Mr. J. Croll on the Physical Cause of Ocean-currents. 179 



Ocean-currents due to the System of Winds. — If we leave out 

 of account a few small inland sheets of water, the globe may be 

 said to have but one sea, just as it possesses only one atmo- 

 sphere. We have accustomed ourselves, however, to speak of 

 parts or geographical divisions of the one great ocean, such as 

 the Atlantic and the Pacific, as if they were so many separate 

 oceans. And we have likewise come to regard the currents of 

 the ocean as separate and independent of one another. This 

 notion has no doubt to a considerable extent militated against 

 the acceptance of the theory that the currents are caused by the 

 winds, and not by difference of specific gravity ; for it leads to the 

 conclusion that currents in a sea must now in the direction of 

 the prevailing winds blowing over that particular sea. The 

 proper view of the matter, as I hope to be able to show, is that 

 which regards the various currents merely as members of one 

 grand system of circulation produced, not by the trade-winds 

 alone, nor by the prevailing winds proper alone, but by the 

 combined action of all the prevailing winds of the globe, regarded 

 as one system of circulation. 



If the winds be the impelling cause of currents, the direction 

 of the currents will depend upon two circumstances, viz. : — (l)the 

 direction of the prevailing winds of the globe, including, of 

 course, under this term the prevailing winds proper and the trade- 

 winds ; and (2) the conformation of land and sea. It follows, 

 therefore, that as a current in any given sea is but a member of 

 a general system of circulation, its direction is determined, not 

 alone by the prevailing winds blowing over the sea in question, 

 but by the general system of prevailing winds. It may conse- 

 quently sometimes happen that the general system of winds 

 may produce a current directly opposite to the prevailing wind 

 blowing over the current. The accompanying Chart (Plate II.) 

 shows how exactly the system of ocean-currents agrees with 

 the system of the prevailing winds. The fine lines indicate the 

 paths of the prevailing winds, and the fine arrows the direction 

 in which the wind blows along those paths. The large arrows 

 show the direction of the principal ocean-currents. 



The directions and paths of the prevailing winds have been 

 taken from Messrs. Johnston's small physical Atlas, which, I 

 find, agrees exactly with the direction of the prevailing winds as 

 deduced from the four quarterly wind charts lately published 

 by the Hydrographic Department of the Admiralty. The direc- 

 tion of the ocean-currents has been taken from the Current- 

 chart published by the Admiralty. 



In every case, without exception, the direction of the main cur- 

 rents of the globe agrees exactly with the direction of the pre- 

 vailing winds. There could not possibly be a more convincing 



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