144 RIVER FOGS. [CHAP. XXIX. 



clear on each side. These fogs are generated in the 

 same manner as ordinary clouds, by the mixture of 

 two currents of air of different degrees of tempera 

 ture. The river cools the air in contact with its 

 surface, and this colder layer of air mingling with the 

 warmer layer immediately over it, causes the fog to 

 begin to form close to the water. Hence it is fre 

 quently confined to the bed of the river, not spread 

 ing at all over the banks. The upper surface is often 

 as well defined as if it were a bed of liquid, instead 

 of vapour, and the cabin, roof, and funnels of a 

 steamer may be seen moving along perfectly unob- 

 scured, while the hull and lower parts are as com 

 pletely hidden as if buried beneath the turbid water 

 on which it floats. The pilot, too, from the upper 

 deck, can often see the shore and landmarks with 

 perfect clearness, and steer his vessel with safety, 

 while the passengers on the cabin deck can see no 

 thing beyond the sides of the boat. The fogs form 

 sometimes whatever be the quarter from which the 

 wind blows, but are more frequent when it is from 

 the south, as the air is then the warmest. Pieces of 

 ice are rarely floated down below Natchez, 350 miles 

 above the Balize ; but, in some seasons, they have 

 been known to reach the Gulf itself. 



Next morning we weighed anchor, and passed Fort 

 Jackson, formerly Fort St. Philip, thirty -three miles 

 above the Balize. At several points, where we stopped 

 for passengers, Dr. Carpenter and I landed. The wood 

 consisted of live oaks bearing bunches of misletoe, 

 cypress hung with Spanish moss, elms, alders, and the 

 red maple ; also a species of myrica, twenty feet high, 



