LONDON FOG. 193 



of attention to the observer of nature. The fog 

 is a natural production, though some of the ele- 

 ments of it are brought together by artificial 

 means ; and thus, though they be somewhat dis- 

 mal charms, it has still some of the charms that be- 

 long to all natural phenomena. It is curious to find 

 a sort of twilight representation of London in that 

 very substance which completely hides London 

 itself ; and yet such is the case. It is not to be 

 understood that the wards, and cities, and bo- 

 roughs which compose the metropolis, are as well 

 represented by their several fogs as they are by 

 other means; but still they are represented by 

 these. 



The air over London moves upwards and down- 

 wards with the tide of the river ;, and over rivers 

 of such magnitude the light winds are more fre- 

 quently in the direction of the tides, than in the 

 cross direction. The light winds that accompany 

 the fog, though they barely reach the streets, and 

 are not indeed very perceptible, when so little can 

 be seen, are usually from the east. Hence, if 

 the tide is upward and the wind at east, the fog 

 will be borne slowly westward, until the fog, 

 which is produced at Blackwall may reach as far 

 as Chelsea before the turn of the tide. That is 

 one of the causes which produces, or at least en- 

 ables a person at Chelsea to see, the " fog map." 



But again, as the heat of the population and 

 their fires, and the smoke of the latter, produce the 

 smoke, the smoke must be most dense where 

 these are most abundant ; and though the quan- 

 tity added as the moving mass creeps westward, 

 must, to some extent, weaken the shades of den- 

 sity as first produced, yet these are not altoge- 



