TIDE-RIPS AND SEA DEIFT. 397 



from the equatorial regions. On tlio 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 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. 

 Accordingly, therefore, as the immense vohime 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 (§ 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 difference of the specific gravity at the two places. 

 If, for instance, these waters, when they commence their flow 

 from the hj^perborean regions, were at 30°, their specific gravity 

 will correspond to that of sea water at 30°. But when they 

 Eirrive in the Gulf of Guinea or the Bay of Panama, having risen 

 hy 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 in- 

 stnictive. 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 

 wann to escape along the land at the sides, as well as occasionally 



