CHAP. VIII 



THE CAUSES OF GLACIAL EPOCHS 



143 



partially reversed. The south polar area, having its winter 

 in perihelion, would probably have had less ice, while the 

 north-temperate and Arctic regions would have been 

 largely ice-clad ; and the north-east trades would therefore 

 be stronger than they are now. The south-westerly anti- 

 trades would also be stronger in the same proportion, and 

 would bring with them a greatly increased quantity of 

 moisture, which is the prime necessity to produce a con- 

 dition of glaciation. 



But this is only one-half of the effect that would be 

 produced, for the increased force of the trades sets up 

 another action which still further helps on the accumula- 

 tion of snow and ice. It is now generally admitted that 

 we owe much of our mild climate and our comparative 

 freedom from snow to the influence of the Gulf Stream, 

 which also ameliorates the climate of Scandinavia and 

 Spitzbergen, as shown by the remarkable northward cur- 

 vature of the isothermal lines, so that Drontheim in N. 

 Lat. 62° has the same mean temperature as Halifax (Nova 

 Scotia) in N. Lat. 45°. The quantity of heat now brought 

 into the North Atlantic by the Gulf Stream depends mainly 

 on the superior strength of the south-east trades. When 

 the north-east trades were the more powerful, the Gulf 

 Stream would certainly be of much less magnitude and 

 velocity ; while it is possible, as Dr. Croll thinks, that a 

 large portion of it might be diverted southward owing to 

 the peculiar form of the east coast of South America, and 

 so go to swell the Brazilian current and ameliorate the 

 climate of the southern hemisphere. 



That effects of this nature would follow from any in- 

 crease of the Arctic, and decrease of the Antarctic ice, may 

 be considered certain ; and Dr. Croll has clearly shown that 

 in this case cause and effect act and react on each other in 

 a remarkable way. The increase of snow and ice in the 

 northern hemisphere is the cause of an increased supply of 

 moisture being brought by the more powerful anti-trades, 

 and this greater supply of moisture leads to an extension 

 of the ice, which reacts in still further increasing the 

 supply of moisture. The same increase of snow and ice, 

 by causing the north-east to be stronger than the south-east 



