190 THE FOG-MAP 



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 enables 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 quantity added as 

 the moving mass creeps westward must, to some 

 extent, weaken the shades of density as first pro- 

 duced, yet these are not altogether obliterated. 

 Hence if one takes post somewhere about EaiTs- 

 court, on a morning with the wind at east, first 

 comes the fog of Brompton, and part of Chelsea and 

 Knightsbridge ; then comes the Green Park, a great 

 deal lighter. St. James's is not very dense, because 

 the houses there are large, and the fires not many. 

 It then gradually thickens to St. Giles's, and the 

 hundreds of Drury. Lincoln's Inn Fields lighten 

 the prospect a little ; but the thick mass of buildings 

 all the way to St. Paul's make it soon dark again. 

 St. Paul's is but a speck ; and after that it is usually 

 dark as Erebus till you are quite tired of it. If the 

 fog of one of the great breweries, or other works, 

 which bountifully bestow all their smoke on the 

 neighbourhood, happens to pass over you, it is per- 

 fect obscurity, more especially if the air which is 

 now passing over you happened to be there when 

 they were feeding the fire. 



The London fog is no indication of rain, or, in- 

 deed, are any of the creeping fogs that are formed 

 in the hollows. They are, indeed, the very re- 

 verse they show that the upper air resists and 

 keeps down the fog, so that the temperature of its 

 own humidity is not altered. But the London fog 

 has a rain of its own, and that rain is filthy to man 

 and pernicious to vegetation. It rains soot and a 



