310 THE PHYSICAL GEOGRAPHY OF THE SEA. 



894. 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. 



895. The arrow-heads point to the direction in which the wa- 

 ters are supposed to flow. Their rate, according to the best in- 

 formation that I have obtained, is, at a mean, only about four knots 

 a day — rather less than more. 



896. 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 (§ 455) ; 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 trop- 

 ical in their temperature. They do not tlien, 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 op- 

 erates to send them back. This cause is to be found in the dif- 

 ference of the specific gravity at the two places. If, for instance, 

 these waters, when they commence their flow from the hyperbo- 

 rean regions, 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 difiering 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 restore that equilibrium which it has destroyed in the 

 sea by rising from 30° to 80° or 85°. 



897. 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. 



898. Plate IX., therefore, is only introduced to give general 

 ideas ; nevertheless, it is very instructive. See how the influx of 



