CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 127 



and we were also three million miles further from the sun 

 at the former period, a very decided difference of climate 

 would result — our winter would be colder and longer, our 

 summer hotter and shorter. Now there is a combination 

 of astronomical revolutions (the precession of the equinoxes 

 and the motion of the aphelion) which actually brings this 

 change about every 10,500 years, so that after this interval 

 the condition of the two hemispheres is reversed as regards 

 nearness to the sun in summer, and comparative duration 

 of summer and winter ; and this change has been going 

 on throughout all geological periods. {See Diagram.) The 

 influence of the present phase of precession is perhaps 



N.HEMISPHERE WINTER IN APHELION S .HEMISPHERE WINTER IN APHELION 



GLACIAL EPOCH IN GLACIAL EPOCH IN 



N.HEMISPHERE S.HEMISPHERE 



DIAGRAM SHOWING THE ALTERED POSITION OF THE POLES AT INTERVALS OF 10,500 YEARS 

 PRODUCED BY THE PRECESSION OF THE EQUINOXES AND THE MOTION OF THE APHELION ; 

 AND ITS EFFECT ON CLIMATE DURING A PERIOD OF HIGH EXCENTRICITY. 



seen in the great extension of the antarctic ice-fields, and 

 the existence of glaciers at the sea-level in the southern 

 hemisphere, in latitudes corresponding to that of England ; 

 but it is not supposed that similar effects were produced 

 with us at the last cold period, 10,500 years ago, because 

 we are exceptionally favoured, by the Gulf-stream warming 

 the whole North Atlantic ocean and by the i3revalence of 

 westerly winds which convey that warmth to our shores ; 

 and also by the comparatively small quantity of high land 

 around the North Pole which does not encourage great 

 accumulations of ice. But besides this change in the re- 

 lation of our seasons to the earth's ajjJielion and 'perihelion 

 there is another and still more important astronomical 



