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It is a well known fact that fogs are of very common occur- 

 rence over the shoal water of the Southern seas. Their outline 

 is so definite, that according to an expression of Humboldt, they 

 present themselves to the eye like air pictures, in which the 

 fashion of the boltom is reflected. Dr. Franklin first noticed 

 that the water in such places is colder than the surrounding sea. 

 As to the cause of this difference of temperature there have 

 been various opinions. None of them, however, seem to be 

 satisfactory. 



Mr. Desor then proceeded to unfold his own views as follows : 

 It is not, he said to foreign causes that we must look for an 

 explanation of this phenomenon, but to the shoals themselves. 

 The atmosphere over the water is cooled down by the abstrac- 

 tion of caloric, caused by the radiation through the water of heat 

 from the bottom. The suspended moisture is consequently con- 

 densed in the form of fog. That such a radiation takes place is 

 sufficiently evident from the formation of ice at the bottom of 

 ponds and rivers, called ground ice. This sometimes accumu- 

 lates in such a quantity, as when detached from the bottom to 

 raise and bear off from their bed large stones and boulders. 

 There is little doubt that the extensive ice fields found in the 

 spring in the vicinity of the Grand Banks, and which Mr. Desor 

 himself saw covering the sea for many miles, in March, 1847, 

 are made up of ground ice which has floated to the surface. It 

 will be seen that the agency of ground ice furnishes a means of 

 solving many difficulties in the distribution of the drift, such as 

 the transportation of those blocks and boulders which are found 

 at a level higher than their source. 



Mr. Burnett exhibited a series of drawings representing 

 the progressive development of the eggs of a spider, of a 

 species unknown to hip. They were magnified to one 

 hundred and sixty diameters. He stated that there was a 

 difficulty in studying the embryonic development of the 

 Arachnidse, growing out of the opacity of the envelope of 

 the egg, and the extreme delicacy and semi-transparency of 

 the germ. By the appfication of acetic acid he succeeded 

 in removing the horny envelope without injuring the vitel- 

 line membrane. Mr. Burnett's observations were found to 

 coincide very exactly with Herold's. 



