THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



OCTOBER 1871. 



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

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

 of Scotland. 



[Continued from vol. xl. p. 259.] 



Dr. Carpenter's Theory of a General Oceanic Circulation. 



THE two great causes which have been assigned for ocean- 

 currents are the influence of the winds and the difference of 

 specific gravity between the ocean in equatorial and polar regions. 

 But even amongst those who adopt the former theory, it is ge- 

 nerally held that the winds are not the sole cause, but that, to a 

 certain extent at least, difference of specific gravity contributes 

 to produce motion of the waters. This is a very natural con- 

 clusion ; and in the present state of physical geography on this 

 subject one can hardly be expected to hold any other view. It 

 is only when we adopt the more rigid method of determining in 

 absolute measure the amount of the forces resulting from differ- 

 ence of specific gravity that we become aware that this is a cause 

 utterly insufficient to produce the motions attributed to it. In my 

 last paper* I examined at considerable length Lieut. Maury's 

 theory, and endeavoured to show that difference of specific gravity 

 between the sea in equatorial and polar regions could not, as he 

 supposed, be ..:e cause of the Gulf-stream and other currents. 

 Since the publication of that paper, an interesting and elaborate 

 memoir on ocean -currents has been read before the Royal Geogra- 

 phical Society by Dr. Carpenter. In this memoir he states that my 

 objections do not apply to the slow movement, imperceptible to 



* Phil. Mag. for October 1870. 

 Phil Mag. S. 4. Vol. 42. No. 280. Oct. 1871. R 



