chap. viii. J THE GULF-STREAM. 367 



Baltic and the northern seas contain." l " Now, here 

 we have on one side the Caribhean Sea and Gulf of 

 Mexico with their waters of brine ; on the other, 

 the great Polar Basin, the Baltic, and the North Sea, 

 the two latter with waters that are but little more 

 than brackish. In one set of these sea-basins the 

 water is heavy, in the other it is light. Between 

 them the ocean intervenes ; but water is bound to 

 seek and to maintain its level; and here, therefore, 

 we unmask one of the agents concerned in causing 

 the Gulf-stream." 2 



As Mr. James Croll has very clearly pointed out, 

 Captain Maury's two causes tend to neutralize each 

 other. 



" Now it is perfectly obvious that if difference in 

 saltness is to co-operate Avith difference in tempera- 

 ture in the production of ocean currents, the saltest 

 waters, and consequently the densest, must be in 

 the polar regions; and the waters least salt, and 

 consequently lightest, must be in equatorial and in- 

 tertropical regions. Were the saltest water at the 

 equator and the freshest at the poles, it would tend 

 to neutralize the effect due to heat, and, instead of 

 producing a current, would simply tend to prevent 

 the existence of the currents which otherwise would 

 result from difference of temperature." "According 

 to both theories it is the differences of density be- 

 tween the equatorial and polar waters that gives rise 

 to currents ; but according to the one theory, the 

 equatorial waters are lighter than the polar, whilst 

 according to the other theory they are heavier than 

 the polar. Either the one theory or the other may 



Captain Maury, op. cit. 2 Ibid. 



