130 
THE ICE AGES, PAST AND FUTURE. 
June, 1892. 
million years in advance, and he found that near that time 
there will occur in the southern hemisphere two other 
Ice Ages within 100,000 years of each other, intermediate in 
severity between the previous ones ; these Ice Ages will apply, 
however, to the southern hemisphere only, and will not 
affect the northern hemisphere. 
The general results of the investigation into the causes of 
the Ice Ages are :— 
First, that an Ice Age could occur every 21.000 years in 
this northern hemisphere, if the eccentricity of the earth’s 
orbit were also suitable at the time ; and also every 21.000 
years in the southern hemisphere, alternately with the Ice 
Age in the northern hemisphere. 
Second, that an Ice Age can only occur when the earth’s 
orbit is changed to near the limit of its extreme eccentricity, 
and the earth is consequently removed to a farther distance 
from the sun than usual during the winter season in the 
northern hemisphere, amounting to an increase of eccen¬ 
tricity to the extreme of l-18th part; and that the periods of 
these changes are enormously large, amounting to from half 
a million to one and a half million years. 
Third, that when a Ice Age occurs in one hemisphere 
there is during the whole of the time a genial climate in the 
other hemisphere, with a short mild winter and a long moderate 
summer. 
A question that naturally arises is as to the duration of 
the Ice Ages—the length of time during which each Ice Age 
may be considered to have lasted—and there is some definite 
information towards an answer to this question. The time is 
necessarily limited within one of the 10,000 years periods 
during which the earth’s axis is inclined on the same side, 
and after which it becomes inclined on the opposite side, and 
reverses the seasons from the one hemisphere to the other. 
The Ice Age must come on gradually after the beginning of 
the 10,000 years period, and gradually pass off towards the 
end of that period ; so that about half the time, say 5,000 
years, may probably be taken as the approximate time of the 
continuance of an Ice Age. This is a period of which we 
can form some definite idea, as it corresponds with about the 
length of time that our knowledge of the history of the 
inhabitants of the earth extends ; and we have consequently 
to think of this country as having been for a similar time 
without a history, and most of the time buried under a glacier, 
as Greenland is at the present time. 
The gradual coming on of the Ice Age and the advance of 
the glacial sheet would thus extend over some 2,000 years; that 
is a longer time than has now elapsed since the beginning of 
