144 ISLAND LIFE 



trade-winds, diminishes the force of the 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 and 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 hemi- 

 sphere, increasing yet further the supply of moisture by 

 the more powerful south-westerly winds, while still fur- 

 ther lowering the temperature by the southward diversion 

 of the Gulf Stream. 



Summary of Principal Causes of Glaciation. — I have now 

 sufficiently 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 

 produce 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 



