246 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
channels of circulation in the ocean. If too warm, it is supposed 
528) that it had its temperature raised in warmer latitudes, and 
therefore the channel in which it is found leads from the equato- 
rial regions. 
532. On the other hand, if the water be too cool for the latitude, 
then the inference is that it has lost its heat in colder climates, and 
therefore is found in channels which lead from the polar regions. • 
The arrow-beards point to the direction in which the waters are 
supposed to flow. Their rate, according to the best information 
that I have obtained, is, at a mean, only about four knots a day — 
rather less than more. 
533. Accordingly, therefore, as the immense volume of water in 
the Antarctic regions is cooled down, it commences to flow north. 
As indicated by the arrow-heads, it strikes against Cape Horn, and 
is divided by the continent, one portion going along the west coast 
as Humboldt's Current (§ 267) ; the other, entering the South At- 
lantic, flows up into the Gulf of Guinea, on the coast of Africa. 
Now, as the waters of this polar flow approach the torrid zone, they 
grow warmer and warmer, and finally themselves become tropical 
in their temperature. They do not then, it may be supposed, stop 
their flow ; on the contrary, they keep moving, for the very cause 
which brought them from the extra-tropical regions now operates 
to send them back. This cause is to be found in the difference of 
the specific gravity at the two places. If, for instance, these wa- 
ters, when they commence their flow^ from the hyperborean re- 
gions, were at 30°, their specific gravity will correspond to that of 
sea water at 30°. But when they arrive in the Gulf of Guinea or 
the Bay of Panama, having risen by the way to 80°, or perhaps 
85°, their specific gravity becomes such as is due sea water of this 
temperature ; and, since fluids differing in specific gravity can no 
more balance each other on the same level than can unequal 
weights in opposite scales, this hot water must now return to re- 
store that equilibrium which it has destroyed in the sea by rising 
from 30° to 80° or 85°. 
534. Hence it will be perceived that these masses of water 
which are marked as cold are not always cold. They gradually 
pass into warm ; for in traveling from the poles to the equator they 
partake of the temperature of the latitudes through which they 
flow,. and grow warm. 
