Between Cape S. Roque and Tristan d ' Acunha. 81 



ceeding in a south-south-easterly direction from a point to the 

 eastward of St. Paul Rocks and intersecting the meridian of 

 Ascension Island on the parallel of St. Helena, will divide the 

 South Atlantic into two halves — a colder western half as regards 

 the temperature of the lower strata, and a warmer eastern half. 

 The contrast is the same as that found in the North Atlantic, 

 and arises from a similar cause — a warming of the lower strata 

 in the eastern half caused by the return eastward as an under- 

 current of the South Atlantic Equatorial Current. There exists 

 probably an additional cause of the higher temperature of the 

 lower strata of the eastern half of the South Atlantic in com- 

 parison with the western half, namely, a return under-current of 

 warmer water from the North Atlantic, which, finding the 

 depths of the western half occupied by the" Antarctic Current, 

 flows down the eastern valley of the South Atlantic between 

 the central plateau and the coast of South Africa. 



We possess, in this part of the world, only a few soundings 

 of the German frigate " Gazelle," which, however, prove the 

 higher temperature of the lower strata in the eastern half of the 

 South Atlantic, but further observations between Ascension, 

 St. Helena, and the Cape of Good Hope are necessary. 



Section between Cape S. Roque and Tristan d'Acunha 

 (Plate 10, Table V.). — This section passes in a south-easterly 

 direction from Cape S. Roque to Tristan d'Acunha. The arrange- 

 ment of its isotherms marks the progress of the South Atlantic 

 Equatorial Current along the coast of South America, and 

 especially of that branch of it which, turning eastwards, occupies 

 the space between Station 339 and Tristan d'Acunha (Plate 9), 

 and between Stations 131 and 132 (Plate 10). As seen in both 

 sections, this branch attains its maximum depth between lat. 30° 

 and 35° S. A comparison of the isotherm of 2°. 5 C. in the two 

 sections shows that, while in the western half of the South 

 Atlantic it is found not lower than at 1000 fathoms from the 



