160 A TEXTBOOK OF OCEANOGRAPHY 



70 W. Long. Between these two banks are a chain of con- 

 necting- banks. Humboldt thought they grew on the bottom 

 in situ, but there can be no doubt they are all derived from 

 shallow water of the American coasts. Humboldt's banks are 

 really the trade routes for sailing vessels, and since his infor- 

 mation was mainly, if not solely, derived from shipmasters, 

 his notifications of the presence of the Gulf-weed are naturally 

 most abundant from the routes frequented by sailing ships 

 making their passage across the Atlantic. The prospect of a 

 ship reporting the Gulf-weed in an area of 11,000 square kilo- 

 metres is only i in 8, and for the greater part of the surface 

 i in 12 or i in 20 only. The maximum prevalence of the 

 Sargassum is in the oval area between 21 and 35 N. Lat. 

 and 40 and 73 W. Long. that is, in the " Sargasso Sea." 



Rarely the Sargasso "weed " is found in Irish, English, 

 and French coastal waters, and even occasionally in the North 

 Sea and Western Mediterranean. 



The Canaries Current. The Canaries or North African 

 Current runs south along the West Coast of Africa at the 

 eastern boundary of the north-east trades, between Madeira 

 and Cape Verde Islands. Its velocity is moderate, from 8 to 

 30 miles per day, but records above 15 miles are exceptional. 

 The current carries water from high latitudes into the equa- 

 torial regions, and consequently is a cold current. The main 

 body of the Canaries Current runs into the North Equatorial 

 Current, and only a small part is continued along the African 

 coast in a south-easterly and easterly direction as the Guinea 

 Current. There are considerable differences in temperature 

 between the Canaries and Guinea Streams. The southern 

 boundary of the Canaries Current is much farther south in 

 March than in September. The origin of this current is to be 

 sought in the prevailing winds, together with the configuration 

 of the West Coast of Africa. The prevailing winds are north- 

 west and north. 



The Canaries Current is not a pure drift current, but is 

 caused by the diversion of the North Equatorial Current from 



