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to liquefy the ice supposed to be derived wholly or principally 

 from the atmosphere. But this I do not beheve to be the fact. 

 The process of liquefaction is aided, more or less, according to the 

 temperature, by the warmth of the atmosphere, and, to the same 



extent, the air is cooled ; but the great supply of caloric for the 



liquefaction of the ice is derived from the waters beneath. This 

 absorption of heat from the waters below has no effect in dimin- 

 ishing the temperature of the air above. By it, the great mass of 

 water may be cooled down a k\v degrees, still leaving the surface 

 of the lake, in contact with the atmosphere, warmer by some de- 

 grees than the previous covering of ice. If we assume, for exam- 

 ple, the average depth of Lake Champlain to be fifty feet, and the 

 thickness of the ice to be eight inches, and the temperature of the 

 water below to be 36^, immediately before the sudden disappear- 

 ance of the ice, (and these do not, probably, differ very much 

 from the usual conditions,] then, since the heat of fluidity in water 

 IS 140^, it is easy to calculate that the sudden liquefaction of the 

 ice, by heat derived wholly from the water, would diminish the 

 temperature of the whole mass of water less than two degrees, 

 leaving the surface in contact with the atmosphere more than two 

 degrees warmer than the ice. 



There are some other phenomena connected with this lake, 

 which I regard as interesting, but I defer touching upon ihem to 

 another time. 



Burlington, Vermont, April, 1851. 



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Art. YI. — Oji Coral Beefs and Islands ; by James D. Dana. 



Part II. 



I'rom the Report on Geology of the Exploring Expedition under Capt. Wilke=, U. S. N". 



3, Coral Islands. 



A. Forms and general features of Coral Islands. 

 4 A barrier reef, and a lagoon enclosed by it, are the prominent 



features of a coral island ,• yet there are a ievf of small size in 

 which the lagoon is wanting. In the larger islands, the waters 

 within look like the ocean, and are similarly roughened by the 

 wind, though not to the same extent. Standing on the north 

 shore of the Raraka lagoon, (in the Paumotus,) and looking south- 

 west, nothing is descried but blue waters; — far in the distance, to 

 the ri^ht or left, a few faint dots are distinguished; and as the eye 

 sweeps around, these gradually enlarge into lines of palms and 

 other t'erdure, wliich finally become distinct groves on nearing the 

 observer. At Dean's Island, another of the Paumotus, and at 

 many of the Carolines, the resemblance to the ocean is still more 

 striking, 'rlie lagoon is in fact but a fragment of the ocean cut 



Secoxd Series, VoL^II, No, 34-rJ%> ^S^l- * 



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