From Madeira to Tristan d'Acunha. 75 



water within a few hundred fathoms from the surface. It 

 appears from the diagram of the section between Cape Palmas 

 and Cape S. Roque (Plate 10) that this stratum of cold water 

 stretches right across the Atlantic, occupying the whole length 

 of the equatorial belt, and that it is colder in the western half 

 than in the eastern. The isotherm of 5° C. rises between 

 Station 104 and Station 106 from 460 fathoms to 260 fathoms, 

 its average level in the western portion towards Cape S. Roque 

 being at a depth of 300 fathoms, and in the eastern portion 

 towards Cape Palmas at 400 fathoms. On the other hand, the 

 isotherms in the northern belt, from Station 95 to Station 84, 

 show that the stratum between 100 and 500 fathoms is warmer 

 than in the equatorial belt ; while in the southern belt, between 

 Station 339 and Station 342, the water between 250 and 500 

 fathoms is colder than in the equatorial belt. Between 500 

 fathoms and 1000 fathoms the same difference maybe observed, 

 the northern belt being warmer, the- southern colder, than the 

 central belt. The conclusion we arrive at seems obvious. As 

 the cold stratum between 100 and 500 fathoms in the equatorial 

 belt cannot come from the northern belt, which is warmer, it 

 must come from the southern belt, which is colder at the same 

 depths, and especially it must come from that portion of it which is 

 situated between Station 339 and Station 342, or, more correctly 

 speaking, from the belt which lies immediately to the southward 

 of the equatorial belt, between lat. 10° S. and lat. 17° or 18° S. 

 There must, therefore, be a current of cold water flowing from 

 the southern belt into the equatorial belt above the level of 

 500 fathoms, and it is this current which forms the stratum of 

 cold water found between 100 and 500 fathoms below the sur- 

 face of the equatorial belt. According to the theory of thermal 

 circulation, the superheated water of the equatorial region is con- 

 stantly being transferred to higher latitudes, and is as constantly 

 replaced by cold water pouring in from the polar regions. To 



