234 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
lels of latitude — at least there would be no breaks in them, like that 
which we 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, with 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, without 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, which 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- 
