44 Remarks on the Formation of Fogs. 



perceived that fine wisps of vapor were forming in detached spots 

 over the surface of the water. After examining the thermome- 

 ter, which hung in a good exposure, I hastened to the water's 

 edge, noted the temperature there, and again over the surface of 

 the river, at the distance of about two hundred yards from the 

 shore, where the temperature of the water was also observed. 

 The results are given in the fourth line of the table, for March 

 27th, 1840, except that in addition, the temperature at the house 

 on the bluff was 64°, being higher than on the bank. 



The fog commenced to form in almost immediate contact with 

 the surface of the water; indeed, when first observed, it had 

 much the appearance of a very fine fleece floating in spots on the 

 water. It thickened very fast, however, and before the observa- 

 tions over the water were concluded, which required perhaps ten 

 minutes, the river was veiled in an impenetrable cloud, which 

 stood over its surface to the depth of about fifteen feet. Although 

 the upper part of the fog was elevated considerably above the 

 level of the banks, it remained stationary over the bed of the 

 river, and did not flow over the adjoining plain at all. The up- 

 per surface of the fog was level, and though denned with con- 

 siderable accuracy, it terminated above in a thin haze ; and al- 

 though we could see nothing of a steamboat which passed near 

 us, when we were on the bank, we had a fine view of all the 

 upper parts of it after we had mounted upon a pile of wood, 

 which raised us above the thickest, part of the fog. The fog re- 

 mained stationary over the surface of the river about* an hour, 

 and was then swept off by a brisk breeze which sprang up from 

 the northwest. 



The observations given above indicate the absence of those 

 conditions which have generally been considered as requisite to 

 the production of the phenomena under consideration. The 

 temperature of the air was higher than that of the water, and 

 that of the air over land, both on the immediate bank and on 

 the higher lands back, was more elevated than that over the sur- 

 face of the water. The fog could not have resulted from the 

 mixture of the air from over land with that over the water, as 

 the former was lighter than the latter, and could not flow down 

 to the same level with it ; neither could it have been produced 

 by colder air from regions above the surface of the river settling 

 down and mixing with that near the surface ; for in this case we 



