Mr. J. Croll on the Physical Cause of Ocean-currents. 103 



reality he could hardly have selected a case more hostile to that 

 theory. In short it is evident that, if a polar current impelled 

 by a force other than that of gravity can pass from the banks of 

 Newfoundland to the Gulf of Mexico (a distance of some thou- 

 sands of miles) under a current flowing in the opposite direction 

 and, at the same time, so powerful as the Gulf-stream, it could 

 pass much more easily under comparatively still water, or water 

 flowing in the same direction as itself. And if this be so, then 

 all our difficulties disappear, and we satisfactorily explain the 

 presence of cold polar water at the bottom of intertropical seas 

 without having recourse to t the hypothesis advanced by Dr. 

 Carpenter. 



But we have an example of an under current more inexplicable 

 on the gravitation hypothesis than even that of the polar cur- 

 rent, viz. the warm under current of Davis Strait. 



There is a strong current flowing north from the Atlantic 

 through Davis Strait into the Arctic Ocean underneath a sur- 

 face-current passing southwards in an opposite direction. Large 

 icebergs have been seen to be carried northwards by this under 

 current at the rate of four knots an hour against both the wind 

 and the surface- current, ripping and tearing their way with ter- 

 rific force through surface-ice of great thickness. (See Physical 

 Geography of the Sea, chap. ix. new edition, and Dr. A. Miihry 

 { On Ocean-currents in the Circumpolar Basin of the N. Hemi- 

 sphere/) A current so powerful and rapid as this cannot, as Dr. 

 Carpenter admits, be referred to difference of specific gravity. But 

 even supposing that it could, still difference of temperature be- 

 tween the equatorial and polar sea swould not account for it ; for 

 the current in question flows in the wrong direction. Nor will it 

 help the matter the least to adopt Maury's explanation, viz. that the 

 warm under current from the south, in consequence of its greater 

 saltness, is denser than the cold one from the polar regions. 

 For if the water of the Atlantic, notwithstanding its higher tem- 

 perature, is in consequence of its greater saltness so much denser 

 than the polar water on the west of Greenland as to produce an 

 under current of four knots an hour in the direction of the pole, 

 then surely the same thing to a certain extent will hold true in 

 reference to the ocean on the east side of Greenland. Thus in- 

 stead of there being, as Dr. Carpenter supposes, an underflow 

 of polar water south into the Atlantic in virtue of its greater den- 

 sity, there ought, on the contrary, to be a surface-flow in conse- 

 quence of its lesser density. 



The true explanation no .doubt is, that the warm under current 

 from the south and the cold upper current from the north are 

 both parts of one grand system of circulation produced by the 

 winds, difference of specific gravity having no share whatever 



