122 DISCUSSIONS IN CLIMATOLOGY. 



out their aid, would be altogether unattainable." Mr. 

 Wallace thus does full justice to these mutual reactions 

 in so far as the production of glaciation is concerned ; 

 but I am convinced that he must have under-estimated 

 their importance as regards the removal of the glacia- 

 tion. He, however, recognises the fact that these 

 mutual reactions produce an opposite effect on the 

 warm hemisphere whose winters are in perihelion. 

 " These agencies," he says, " are at the same time 

 acting in a reverse way in the southern hemisphere, 

 diminishing the supply of the moisture carried by the 

 anti-trades, and increasing the temperature by means 

 of more powerful southward ocean-currents ; and all 

 this again reacts on the northern hemisphere, increas- 

 ing yet further the supply of moisture by the more 

 powerful south-westerly winds, while still further 

 lowering the temperature by the southward diversion 

 of the Gulf Stream." 



Now, if, during the glaciation of the northern hemi- 

 sphere, these mutual relations produce the opposite 

 effect on the southern hemisphere, it is evident that 

 they must produce this same opposite effect on the 

 northern hemisphere when its winter solstice is in 

 perihelion. Their effect then would be to increase the 

 temperature and melt the ice. When the winter sol- 

 stice is moving towards the aphelion, the physical 

 agents begin to act and react on one another, and 

 this action and reaction go on increasing in inten- 

 sity till the solstice-point reaches the aphelion ; but 

 an exactly similar thing is going on in the other 

 hemisphere, only the effects are the reverse. While 

 the actions and reactions leading to an accumulation 

 of ice are increasing in intensity, we shall suppose, on 

 the northern hemisphere, the same increase is taking- 

 place on the southern hemisphere ; but the result is a 



