266 TEE ATLANTIC. [chap. v. 



Tristan d'Aciniha to tlie equator, the isothermobaths between 

 5° and 20° C. altered very slightly in position : the surface-lay- 

 ers, of course, became steadily and rapidly warmer. 



Bearing in mind that at a certain depth below the surface, 

 varying only slightly in different regions, there is a thick belt 

 of water at a pretty nearly uniform temperature from 4° to 5° 

 C, it is evident that the much higher temperature of the sur- 

 face-layers must be due, for each position, directly or indirectly, 

 to the heat of the sun. Normally the surf ace - temperature 

 would attain its maximum near the equator, and would decrease 

 uniformly toward the poles; and the very abnormal distribu- 

 tion of temperature which actually exists must depend upon 

 some disturbing cause or causes. That several such causes 

 come into play, and many complicated combinations of these 

 causes, there appears to be little doubt ; but one disturbing 

 cause seems to be so paramount, so sufhcient in itself to account 

 for the observed phenomena, that I do not think it necessary in 

 this preliminary sketch to pursue the inquiry beyond it. 



The permanent winds, blowing eternally in one direction 

 where the water is hottest, send the heated surface-water in a 

 constant stream to the westward. This " equatorial current," 

 impinging upon the coast of South America about Cape St. 

 Bogue, splits in two. A considerable portion of the northern 

 branch coursing round the Gulf of Mexico, and becoming con- 

 tracted and condensed by the Strait of Florida, makes itself 

 manifest as the celebrated Gulf-stream ; while the remainder, 

 moving outside the islands in a gentler and less obvious cur- 

 rent, spreads over the great bight between North and South 

 America, and gives an indication of its presence in the high 

 thermometer -readings round Bermudas, and westward to the 

 Azores. The cause of the second and deei3er hump on the 

 temperature-curves (vol. i.. Fig. 100), in a section between Ber- 

 mudas and the coast of Europe, is perhaps not so evident. The 

 explanation which I have suggested elsewhere is that the warm 



