CHAP. VIII.] THE CAUSES OF GLACIAL EPOCHS. 
1B9 
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 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 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. 
Summary 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 
