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



possible, how then can an ocean-current be the result of both 

 causes ? 



The only explanation of the matter appears to be that Maury 

 has failed to perceive the contradictory nature of his two theories. 

 This fact is particularly seen when he comes to apply his two 

 theories to the case of the Gulf-stream. He maintains, as has 

 already been stated, that the waters of the Gulf-stream are 

 Salter than the waters of the sea through which they flow (see 

 §§3, 28, 29, 30, 34, and several other places). And he states 

 that one of the chief causes of the Gulf-stream is this, that " 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" (§ 38). There can be no 

 doubt whatever that it is the density of the waters of the Gulf- 

 stream at its fountain-head, the Gulf of Mexico, resulting from 

 its superior saltness, and the deficiency of density of the waters 

 in polar regions and the North Sea &c., that is here considered 

 to be unmasked as one of the agents. If this be a cause 

 of the motion of the Gulf-stream, how then can the difference 

 of temperature between the waters of intertropical and polar 

 regions assist as a cause? This difference of temperature will 

 simply tend to undo all that has been done by difference of 

 saltness; for it will tend to make the waters of the Gulf of 

 Mexico lighter, and the waters of the polar regions heavier. 

 But Maury maintains, as we have seen, that this difference of 

 temperature is also a cause, which shows that he does not per- 

 ceive the contradiction. 



This is still further apparent. Maury maintains, as stated, 

 that " the waters of the Gulf-stream are salter than the waters 

 of the sea through which they flow/' and that this excess in 

 saltness, by making the water heavier, is a cause of the motion 

 of the stream. But he maintains that, notwithstanding the 

 effect which greater saltness has in increasing the density of the 

 waters of the Gulf-stream, yet, owing to their higher tempera- 

 ture, they are actually lighter than the water through which 

 they flow ; and as a proof that this is the case, he adduces the fact 

 that the surface of the Gulf-stream is roof-shaped (§§ 39-41), 

 which it could not be were its waters not actually lighter than 

 the waters through which the streams flow. So it turns out, in 

 contradiction to what he had already stated, that it is the lesser 

 density of the waters of the Gulf-stream that is the real cause of 



