170 THE PHYSICAL GEOGRAPHY OF THE SEA. 



difference of specific gravity between sea water at one place and 

 sea water at another ; for wherever there is such a difference, 

 whether it be owing to difference of temperature or to difference 

 of saltness, etc., it is a difference that disturbs equilibrium, and 

 currents are the consequence. The heavier water goes toward 

 the lighter, and the lighter whence the heavier comes; for two 

 fluids differing in specific gravity (§ 36), and standing at the same 

 level, can no more balance each other than unequal weights in op- 

 posite scales. It is immaterial, as before stated, whether this dif- 

 ference of specific gravity be caused by temperature, by the matter 

 held in solution, or by any other thing ; the effect is the same, 

 namely, a current. 



468. That the sea, in all parts, holds in solution the same kind 

 of solid matter ; that its waters in this place, where it never rains, 

 are not salter than the strongest brine ; and that in another place, 

 w^here the rain is incessant, they are not entirely without salt, 

 may be taken as evidence in proof of a system of currents or of 

 circulation in the sea, by which its waters are shaken up and kept 

 mixed together as though they were in a phial. Moreover, we 

 may lay it down as a law in the system of oceanic circulation, 

 that every current in the sea has its counter current ; in other 

 words, that the currents of the sea are, like the nerves of the hu- 

 man system, arranged in pairs ; for wherever one current is found 

 carrying off w^ater from this or that part of the sea, to the same 

 part must some other current convey an equal volume of water, 

 or else the first would, in the course of time, cease for the want 

 of water to supply it. 



469. CuERENTS OF THE ATLANTIC. — The principal currents of 

 the Atlantic have been described in the chapter on the Gulf Stream. 

 Besides this, its eddies and its offsets, are the equatorial current 

 (Plate A^I.), and the St. Eoque or Brazil Current. Their fountain- 

 head is the same. It is in the warm waters about the equator, 

 between Africa and America. The former, receiving the Amazon 

 and the Oronoco as tributaries by the way, flows into the Carib- 

 bean Sea, and becomes, with the waters (§ 34) in which the vapors 

 of the trade-winds leave their salts, the feeder of the Gulf Stream. 

 The Brazil Current, coming from the same fountain, is supposed 



