186 Mr. J. Croll on the Physical Cause of Ocean- cur rents. 



currents of the former hemisphere would be stronger than 

 those of the latter. This would be more especially the case 

 with the trade-winds. The N.E. trades being stronger than 

 the S.E. trades would blow across the equator, and the medial 

 line between them would therefore be at some distance to the 

 south of the equator. Thus the equatorial waters would be 

 impelled more to the southern than to the northern hemi- 

 sphere ; and the warm water carried over in this manner to the 

 southern hemisphere would tend to increase the difference of 

 temperature between the two hemispheres. This change, again, 

 would in turn tend to strengthen the N.E. and to weaken the 

 S.E. trades, and would thus induce a still greater flow of equa- 

 torial waters into the southern hemisphere — a result which 

 would still more increase the difference of temperature between 

 the northern and southern hemisphere, and so on — the one cause 

 so reacting on the other as to increase its effects, as was shown at 

 length on a former occasion (Phil. Mag. March 1870). 



It was this mutual reaction of those physical agents which led, 

 as I have pointed out (Phil. Mag. March 1870), to that extra- 

 ordinary condition of climate which prevailed during the glacial 

 epoch. 



There is another circumstance to be considered which perhaps 

 more than any thing else would tend to lower the temperature 

 of the one hemisphere and to raise the temperature of the other; 

 and this is the displacement of the great equatorial current. 

 During a glacial period in the northern hemisphere the medial 

 line between the trades would be shifted very considerably south 

 of the equator ; and the same would necessarily be the case with 

 the great equatorial currents, the only difference being that the 

 equatorial currents, other things being equal, would be deflected 

 further south than the medial line. For the water impelled by 

 the strong N.E. trades would be moving with greater velocity 

 than the waters impelled by the weaker S.E. trades, and, of 

 course, would cross the medial line of the trades before its pro- 

 gress southwards could be arrested by the counteracting influ- 

 ence of the S.E. trades. Let us glance briefly at the results 

 which would follow from such a condition of things. In the 

 first place, as was shown on former occasions (Phil. Mag. for 

 August 1864, February 1867, March ] 870), were the equatorial 

 current of the Atlantic (the feeder of the Gulf- stream) shifted 

 considerably south of its present position, it would not bifurcate, 

 as it now does, off Cape St. Roque, owing to the fact that the 

 whole of the waters would strike obliquely against the Brazilian 

 coast and thus be deflected into the Southern Ocean. The effect 

 produced on the climate of the North Atlantic and North-western 

 Europe by the withdrawal of the water forming the Gulf-stream, 



