REPLY TO CRITICS. 57 



as the accumulation of snow and ice is concerned, 

 mutually strengthen each other. 



The same process of mutual action and reaction 

 takes place among the agencies in operation on the 

 warm hemisphere; only the result produced is dia- 

 metrically opposite to that produced in the cold 

 hemisphere. On this warm hemisphere action and 

 reaction tend to raise the mean temperature and 

 diminish the quantity of snow and ice existing in 

 temperate and polar regions. 



The primary cause of all these physical agencies 

 being set in operation is a high state of eccentricity 

 of the earth's orbit ; and with a continuance of that 

 state a glacial epoch becomes inevitable. 



TJie Explanation begins with Winter. — Mr. Hill 

 asks why I always begin in my explanation with the 

 aphelion winter rather than with the perihelion sum- 

 mer. The reason is that the character of the summer 

 is determined by that of the winter, and not the winter 

 by that of the summer. It is true that, to a certain 

 extent, the influence is mutual ; but the effect of the 

 summer on the winter is trilling in comparison with 

 that of the winter on the summer. To begin our 

 explanation with the summer would be like beginning 

 at the end of a story and telling it backward. 



31. Woeikof on the Cause of Glaciation. — In an 

 article by A. AVoeikof on " Glaciers and Glacial Periods 

 in their Relations to Climate " (" Xature," March 2nd, 

 1882), it is maintained that the chief cause which leads 

 to the formation of snow, and consequently to a glacial 

 condition, is a low surface-temperature of the sea sur- 

 rounding or adjoining the land. When the surface- 

 temperature of the water much exceeds the freezing- 

 point, the vapour, he says, evaporated from the sea 

 and condensed on the land will be rain and not snow : 



