328 Of the Propagation of Heat 



cient in that part of the North Sea (lat. 67°) where Lord 

 Mulgrave sounded at the depth of 4680 feet, to melt a 

 cake of ice 265 feet thick ! 



But the Heat evolved in the formation of each super- 

 ficial foot of ice would be sufficient to raise the tempera- 

 ture of a stratum of incumbent air 2220 times as thick 

 as the ice (consequently, in the case in question, 265 X 

 2220 feet, or 869 miles thick) 28 degrees, or from the 

 temperature of freezing water to that of 50° of Fahren- 

 heit's thermometer, or to the mean annual temperature 

 of the northern parts of Germany ! 



The Heat given off to the air by each superficial foot 

 of water in cooling one degree is sufficient to heat an in- 

 cumbent stratum of air 44 times as thick as the depth 

 of the water 10 degrees. Hence we see how very power- 

 fully the water of the ocean, which is never frozen over, 

 except in very high latitudes, must contribute to warm 

 the cold air which flows in from the polar regions. 



But the ocean is not more useful in moderating the 

 extreme cold of the polar regions than it is in tempering 

 the excessive heats of the torrid zone ; and what is very 

 remarkable, the fitness of the sea water to serve this last 

 important purpose is owing to the very same cause which 

 renders it so peculiarly well adapted for communicating 

 Heat to the cold atmosphere in high latitudes, namely, 

 to the salt which it holds in solution. 



As the condensation of salt water with cold con 

 tinues to go' on even long after it has been cooled to the 

 temperature at which fresh water freezes, those particles 

 at the surface which are cooled by an immediate contact 

 with the cold winds must descend, and take their places 

 at the bottom of the sea, where they must remain, till, 

 by acquiring an additional quantity of Heat, their spe- 



