the 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



OCTOBER 1870. 



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. xxxix. p. 194.] 



THERE is no point connected with ocean-currents on which 

 more diversity of opinion has existed than in regard to 

 their origin. At present, however, there may be said to be only 

 two theories held on the subject, viz. that which attributes 

 the currents to the influence of the trade and other winds, and 

 that which attributes them to differences in specific gravity be- 

 tween the waters of intertropical and polar regions. The latter 

 theory appears at present to be the more prevalent of the two, 

 although, perhaps, not so among scientific men. It is difficult 

 to conceive how a theory so manifestly erroneous should have 

 gained such general acceptance. Its popularity is no doubt 

 chiefly owing to the very great prominence given to it by Lieut. 

 Maury in his interesting and popular work e The Physical Geo- 

 graphy of the Sea/ Another cause which must have favoured 

 the reception of this theory is the ease with which it is per- 

 ceived how, according to it, circulation of the waters of the 

 ocean is supposed to follow. One has no difficulty, for ex- 

 ample, in perceiving that if the intertropical waters of the ocean 

 are expanded by heat, and the waters around the poles contracted 

 by cold, the surface of the ocean will stand at a higher level at 

 the equator than at the poles. Equilibrium being thus disturbed, 

 the water at the equator will tend to flow towards the poles as a 

 surface-current, and the water at the poles towards the equator 

 as an undercurrent. This, at first sight, looks well, especially 

 to those who take but a superficial view of the matter. 

 Phil. Mag. S. 4. Vol. 40. No. 267. Oct. 1870. R 



