LONDON FOG. 189 



slackening of the fires during the hours of rest, and the 

 upper air, which may be very dry and tranquil with- 

 out the limits of the city heat both upwards and lat- 

 erally, may have melted the fog of the preceding day, 

 the air may be moderately clear. But when the half- 

 million of fires are lighted, and send up their heat, 

 the whole moisture of the surrounding air is poured 

 over the city ; and that, mingling with the evapora- 

 tion from the city itself, becomes so dense, that the 

 charcoal, and the nitrate of ammonia, and all the 

 other matters which, at ordinary times, the air dis- 

 perses in great part, float mixed with the watery 

 vapour, and produce an atmosphere approaching as 

 nearly to the consistency of a quagmire in the air 

 as it is perhaps possible to obtain. 



But unpleasant and inconvenient as the London 

 fog is, and much as it prevents all means of obser- 

 vation, there is still something in it worthy of at- 

 tention to the observer of nature. The fog is a 

 natural production, though some of the elements of 

 it are brought together by artificial means ; and thus r 

 though they be somewhat dismal charms, it has still 

 some of the charms that belong to all natural phe- 

 nomena. It is curious to find a sort of twilight rep- 

 resentation 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 boroughs which compose the me- 

 tropolis, 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 frequently 

 in the direction of the tides than in the cross direc- 

 tion. 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 



