52 AUTUMN AND WINTER 



springs up. The fugitiveness of land-fogs is probably due 

 to the constant slight difference of temperatures over any 

 tract of dry land, with its varying elevation and exposure, 

 and alternations of meadow, woodland and marsh. A slight 

 inrush of air from a warmer quarter is thus often sufficient to 

 dissipate the mist. At sea, on the other hand, there is far 

 greater uniformity in the temperature of the whole body of 

 water and of the air which sweeps over it. If the balance 

 of temperatures once sets up condensation, the process will 

 be so general and widespread, that the wind will not affect it. 

 Belts and islands of fog are often to be seen wandering out 

 at sea, hiding distant vessels and releasing them again, and 

 rousing the sirens of distant lighthouses to irregular bursts 

 of warning sound. These isolated volumes of mist are 

 perfect examples of the identity of fog and cloud ; they are 

 exactly like the blurred and formless dawn-clouds, though 

 they rest on the sea. Unlike the more widespread sea-fogs, 

 these banks only occur in calm weather, when there is little 

 wind to change their temperature and disperse them. 



