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-heads point to the direction in w^hich 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 w^armer 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 w^ater 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 diff'ering 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 

 Avhich 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. 



