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XLIX. Some Observations on the Formation of Mists in parti- 

 cular Siluatio?is. By Sir H. Davy, Bart. F.R.S. F.P.R.L* 



Alj. persons wlio have been accustomed to the observation of 

 Nature, must have frequently witnessed the formation of mists 

 over the beds of rivers and lakes in calm and clear weather after 

 sun-set J and whoever has considered these phsenomena in rela- 

 tion to the radiation and communication of heat and nature of 

 vapour, since the publication of the researches of MM. Rumford, 

 Leslie, Dalton, and Wells, can hardly have failed to discover the 

 true cause of them. As, however, I am not aware that any work 

 has yet been published in which this cause is fully discussed, and 

 as it involves rather complicated principles, I shall make no apo- 

 logy for offering a few remarks on the subject to the Royal So- 

 ciety. 



As soon as the sun has disappeared from any part of the globe, 

 the surface begins to lose heat by radiation, and in greater pro- 

 portions as the sky is clearer ; but the land and water are cooled 

 by this operation in a very different manner : the impression of 

 cooling on the land is limited to the surface, and very slowly 

 transmitted to the interior ; whereas in water above 45° Fahren- 

 heit, as soon as the upper stratum is cooled, whether by radiation 

 or evaporation, it sinks in the mass of fluid, and its place is sup- 

 plied by warmer water from below, and till the temperature of 

 the whole mass is reduced nearly to 40° F. the surface cannot be 

 the coolest part. It follows, therefore, that wherever water exists 

 in considerable masses, and has a temperature nearly equal to that 

 of the land, or only a few degrees below it, and above 45° F. at 

 sun-set, its surface during the night, in calm and clear weather, 

 will be warmer than that of the contiguous land; and the air 

 above the land will necessarily be cooler than that above the wa- 

 ter; and when they both contain their due jjroportion of aqueous 

 vapour, and the situation of the ground is such as to permit the 

 cold air from the land to mix with the warmer air above the 

 water, mist or fog will be the result ; which will be so much the 

 greater in quantity, as the land surrounding or inclosing the 

 water is higher, the water deeper, and the temperature of the 

 ■water, which will coincide with the quantity or strength of vapour 

 in the air above it, greater. 



I shall detail some observations which appear to me to show 

 the correctness of this view. June 9th, 10th, 11th, the tempe- 

 rature of the atmosphere and of the Danube was repeatedly ex- 

 amined during a voyage that I made upon this river from Ratis- 

 bonne to Vienna, and on each of these days, the sky being per- 



• From Philosophical Transactions for 1819, Part I. 



fectly 



