382 PHYSIC^VL GEOGEAPHY OF THE SEA, AND ITS METEOEOLOGY. 



lead from the polar regions. The arrow-heads point to the direc- 

 tion in which the waters are supposed to flow. Their rate, accord- 

 ing to the best information that I have obtained, is, at a mean, 

 only about four knots a day — rather less than more. Accord- 

 ingly, 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 Hum- 

 boldt's Current (§ 398) ; the other, entering the South Atlantic, 

 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 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 to 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 the opposite scales of a true balance, 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^. 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 travelling from the poles to the equator they partake of 

 the temperature of the latitudes through which they flow, and 

 grow warm. Plate IX., therefore, is only introduced to give 

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

 influx of cold water into the South Atlantic appears to divide 

 the warm water, and squeeze it out at the sides, along the 

 coasts of South Africa and Brazil. So, too, in the North Indian 

 Ocean, the cold water again compelling the warm to escape 

 along the land at the sides, as well as occasionally in the middle. 

 In the North Atlantic and North Pacific, on the contrary, the 

 warm water appears to divide the cold, and to squeeze it out 

 along the land at the sides. The impression made by the cold 



