Mr. J. Croll on the Physical Cause of Ocean-currents. 95 



the fact that the question is one which properly belongs to the 

 domain of physics and mechanics, while as yet no physicist of 

 note (if we except Dr. Colding, of Copenhagen) has given, as far 

 as I know, any special attention to the subject. It is true that in 

 works of meteorology and physical geography reference is con- 

 tinually made to such eminent physicists as Herschel, Pouillet, 

 Buff, and others ; but when we turn to the writings of these 

 authors we find merely a few remarks expressive of their opinions 

 on the subject, and no special discussion or investigation of the 

 matter, nor any thing which could warrant us in concluding 

 that such investigations have ever been made. At present the 

 question cannot be decided by a reference to authorities. 



The various theories on the subject may be classed under two 

 divisions : the first of these attributes the motion of the water to 

 the impulse of the wind, and the second to the force of gravity 

 resulting from difference of density. The latter may be subdi- 

 vided into two classes. The first of these (of which Maury may 

 be regarded as the representative) attributes the Gulf-stream 

 and other sensible currents of the ocean to difference of specific 

 gravity. The other class (at present the more popular of the 

 two, and of which Dr. Carpenter may be considered the repre- 

 sentative) denies altogether that such currents can be produced 

 by difference of specific gravity*, and affirms that there is a 

 general movement of the upper portion of the ocean from the 

 equator to the poles, and a counter movement of the under por- 

 tion from the poles to the equator. This movement is attri- 

 buted to difference of specific gravity between equatorial and 

 polar water, resulting from difference of temperature. 



The former theory I examined at some length in a paper in 

 the Philosophical Magazine for October 1870, and the latter 

 theory in a paper in the same journal for October 1871. Since 

 then Dr. Carpenter has done me the honour, in a paper read 

 before the Royal Society f, to discuss at considerable length the 

 various objections advanced by me to his theory. He has also in 

 this memoir stated and explained his views on several points 

 more fully than on former occasions. He further restates at 

 some length the various facts for which his theory is designed to 

 account, facts which he considers I have never attempted to ex- 

 plain. This to a certain extent is true; for as yet I have not 

 reached that part of my paper " On Ocean-currents " in which 

 these points fall to be discussed. One of the objects of the pre- 

 sent paper is to endeavour to show that all the facts to which Dr. 

 Carpenter refers can be perfectly well explained without having 

 recourse to any such general movement of the ocean as he 



* Proceedings of the Royal Societv, No. 138, p. 596, foot-note. 

 t See Proc. Rov. Soc. No. 138. 



