THE STORY OF NINETEENTH-CENTURY SCIENCE 



period ; but it was ably and vigorously combated by Dr. 

 James Croll, the Scottish geologist, in his Climate and 

 Time, and latterly the old theory that ocean currents 

 are due to the trade- winds has again come into favor. 

 Indeed, very recently a model has been constructed, with 

 the aid of which it is said to have been demonstrated 

 that prevailing winds in the direction of the actual trade- 

 winds would produce such a current as the Gulf Stream. 



Meantime, however, it is by no means sure that gravi- 

 tation does not enter into the case to the extent of pro- 

 ducing an insensible general oceanic circulation, inde- 

 pendent of the Gulf Stream and similar marked currents, 

 and similar in its larger outlines to the polar-equatorial 

 circulation of the air. The idea of such oceanic circula- 

 tion was first suggested in detail by Professor Lenz of 

 St. Petersburg, in 1845, but it was not generally recog- 

 nized until Dr. Carpenter independently hit upon the 

 idea more than twenty years later. The plausibility of 

 the conception is obvious ; yet the alleged fact of such 

 circulation has been hotly disputed, and the question is 

 still siib judice. 



But whether or not such general circulation of ocean 

 water takes place, it is beyond dispute that the recog- 

 nized currents carry an enormous quantity of heat from 

 the tropics towards the poles. Dr. Croll, who has per- 

 haps given more attention to the physics of the subject 

 than almost any other person, computes that the Gulf 

 Stream conveys to the North Atlantic one-fourth as 

 much heat as that body receives directly from the sun, 

 and he argues that 'were it not for the transportation of 

 heat by this and similar Pacific currents, only a narrow 

 tropical region of the globe would be warm enough for 

 habitation by the existing faunas. Dr. Croll argues that 



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