Mr. J. Croll on the Physical Cause of Ocean-currents. 239 



A very considerable portion of Maury's book, however, is 

 devoted to proving that the waters of equatorial and intertropical 

 regions are salter and heavier than those of the polar regions ; 

 and yet, notwithstanding this, he endeavours to show that this 

 difference in respect to saltness between the waters of the equato- 

 rial and the polar regions is one of the chief causes, if not the chief 

 cause, of ocean-currents. In fact, it is for this special end that 

 so much labour is bestowed in proving that the saltest water is 

 in the equatorial and intertropical regions, and the freshest in 

 the polar. 



" In the present state of our knowledge," he says, " concern- 

 ing this wonderful phenomenon (for the Gulf- stream is one of the 

 most marvellous things in the ocean) we can do little more than 

 conjecture. But we have two causes in operation which we may 

 safely assume are among those concerned in producing the Gulf- 

 stream. One of these is the increased saltness of its water after 

 the trade- winds have been supplied with vapour from it, be it 

 much or little ; and the other is the diminished quantum of salt 

 which the Baltic and the Northern Seas contain "■ (§ 37) . " 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 con- 

 cerned in causing the Gulf-stream " (§ 38). To the same effect 

 see §§ 52, 522, 523, 524, 525, 526, 528, 530, 554, 556. 



Lieut. Maury's two causes neutralize each other. Here we 

 have two theories put forth regarding the cause of ocean-cur- 

 rents, the one in direct opposition to the other. According to 

 the one theory, ocean-currents exist because the waters of equa- 

 torial regions, in consequence of their higher temperature, are 

 less* dense than the waters of the polar regions ; but according 

 to the other theory, ocean-currents exist because the waters of 

 equatorial regions, in consequence of their greater saltness, are 

 more dense than the waters of the polar regions. If the one 

 cause be assigned as a reason why ocean-currents exist, then the 

 other can be equally assigned as a reason why they do not exist. 

 According to both theories it is the difference of density between 

 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 

 be true, or neither; but it is logically impossible that both of them 

 can, for the simple reason that the waters of the equator cannot 



