DAMP AND FOG. 195 



density of the fog must vary with that. Some 

 parts of London are on a thick bed of fine dry sand 

 and gravel, which allows the water to sink into 

 the ground, so that it is not there to cause fog. 

 Others are on sludge or mud, natural or artificial, 

 and that works up between the stones of the pave- 

 ment, forms mire on the surface, and converts the 

 street into a very successful manufactory of fog ; 

 and other parts again are on an exceedingly tough 

 clay, the surface of which is kept cold by continual 

 humidity and evaporation. 



We may here find a use in observing the effects 

 of the London fog ; for it will be found, where 

 other circumstances are the same, to be no bad 

 indicator of the healthiness of the different places. 

 When the air is more than usually humid, and the 

 surfaces of the walls in consequence cold, they 

 melt dew out of the warmer and humid air, just 

 as the windows of a room in which there are 

 many people, melt dew out of the moist and warm 

 air within ; or as the surface of the earth and of 

 vegetables melts dew out of the warm air of the 

 evening, which does not cool so fast as these solid 

 substances. The dew of the fog takes the coat of 

 the fog along with it ; and thus, wherever the 

 bricks and stones become soonest discoloured, and 

 the former show symptoms of decay, and the latter 

 get discoloured with green mould, and other little 

 plants, the place, whatever may be its height 

 above the mean level, is always the most damp 

 and unwholesome. Wherever the bricks lose their 

 colour fast, and become granular at the edges, it 

 will be found that the mortar is most decomposed, 

 and has an efflorescence of salts of lime on it ; and 

 it will be found that the buds of the trees are black, 

 s 2 



