Remarks on the Formation of Fogs. 41 



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ing remarks will be to render it probable that other influences 

 may, under favorable conditions, give rise to the same phenom- 

 enon ; and also to show that the prevailing opinions respecting 

 the conditions requisite to its production, are in some measure 

 erroneous. 



In the year 1819, a paper was published by Sir H. Davy, in 

 the Philosophical Transactions, giving his views, and the result 

 of his observations on the formation of fogs and mists. The fol- 

 lowing is a summary of the results at which he arrived. " After 

 sunset, the earth commences to cool in consequence of radiation, 

 but land and water are cooled by this operation at very different 

 rates : the surface of the land cools much more rapidly than that 

 of the water, and the air over the land becomes colder than that 

 over the water, and when they both contain their due proportion 

 of aqueous vapor, and the land is so situated as to permit the 

 cold air from the land to mix with the warmer air over the water, 



the production of mist or fog will result. The density of such 



fog will moreover be greater as the land surrounding the water 

 is higher, and the water deeper and warmer." 



Here Davy follows Hutton, in attributing the formation of fog 

 to the mixture of masses of air of different temperatures, and all 

 the forms of visible vapor are now supposed to have their origin 

 in some modification of the same influences. 



He goes on to say, that in no case that came under his obser- 

 vation, " was fog formed on a river or lake, when the temperature 

 of the water was lower than that of the air over it, even though 

 the air was saturated with vapor." These remarks have been 

 generalized and a law established, which I believe is generally 

 received as true, that u the formation of fog never takes place 

 over water, when its temperature is lower than that of the atmos- 

 phere, not even though the air should be saturated with vapor." 

 Now we cannot hesitate about considering the observations of 

 Davy as perfectly accurate, but it will be shown that the law is 

 not of universal application, and that we should, consequently, 

 protest against the too hasty generalization. 



From a comparison of my own conclusions, drawn from obser- 

 vations on the fogs of our southern rivers, with those of Davy, 

 drawn from observation on many of the great rivers of Europe, 

 I should infer, either that the agencies governing their formation 

 there must be different from those in which they originate here ; 



Vol. xliv, No. L— Oct.-Dec. 1842. 6 



