§ 742. TIDE-KIPS AND SEA DRIFT. 397 



across the seas, and taken the registrations of it exclusively for 

 mj guide, without regard to the reported set of the currents. 

 When, in any latitude, the temperature of the water has appear- 

 ed too high or too low for the latitude, the inference has been 

 that such water was warmed or cooled, as the case may be, in oth- 

 er latitudes, and that it has been conveyed to the place where 

 found through the great channels of oceanic circulation. If too 

 warm, it is supposed that it had its temperature raised in warmer 

 .latitudes, and therefore the channel in which it is found leads 

 from the equatorial regions. On the other hand, if the water be 

 too cool for the latitude, then the inference is that it has lost its 

 lieat in colder climates, and therefore is found in channels which 

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

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

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

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

 ly, therefore, as the immense volume of water in the antarctic re- 

 gions 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 (§ 898) ; 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 80°, their specific gravity will correspond to 

 that of sea water at 80°. 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 wa- 

 ter of this temperature ; and, since fluids differing in specific grav- 

 ity 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 80° to 80° or 85°. Hence it 



