1872.] 



* Shearwater' Scientific Researches. 



indraughts maintained by a vis a f route rather than by a vis d, tergo. For 

 it is obvious that the Trade-winds cannot be continually driving westwards 

 the surface-water of the Intertropical region, unless that water is as con- 

 tinually re-supplied ; and it is very curious to observe that while the por- 

 tions of the North- and South-African currents which lie furthest from the 

 coast are drawn into the general westerly drift that prevails over the Inter- 

 tropical area, the part of each that lies nearest the coast follows its curva- 

 ture, so that the two meet in the Bight of Biafra, which maybe considered 

 the "head water" of the great Equatorial current. The eastward de- 

 flection of a large part of the North-iVfrican current, along the whole line 

 of the Guinea Coast, from Cape Verde and Sierra Leone to Fernando Po, 

 only a little to the north of the west-floxv'mg Equatorial current (the boun- 

 dary between the two being almost as sharply defined as the " cold-wall" 

 which separates the Gulf-stream from the Arctic current), is a fact of pe- 

 culiar significance. And this significance becomes still more marked, as we 

 find that this Guinea current is fed, not merely by the North-African cur- 

 rent, but by an Easterly flow that is traceable across the Atlantic, usually 

 between 6° and 8° N. Lat., on an average as far as 35° West, and some- 

 times almost to the coast of Guiana. This flow intervenes between the 

 well-defined Equatorial current which is driven on by the S.E. Trade, and 

 the more expanded Westerly Drift impelled by the N.E. Trade, of which 

 the southern border has nearly the same rate as the Equatorial Current ; 

 and the area over which it takes place appears to correspond pretty closely 

 with that of the "Equatorial Calms," — the Thermic Equator, as already 

 stated, lying a few degrees to the north of the Geographical Equator. 

 Within this area the winds are variable ; and neither here nor along the 

 Guinea Coast does it seem that the easterly flow can be sustained by any 

 prevalent movement of the Air in that direction. I would suggest, there- 

 fore, that the " Guinea Current " is mainly an indraught or supply-cur- 

 rent, contributing, with the South-African current, to keep up the level of 

 the head-water of the Equatorial current in the Bight of Biafra ; and that, 

 in like manner, the Guinea current itself draws upon (1) the North - 

 African current, and (2) the " still " region of the Atlantic. 



121. But besides the indraught propagated backwards (so to speak) 

 from the Guinea Current, the still water that lies between the two great 

 westerly Drift-currents will be likely to take on an Easterly movement of 

 the kind known as back-water*; for as the part of it in proximity to the 

 current on either side is carried along by the friction of that current, a void 

 will be created, which must be supplied by a flow of water in the opposite di- 

 rection. And that this is partly the explanation of the more or less constant 

 easterly movement of the band of Atlantic water which lies immediately to 

 the North of the Equatorial Current, from the Guiana coast to the Bight of 



* The phenomenon of " back-water" ha3 been well explained and illustrated by Mr. 

 Laughton in his 1 Physical Geography in its Relation to the prevailing Winds and 

 Currents.' 



