192 LONDON FOG. 



the land, especially where it is bare and dry, cools 

 much sooner than the water; and as it is the 

 change of temperature, and not the absolute tem- 

 perature that produces the change of evaporation, 

 vapour then gathers, over the pools and marshes, 

 and the courses of the rivers; and among bare 

 hills with deep valleys, and lakes and rivers, the 

 fog is often seen white and dense, in the hollows, 

 as if some white fluid had been poured into 

 them. 



City fogs, such as the fog of London, which is 

 at times very annoying, and always very offen- 

 sive, are owing to a similar cause, only in the 

 case of these, that cause is in the city. In the 

 early morning, when the production of fog has 

 been lessened by the slackening of the fires dur- 

 ing the hours of rest, and the upper air, which 

 may be very dry and tranquil without the limits 

 of the city heat both upwards and laterally, 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 

 evaporation 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 disperses 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 Lon- 

 don fog is, and much as it prevents all means of 

 observation, there is still something in it worthy 



