234 THE PHYSICAL GEOGRAPHY OF THE SEA. 



lels of latitude — at least there would be no breaks in them, like that 

 which w^e see in the isotherm of 80° for September. It appears 

 from this line that there is a part of the ocean near the equator, 

 and about midway the Atlantic, which, w^ith its waters, never does 

 attain the temperature of 80° in September. Moreover, this iso- 

 therm of 80° will pass, in the North Atlantic, from its extreme 

 southern to its extreme northern declination — nearly two thou- 

 sand miles — in about three months. Thus it travels at the rate 

 of about twenty-two miles a day. Surely, w^ithout the aid of cur- 

 rents, the rays of the sun could not drive it along that fast. 



499. Being now left to the gradual process of cooling by evap- 

 oration, atmospherical contact, and radiation, it occupies the other 

 eight or nine months of the year in slowly returning 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. Hence the slow rate of move- 

 ment at which this line travels on its march south. 



500. Between the meridians of 25° and 30° west, the isotherm 

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

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

 found between the parallels of 45° and 47°, and by December it 

 has nearly reached its extreme southern descent between these 

 meridians, w^hich it accomplishes in January, standing then near 

 the parallel of 40°. It is all the rest of the year in returning 

 northward to the parallel whence it commenced its flow to the 

 south in September. 



501. 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 there- 

 fore 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 mo- 

 tion, and in the season of the slow motion principally by a grad- 



