CHAP. VIII.] THE CAUSES OF GLACIAL EPOCHS. 



123 



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 com- 

 parative 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 

 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 would be produced with us at the 



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, 



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 prevalence 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 the amount of 

 excentricity itself varies very largely, though very slowly, 

 and it is now nearly at a minimum. It also varies very 

 irregularly ; but its amount has been calculated for several 

 million years back. Fifty thousand years ago it was rather 

 less than it is now, but it then increased, and when we come 

 to a hundred thousand years ago there is a difference of 



