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XXIII. On Ocean-currents. — Part III. On the Physical Cause of 

 Ocean-currents. By James Croll, of the Geological Survey 

 of Scotland. 



[With a Plate.] 



[Concluded from p. 122.] 

 The Gibraltar Current. 



IN my last paper I proved that it was only the water lying 

 above the level of the submarine ridge which crosses the 

 Strait of Gibraltar that could exercise any influence in producing 

 circulation between the Atlantic and the Mediterranean*. The 

 water of the Atlantic below the level of this ridge might be as 

 light as air, and that of the Mediterranean as heavy as molten 

 lead ; but this could produce no disturbance of equilibrium. It 

 is only the waters lying above the level of this ridge in the two 

 seas that require to balance each other ; and if there is no dif- 

 ference of density between the Atlantic and the Mediterranean 

 waters from the surface down to the level of the top of the ridge, 

 then there is nothing that can produce the circulation which Dr. 

 Carpenter supposes. This submarine ridge comes up to within 

 very nearly 100 fathoms of the surface ; and according to Dr. 

 Carpenter's own admission, little or no difference of density ex- 

 ists down to that depthf; it follows therefore that there is nothing 

 to produce disturbance of equilibrium, or any such circulation as 

 that which he infers. It is true that in his last expedition he 

 found the bottom-water on the ridge somewhat denser than 

 Atlantic water at the same depth, the former being 1*0292 and 

 the latter 1*0265 ; but it also proved to be denser than Medi- 

 terranean water at the same depth. He found, for example, that 

 " the dense Mediterranean water lies about 100 fathoms nearer 

 the surface over a 300-fathoms bottom, than it does where the 

 bottom sinks to more than 500 fathoms" (§51). But any excess 

 of density which might exist at the ridge could have no tendency 

 whatever to make the Mediterranean column preponderate over 

 the Atlantic column, any more than could a weight placed over 

 the fulcrum of a balance have a tendency to make the one scale 

 weigh down the other. 



Although Dr. Carpenter has done me the honour to discuss 

 nearly all the objections which I have advanced against his 

 theory, he nevertheless makes no reference whatever to this ob- 

 jection ; and this is the more singular, seeing that the expedition, 

 of which his memoir is a report, was chiefly if not solely under- 

 taken for the purpose of establishing the correctness of his theory 

 of the Gibraltar current. If, therefore, there was any one objec- 

 tion advanced by me which he might have been expected to 



* Phil. Mag., October 1871, pp. 269-272. 

 t Proc. Roy. Geog. Soc. Jan. 9, 1871, § 13. 



