Mr. J. Croll on the Physical Cause of Ocean-currents. 187 



may be conceived from what has already been stated concerning 

 the amount of heat conveyed by that stream. The heat thus 

 withdrawn from the North Atlantic would go to raise the tem- 

 perature of the Southern Ocean and Antarctic regions. A similar 

 result would take place in the Pacific Ocean. Were the equato- 

 rial current of that ocean removed greatly to the south of its 

 present position, it would not then impinge and be deflected 

 upon the Asiatic coast, but upon the continent of Australia ; and 

 the greater portion of its waters would then pass southward 

 into the Southern Ocean, while that portion passing round the 

 north of Australia (owing to the great strength of the N.E. 

 trades) would rather flow into the Indian Ocean than turn round, 

 as now, along the east coast of Asia by the Japan Islands. The 

 stoppage of the Japan current, combined with the displacement 

 of the equatorial current to the south of the equator, would 

 greatly lower the temperature of the whole of the North Pacific 

 and adjoining continents, and raise to a corresponding degree 

 the temperature of the South Pacific and Southern Ocean. 

 Again, the waters of the equatorial current of the Indian Ocean 

 (owing to the opposing N.E. trades), would not, as at pre- 

 sent, find their way round the Cape of Good Hope into the 

 North Atlantic, but would be deflected southwards into the 

 Antarctic sea. 



We have in the present state of things a striking example of 

 the extent to which the medial line between the two trades may 

 be shifted, and the position of the great equatorial currents of 

 the ocean may be affected by a slight difference in the relative 

 strength of the two aerial currents. The S.E. trades are at pre* 

 sent a little stronger than the N.E. ; and the consequence is that 

 they blow across the equator into the northern hemisphere to a 

 distance sometimes of 10 or 15 degrees, so that the mean posi- 

 tion of the medial line lies at least 6 or 7 degrees north of the 

 equator. 



And it is doubtless owing to the superior strength of the 

 S.E. trades that so much warm water crosses the equator from 

 the South to the North Atlantic, and that the main portion of 

 the equatorial current flows into the Caribbean Sea rather than 

 along the Brazilian coast. Were the two trades of equal strength, 

 the transference of heat into the North Atlantic from the south- 

 ern hemisphere by means of the Southern Atlantic and Equa- 

 torial currents would be much less than at present. The same 

 would also hold true in regard to the Pacific. 



Ocean-currents in relation to the Distribution of Plants and 

 Animals. — In the fifth and last editions of the ' Origin of Species/ 

 Mr. Darwin has done me the honour to express his belief that 

 the foregoing view regarding alternate cold and warm periods 



