CHAP, viii.] THE CAUSES OF GLACIAL EPOCHS. 



139 



increasing the supply of moisture. The same increase of snow 

 and ice, by causing the north-east to be stronger than the south- 

 east trade-winds, diminishes the force of tlie Gulf Stream, and 

 this diminution lowers the temperature of the North Atlantic 

 both in summer and winter, and thus helps on still further the 

 formation und perpetuation of the icy mantle. It must also be 

 remembered that these agencies 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, 

 increasing 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. 



Nummary of principal Causes of Glaciation. — I have now suf- 

 ficiently answered the question, why the short hot summer would 

 not melt the snow which accumulated during the long cold 

 winter (produced by high excentricity and winter in aphelion), 

 although the annual amount of heat received from the sun was 

 exactly the same as it is now, and equal in the two hemispheres. 

 It may be well, before going further, briefly to summarise the 

 essential causes of this apparent paradox. These are — primarily, 

 the fact that solar heat cannot be stored up owing to its being 

 continually carried away by air and water, while cold can be so 

 stored up owing to the comparative immobility of snow and 

 ice ; and, in the second place, because the two great heat- 

 distributing agencies, the winds and the ocean currents, are so 

 affected by an increase of the snow and ice towards one pole 

 and its diminution towards the other, as to help on the process 

 when it has once begun, and by their action and reaction pro- 

 duce a maximum of effect which, without their aid, would be 

 altogether unattainable. 



But even this does not exhaust the causes at work, all tending 

 in one direction. Snow and ice reflect heat to a much greater 

 degree than do land or water. The heat, therefore, of the short 

 summer would have far less effect than is due to its calculated 

 amount in melting the snow, because so much of it would be 

 lost by reflection. A portion of the reflected heat would no 



