Remarks on the Formation of Fogs. 45 



would expect to see the vapor commence to appear at some dis- 

 tance above the surface of the water, instead of in the air in im- 

 mediate contact wall that surface. We can hardly fail to attrib- 

 ute the production of fog in this instance to the refrigeration of 

 the air, in consequence of communicating its heat to the water, 

 and notwithstanding the low conducting power of atmospheric 

 air, we must suppose the extension of refrigeration to depend 

 mainly on the conducting power of the humid air. If the dimi- 

 nution of temperature was confined to the stratum of air in im- 

 mediate contact with the aqueous surface, the deposition of water 

 on that surface would be the only result ; but as the chill becomes 

 general through the air above the river, fog will be formed, and 

 will increase in depth and density as the impression is augmented. 

 The general conclusions at which I have arrived in reference 

 to the occurrence of fogs over the Mississippi River, are as 

 follows : — 



1. That fogs form over this river at all seasons. 



2. That they occur frequently during the day-time as well as 

 at night. 



3. That those occurring during the day-time, though often 

 very dense, are not so extensive as those occurring at night, and 

 rarely extend over land at all. 



4. Fogs originating in day-time are of much more frequent 

 occurrence from February to May than at other seasons, though 

 the nocturnal fogs of winter frequently do not disappjar before 

 10 or 11 o'clock in the morning. 



5. In a large majority of the observations made by me, during 

 the prevalence of fogs on the Mississippi River, in autumn and 

 winter, the water had a higher temperature than the air. These 

 have been exclusively nocturnal fogs, which frequently extend 

 over large regions of country. 



6. In almost every instance in which I have observed the for- 

 mation of nocturnal fogs over the Mississippi River, during the 

 spring and earlier part of the summer, the river water has had 

 a lower temperature than the air. These vary in extent, without 

 any apparent connection with the relative temperature of the 

 water and air ; and the air over the higher lands is by no means 

 invariably colder than that over and near the surface of the water. 



7. In every instance in which I have observed fogs to form 

 over the Mississippi River, in day-time, during the spring and 



