THE CAUSE OF THE GLACIAL PERIOD. 315 



South Atlantic to fill the vacuum caused by the heat of 

 the sun north of the equator. 



But, again, why is this ? Because, says Mr. Croll, we 

 are now in that stage of astronomical development favour- 

 able to the increased warmth of the northern hemisphere. 

 In the northern hemisphere the summers are longer than 

 the winters. Perihelion occurs in winter and aphelion in 

 summer. This is the reason why the North Atlantic is 

 warmer than the South Atlantic, and why the trade-winds 

 of the south are drawn to the north of the equator. Ten 

 thousand five hundred years ago, however, the conditions 

 were reversed, and the greater rarefaction of the atmos- 

 phere would have taken place south of the equator, thus 

 drawing the trade-winds in that direction. 



By again inspecting the map, one will see how far- 

 reaching the effect on the climate of northern countries 

 this change in the prevalences of the trades would have 

 been. Then, instead of having the northwest current 

 leading along the northeast coast of South America into 

 the Gulf of Mexico augmented by the warm currents cir- 

 culating south of the equator, the warm currents of the 

 north would have been pushed down so far that they 

 would augment the current running to the southwest be- 

 yond Cape St. Roque, along the southeast shore of South 

 America ; thus the northern portion of the Atlantic, 

 instead of robbing the southern portion of heat, would 

 itself be robbed of its warm currents to contribute to the 

 superfluous heat of the South Atlantic. 



This theory is certainly very ingenious. There is a 

 weak point in it, however. Mr. Croll assumes that when 

 the winters of the northern hemisphere occur in aphelion, 

 they must necessarily be colder than now. But, evidently, 

 this assertion implies a fuller knowledge than we possess 

 of the laws by which the heat received from the sun is 

 distributed over the earth. 



For it appears from observation that the equator is by 



