CHAP, VIII. ] THE GULF-STREAM. 367 
Baltic and the northern seas contain.’’! “* Now, here 
we have on one side the Caribbean 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.”’ ’ 
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 with 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. Hither the one theory or the other may 
" Captain Maury, op. cit. 2) Ibid: 
