chap, viii.] THE CAUSES OF GLACIAL EPOCHS. 151 
If this argument is valid, then it would follow that, so long as 
excentricity was high, whatever condition of climate was brought 
about by it in combination with geographical causes, would 
persist through several phases of precession ; but this would not 
necessarily be the case when the excentricity itself changed, and 
became more moderate. It would then depend upon the pro- 
portionate effect of climatal and geographical causes in produc- 
ing glaciation as to what change would be produced by the 
changing phases of precession ; and we can best examine this 
question by considering the probable effect of the change in 
precession during the next period of 10,500 years, with the 
present moderate degree of excentricity. 
Probable effect of Winter in aphelion on the Climate of Britain. 
— Let us then suppose the winters of the northern hemisphere 
to become longer and much colder, the summers being propor- 
tionately shorter and hotter, without any other change whatever. 
The long cold winter would certainly bring down the snow-line 
considerably, covering large areas of high land with snow during 
the winter months, and extending all glaciers and ice-fields. 
This would chill the superincumbent atmosphere to such an 
extent that the warm sun and winds of spring and early summer 
would bring clouds and fog, so that the sun-heat would be cut 
off and much vapour be condensed as snow. The greater sun- 
heat of summer would no doubt considerably reduce the snow 
and ice; but it is, I think, quite certain that the extra accumula- 
tion would not be all melted, and that therefore the snow-line 
would be permanently lowered. This would be a necessary result, 
because the greater part of the increased cold of winter would be 
stored up in snow and ice, while the increased heat of summer 
could not be in any way stored up, but would be largely prevented 
can be increased. The only factor available is the Antarctic ice, and if 
this were largely increased, the northward-flowing currents might be so 
increased as to melt some of the Arctic ice. But the very same argument 
applies to both poles. Without some geographical change the Antarctic 
ice could not materially diminish during its winter in perihelion , nor in- 
crease to any important extent during the opposite phase. We there- 
fore seem to have no available agency by which to get rid of the ice 
over a glaciated country, so long as the geographical conditions remained 
vnchanged and the excentricity continued high. 
