as 

 quickly as it is evolved from the chimneys. But if, with the 

 distribution of temperature already supposed to have been 

 attained, a calm supervenes, we have a set of conditions 

 present together which can scarcely fail to produce a smoky 

 fog; because the coldest and heaviest air, being at bottom, 

 must tend to stagnate there, all upward and downward cur- 

 rents, such as often at other times occur, being then arrested. 

 Thus, in the stagnating lower stratum, the smoke emitted as 

 usual from the chimneys must accumulate excessively ; and 

 if the newly arrived warm air be moist enough to produce fog 

 by its mixing with the cooler air below, as must often be the 

 case, the accumulating smoke will be accompanied by fog, and 

 so the smoky fog will be produced. If the newly-arrived air 

 over the substratum of colder air be warm and not very moist, 

 an excessive accumulation of smoke may occur in the atmo- 

 sphere of a town in calm weather without there being neces- 

 sarily any fog — the main conditions tending to an extraordinary 

 accumulation of smoke being, a calm atmosphere with a cool 

 stratum at bottom and warmer air above ; and with the cool 

 stratum extending high enough to cover the chimneys of a 

 town, and receive their smoke. On the other hand, if the 

 upper air be cool, the air of the town may be kept clear and 

 fresh without any perceptible wind, by a constant gentle in- 

 terchange of air upwards and downwards ; the warm air of 

 the town floating upwards and being replaced by the cool air 

 from above. 



