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 
CLACIAL EPOCH IN 
N. HEMISPHERE 
S .HEMISPHERE WINTER IN APHELION 
CLACIAL EPOCH IN 
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 
