100 Mr. J. Croll on the Physical Cause of Ocean-cur rents. 



Although the under currents are parts of one general system of 

 oceanic circulation produced by the impulse of the system of pre- 

 vailing winds, yet their direction and position are nevertheless 

 to a large extent determined by different laws. The water at the 

 surface, being moved by the force of the wind, will follow the 

 path of greatest pressure and traction, — the effects resulting from 

 the general contour of the land, which to a great extent are com- 

 mon to both sets of currents, not being taken into account ; 

 while, on the other hand, the under currents from polar regions 

 (which to a great extent are simply " indraughts " compensating 

 for the water drained from equatorial regions by the Gulf-stream 

 and other surface- currents) will follow, as a general rule, the 

 path of least resistance. 



The Cause assigned for the hypothetical mode of circulation. — 

 Dr. Carpenter assigns a cause for his mode of circulation ; and 

 that cause he finds in the difference of specific gravity between 

 equatorial and polar waters, resulting from the difference of 

 temperature between these two regions. "Two separate ques- 

 tions," he says, " have to be considered, which have not, perhaps, 

 been kept sufficiently distinct either by Mr. Croll or by myself: — 

 first, whether there is adequate evidence of the existence of a 

 general vertical oceanic circulation ; and second, whether, suppo- 

 sing its existence to be provisionally admitted, a vera causa can be 

 found for it in the difference of temperature between the oceanic 

 waters of the polar and equatorial areas " (§ 17). It seems to 

 me that the facts adduced by Dr. Carpenter do not necessarily 

 require the assumption of any such mode of circulation as that 

 advanced by him. The phenomena can be satisfactorily ac- 

 counted for otherwise ; and therefore there does not appear to 

 be any necessity for considering whether his hypothesis be suf- 

 ficient to produce the required effect or not. 



An important consideration overlooked. — But there is one im- 

 portant consideration which Dr. Carpenter seems to have over- 

 looked — namely, the fact that the sea is Salter in intertropical 

 than in polar regions, and that this circumstance, so far as it 

 goes, must tend to neutralize the effect of difference of tempera- 

 ture. It is probable indeed that the effect produced by differ- 

 ence of temperature is thus entirely neutralized, and that no 

 difference of density whatever exists between the sea in intertro- 

 pical and polar regions, and consequently that there is no differ- 

 ence of level nor any thing to produce such a general motion as 

 he supposes. This I am glad to find is the opinion of Professor 

 Wyville Thomson. 



" I am greatly mistaken," says that author, " if the low spe- 

 cific gravity of the polar sea, the result of the condensation and 

 precipitation of vapour evaporated from the intertropical area, 



