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



Ocean being, as Professor Wyville Thomson remarks, a well- 

 nigh closed basin, not permitting of a free outflow into the 

 Pacific Ocean of the water impelled into it, and the general ten- 

 dency of the winds being to prevent or retard the return of the 

 water into the Atlantic, the path of least resistance for the 

 return currents must lie at a considerable distance below the 

 surface. A large portion of the water from the polar regions 

 no doubt leaves those regions as surface-currents ; but a surface- 

 current of this kind, on meeting with some resistance to its on- 

 ward progress along the surface, will dip down and continue its 

 course as an under current. We have an example of this in the 

 case of the polar current, which upon meeting the Gulf- stream 

 on the banks of Newfoundland divides — a portion of it dipping 

 down and pursuing its course underneath that stream into the 

 Gulf of Mexico and the Caribbean Sea. And that this under cur- 

 rent is a real and tangible current, in the proper sense of the term, 

 and not an imperceptible movement of the water, is proved by 

 the fact that large icebergs deeply immersed in it are often car- 

 ried southward with considerable velocity against both the force 

 of the wind and the Gulf- stream. 



Dr. Carpenter refers at considerable length (§ 134) to Mr. 

 Mitchell's opinion as to the origin of the polar current, which is 

 the same as that advanced by Maury, viz. that the impelling 

 cause is difference of specific gravity. But although Dr. Car- 

 penter quotes Mr. Mitchell's opinion, he nevertheless does 

 not appear to adopt it; for in §§ 90-93 and various other 

 places he distinctly states that he does not agree with Captain 

 Maury's view that the Gulf-stream and polar current are caused 

 by difference of density. In fact Dr. Carpenter seems particu- 

 larly anxious that it should be clearly understood that he dis- 

 sents from the theory maintained by Maury. But he does not 

 merely deny that the Gulf-stream and polar current can be 

 caused by difference of density ; he even goes so far as to affirm 

 that no sensible current whatever can be due to that cause, and 

 adduces the authority of Sir John Herschel in support of that 

 opinion: — "The doctrine of Captain Maury," he says, "was 

 powerfully and convincingly opposed by Sir John Herschel ; 

 who showed, beyond all reasonable doubt, first, that the Gulf- 

 stream really has its origin in the propulsive force of the trade- 

 winds, and, secondly, that the greatest disturbance of equilibrium 

 which can be supposed to result from the agencies invoked by 

 Captain Maury would be utterly inadequate to generate and 

 maintain either the Gulf-stream or any other sensible current" 

 (§ 92). This being Dr. Carpenter's belief, it is somewhat singular 

 that he should advance the case of the polar current passing 

 under the Gulf-stream as evidence in favour of his theory ; for in 



