by which Crabs throw off their Claws, 1 1 



the obvious tendency of the muscles to bend the limb instead of break- 

 ing- it, a careful consideration of these drawings, or of the limb of the 

 animal itself with their assistance, will, it is hoped, render it perfectly 

 intelligible. 



Art. II. — On the Fogs of the Polar Seas. By George 

 Harvey, Esq., F.R.S., L. and E. 



[Communicated by the Author.] 



It has. been commonly supposed, that the fogs which cover the 

 Arctic Seas during the greater part of the summer months, are 

 produced by the moist air depositing its vapour, in consequence of 

 being chilled by contact with the sea. But this cause, it is pre- 

 sumed, is not adequate to the formation of mists ; since it has 

 been proved by Dr. Wells*, that dew and hoar frost are the 

 only results which arise from air, either perfectly or imperfectly 

 saturated with moisture, coming in contact with a body colder 

 than itself. To produce mist or fog, as has been satisfactorily 

 demonstrated by Dr. James Huttont, it is necessary, 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 perfect accordance with these 

 conditions. 



Before the end of June, the shoals of ice are commonly divided 

 and scattered ; the temperature of the ocean being at that time 

 necessarily greater than that of the icy masses floating on its 

 bosom. This inequality of temperature will necessarily 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 ice* 

 bergs. The cooling influence of the icy masses also, in conse- 

 quence of their being elevated considerably above the sea, will be 



* Wells on Dew. 



t Transactions of the Royal Society of Edinburgh, Vol. I. 



