316 Mr. Harvey on the Fogs of the Polar Seas. 



satisfactorily demonstrated by Dr. James Hutton*, it is neces- 

 sary that volumes of air, of unequal degrees of temperature, 

 and holding moisture in solution, should be mingled together ; 

 and the circumstances of the Arctic Seas, during the period 

 when these fogs generally prevail, are, it is presumed, in pei'- 

 fect accordance vvith these conditions. 



Before the end of June, the shoals of ice are commonly di- 

 vided and scattered, the temperature of the ocean being at 

 that time necessarily greater than that of the icy masses floats 

 ing on its bosom. This inequality of temperature will neces- 

 sarily impart a corresponding influence to the air, and occasion 

 the portions of the atmosphere resting on the broken surfaces 

 of the water to become warmer than the atmosphere in the 

 vicinity of the icebergs. The cooling influence of the icy 

 masses also, in consequence of their being elevated conside- 

 rably above the sea, will be difliised, not only by radiations 

 from their upper surfaces to the canopy of the sky above them, 

 but by horizontal radiations to the air surrounding their sides. 

 A volume of the atmosphere therefore, between two neighbour- 

 ing masses of ice, will necessarily have its middle portion of a 

 higher temperature than that of either of the portions of air 

 between it and the icebergs f ; and the consequence of such an 

 unequal distribution of temperature must be, to cause the cold 

 air to mingle with that of a higher temperature, and thus to 

 produce mist or fog. The density of such mist or fog will de- 

 pend on the difference between the temperatures of the 

 mingling volumes, and on the quantity of vapour contained in 

 the air. 



The elevation of those mists above the surface of the sea 

 will also be regulated by that of the icebei'gs near which they 

 form ; since the cooling influence of the frozen mass, by rapidly 

 diminishing above its summit, will as rapidly destroy all ten- 

 dency in the portion of the atmosphere above the level of the 

 iceberg to assume a condition favourable to the formation of 

 mist ; thus prescribing to the mist an elevation dependent on 

 that of the iceberg near which it forms. Captain Ross ac- 

 cordingly remarks, in his account of the Polar Voyage, " that 

 the fog was extremely thick on the surface of the sea, but at 

 the mast-head, and at the top of the iceberg, it was perfectly 

 clear." Captain Scoresby also, in his paper on the fogs of the 



• Transactions of the Royal Society of Edinburgh, vol. i. 



-f- If the water in the vicinity of icebergs presents considerable inequalities 

 of temperature, the air which reposes on it must be subject to like varia- 

 tions; and numerous examples of the former are to be met with in the ac- 

 counts of the polar voyages. Thus Captain Franklin remarks, " the tem- 

 perature of the surface water was 35° when among the ice, 38° when just 

 clear of it, and 41°*5 at two miles distant." 



Polar 



