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



storm of wind, the wire screen giving way much in the same 

 way as sails during a hurricane at sea. 



Ocean-currents in relation to Change of Climate. — In my 

 attempts to prove that oceanic circulation is produced by the 

 winds and not by difference of specific gravity, and that ocean- 

 currents are the great distributors of heat over the globe, my 

 chief aim has been to show the bearing which these points have 

 on the grand question of secular changes of climate during geo- 

 logical epochs, more particularly in reference to that mystery 

 the cause of the glacial epoch. 



In concluding this series of papers, I may therefore be allowed 

 briefly to recapitulate those points connected with the subject of 

 ocean-currents which seem to shed most light on the question 

 of changes of climate, referring the reader for fuller details to 

 former papers on the question. 



The complete agreement between the systems of ocean-cur- 

 rents and winds not only shows that the winds are the impelling 

 cause of the currents, but it also indicates to what an extent the 

 directions of the currents are determined by the winds, or, more 

 properly, to what an extent their directions are determined by 

 the direction of the winds. 



We have seen in a former part of this paper (Phil. Mag. Feb. 

 1870) to what an enormous extent the climatic conditions of the 

 globe are dependent on the distribution of heat effected by means 

 of ocean-currents. It has been there pointed out that, if the 

 heat conveyed from intertropical to temperate and polar regions 

 by oceanic circulation were restored to the former, the equa- 

 torial regions would then have a temperature about 55° warmer, 

 and the high polar regions a climate 83° colder than at present. 

 It follows, therefore, that any cause which will greatly affect the 

 currents or greatly change their paths and mode of distribution, 

 will of necessity seriously affect the climatic condition of the 

 globe. But as the existence of these currents depends on the 

 winds, and their direction and form of distribution depend upon 

 the direction and form of distribution of the winds, any cause 

 which will greatly affect the winds will also greatly affect the 

 currents, and consequently will influence the climatic condition 

 of the globe. Again, as the existence of the winds depends 

 mainly on the difference of temperature between equatorial and 

 polar regions, any cause which will greatly affect this difference 

 of temperature will likewise greatly affect the winds ; and these 

 will just as surely react on the currents and climatic conditions 

 of the globe. A simple increase or decrease in the difference of 

 temperature between equatorial and polar regions, though it 

 would certainly produce an increase or a decrease, as the case 

 might be, in the strength of the winds, and consequently in the 



