§ 723. THE CLIALA.TES OF THE SEA. SS5 



in September. Moreover, this isotherm of 80^ will pass, in the 

 North Atlantic, from its extreme southern to its extreme northern 

 declination — nearly two thousand miles — in about three months. 

 Thus it travels at the rate of about twenty-two miles a day. Sure- 

 ly, without the aid of currents, the rays of the sun could not drive 

 it along that fast. Being now left to the gradual process of cool- 

 ing by evaporation, atmospherical contact, and radiation, it occu- 

 pies the other eight or nine months of the year in slowly return- 

 ing south to the parallel whence it commenced 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 required to restore it so rapid. Ilence the 

 slow rate of movement at which this line travels on its march 

 south. Between the meridians of 25° and 80° west, the isotherm 

 of 60° in September ascends as high as the parallel of 56° N. 

 In October it reaches the parallel of 50° north. In November 

 it is found between the parallels of 45° and 47°, and by Decem- 

 ber it has nearly reached its extreme southern descent between 

 these meridians, which it accomplishes in January, standing then 

 near the parallel of 40°. It is all the rest of the year in return- 

 ing northward to the parallel whence it commenced its flow 

 to the south in September. Now it will be observed that this is 

 the season — from September to December — ^immediately succeed- 

 ing that in which the heat of the sun has been playing with great- 

 est activity upon the polar ice. Its melted waters, which are thus 

 j)nt in motion in June, July, and August, would probably occupy 

 tiie 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 w^armer 

 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. "VVe 

 have precisely such phenomena exhibited by the waters of the 

 Chesapeake Bay as they spread themselves over the sea in win- 

 ter. At this season of the year, the charts show that water of 

 very low temperature is found projecting out and overlapping the 



B B 



