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Show : . ( 190 THE FOG-MAP upward and the wind at east, the fog will be borne slowly westward, until the fog, which is produced at Black wall may re::~ch as far as Chelsea before the turn of the tidP-. 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 ab~ndant; and though the quantity added as the movmg mass creeps westward must, to some extent, weaken the shades of density as first produced, yet these are not altogether obliterated. Hence if one takes post somewhere about Earl'scourt, on a morning with the wind ::~t east, first comes the fog of Brompton, and part of Chelsea and Knig~tsbridge ~ then comes the Green P:uk, a great de;1l hghter. 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 D~ury. Lincoln's Inn Fields lighten the prospect a llttle; bnt 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 th~ neighbourh<?od, liappens to pass over you, it is perfect obsc~1nty, more P-specially if the air which is now passmg over you happened to be there when they were feeding the fire. The London fog is no indication of rain, or, in~ eed, are any of the creeping fogs that are formed m the hollows. They are, indeed, the very reverse- 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 ve2'etation. It rains soot and a DAMP AND FOG. 191 " villanous combination" of acrid matters which soil t_he people and the!r proviswns, even whbe they are m the act of eatmg. Broccoli, and also the close-leaved vegetables, always have a nauseous bitter taste in thick fogs. But.th.e fog depends on the quantity of moisture there 1s 111 the earth, ~r mud, or whatever happens to be exposed to the air; and so t.he density of the fog must ~ary with that. Some parts of London are on a thick bed of ~ne ~ry sand and gravel, which ~llows the water to smk. mto the ground, so that it 1s not there to canse fog. Others are on sludge or mud, natural or artificial, and that works up betweP-n the stones of the pavement, forms mire on the surface, and converts the street into a very successful manufa_ctory of fog; and other parts again are on an exceedmgly ~ough cla:y:, ~he surface of which is kept cold_ by contmual hunud1ty and evaporation. We may here find a use in observincr the effects o~ the London fog; for it will be found, ~here other circumstances are the same, to be no bad indicator o~ t~e healthiness of the different places. When the a1r 1s 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 whic~ there are many people melt dew out of the mOist and warm air within; or as the surfa~e of the earth and of verretahles melts dew out of the warm air of the eveni~g, which does not cool so fast as these olid substances. The dew of the fog takes the coat of the fog alono- with it· and thus, wherever the bricks and stone; becom~ soonest discoloured, and the f~rmer show symptoms of decay, and the latter get discoloured with green mould, ~nd ~ther little plants, the place, whatever may be Its hetght above the mean level, is always the most damp and unwholesome. Wherever the bricks lose th~ir ~olour fast, and become granular at the edges, 1t will be found 1·hat the mortar is most de- |