THE CLIMATES OF THE SEA. 385 



aid of cnri'ents, the rays of the sim conld not drive it along that 

 £ist. In this fact we have another link in the chain of proof 

 (Chap. XXII.), going to show that the sea receives more heat 

 than it radiates off again. Being now left to the gradual process 

 •of cooling by evaporation, atmospherical contact, and radiation, 

 this isotherm occupies the other eight or nine months of the 

 year in slowly returning south to the parallel whence it com- 

 menced to flow northward. As it does not cool as rapidly as it 

 was heated, the disturbance of equilibrium by alteration of 

 specific gravity is not so sudden, nor the current which is 

 inquired to restore it so rapid. Hence the slow rate of move- 

 ment at which this line travels on its march south. Between 

 the meridians of 25° and 30° west, the isotherm of 60° in Sep- 

 tember ascends as high as the parallel of 56° N. In October it 

 reaches the parallel of 50° north. In November it is found 

 beneath the parallels of 45° and 47°, and by December it has 

 nearly r-eached its extreme southern descent between these meri- 

 dians, which it accomplishes in Januarj^ standing then near the 

 5^^)arallel of 40°. It is all the rest of the year in returning north- 

 ward to the parallel Avhence it commenced its flow to the south 

 in September. Now it will be observed that this is the season — 

 from September to December — immediately succeeding that in 

 which the heat of the sun has been playing with greatest activity 

 upon the polar ice. Its melted waters, which are thus put in 

 motion in June, July, and August, would probably occupy the 

 fall months in reaching the parallels indicated. These waters, 

 though cold, and rising gradually in temperature as they flow 

 south, are probably fresher, and if so, probably lighter than the 

 sea water ; and therefore it may well be that both the warmer 

 and cooler systems of these isothermal lines are made to vibrate 

 up and down the ocean principally by a gentle surface current in 

 the season of quick motion, and in the season of the slow motion 

 principally by a gradual process of calorific absorption on the 

 one hand, and by a gradual process of cooling on the other. We 

 have precisely such phenomena exhibited by the waters of the 

 Chesapeake Bay as they spread themselves over the sea in winter. 

 At this season of the year, the charts show that water of very 

 low temperature is found projecting out and overlapping the 

 usual limits of the Gulf Stream. The outer edge of this cold 

 water, though jagged, is circular in its shape, having its centre 

 near the mouth of the bay. The waters of the bay, being fresher 



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