42 Remarks on the Formation of Fogs. 



or that his observations, particularly on rivers running towards 

 the south, must have been mainly limited to the last of summer, 

 the autumn, or first part of the winter.* For, were the conditions 

 always observed by him to accompany the formation of fogs ab- 

 solutely requisite for their production, it would preclude the pos- 

 sibility of their occurrence on the Mississippi River during a con- 

 siderable portion of the year, particularly during the latter part of 

 the winter, the spring, and first part of summer. For, the large 

 body of water which comes down that river at those seasons, 

 from much colder regions, traverses the climates through which 

 it passes so rapidly, that it does not acquire their temperature ; 

 and when it arrives here its temperature is always much below 

 the mean of those seasons, and even below the ordinary monthly 

 minima. Convinced that this must be the case, and aware that 

 the water of the Mississippi River is pleasantly cold for drinking 

 during all the spring and first summer months, which required, 

 at those seasons, that it should be considerably colder than the 

 air, I was satisfied that fogs, which are of common occurrence 

 at those seasons, must be formed much more frequently over this 

 body of water, than the temperature of the air was lower than 

 that of the water. In the latter part of the year 1839, I com- 

 menced a series of observations with the view of testing the truth 

 of these conclusions. The following table exhibits the results of 

 observations, made during the formation or prevalence of fogs, 

 when the temperature of the water was lower than that of the air. 



* Since the above was written, this inference has been confirmed by reference 

 to Paris's Life of Davy. I give the names of rivers, en the fogs of which he made 

 observations, with the place of observation, the general course of the rivers as to 

 latitude, and the date of observation. 1st. The Rhine, between Cologne and 

 Coblentz; course, north ; date, 31st of May. 2d. The Danube, between Ratisbon 

 and Vienna; general course of the waters from the sources, the tributaries being 

 taken into account, northwardly; date, 9th, 10th and 11th of June. 3d. The 

 Raab, near Kermond in Hungary ; course, north ; date, 11th July. 4th. The Save 

 in Carniola; course, south ; date, "end of August." 5th. The Ironzo,in the Friul; 

 course, south; date, " middle of September." 6th. The Po, near Ferrara; course, 

 south; date, " end of September." 7th. The Tiber; course, south; date, "be- 

 ginning of October." So that only the Ironzo, the Po, and the Tiber, have south- 

 ern courses to the points of observation, and on all these the observations were 

 made during the autumn. 



