June, 1892. 
THE ICE AGES, PAST AND FUTURE. 
129 
summer period 166 days ; so that one half of the year’s total 
supply of heat will then in winter be spread over 199 days 
instead of 179 days as at the present time, resulting in a 
lower average temperature and a longer period during which 
this low temperature will continue, conditions causing the 
existence of an Ice Age. 
The conditions required for an Ice Age are a long and 
excessively cold winter, during the rigours of which snow and 
ice accumulate to such an extent that the succeeding brief 
though hot summer is not able to thaw as much water as has 
been frozen during the winter. Then the ice grows from year 
to year, and an ice sheet is formed which extends ultimately 
far beyond the limits within which the polar ice sheets are at 
the present time formed. In the southern hemisphere, how¬ 
ever, the conditions will then be just reversed, and instead of 
an Ice Age there will be a genial season, with a short mild 
winter, and a long moderately warm summer ; the winter 
will be only 166 days long, and the summer 199 days long, 
and the heat from the sun during that period will be 
moderated by the circumstance of the earth being during the 
time at greatest possible distance from the sun. 
There appears good reason to believe that the above 
named causes would be amply sufficient for the occurrence of 
an Ice Age; and the conclusion also arises that, as the causes 
are continuous in action, there will not only be a repetition 
of the Ice Age at some future time, but that in the past there 
has been more than one such occurrence ; and in reference 
to this last point there has been received some definite 
geological evidence in its confirmation by the discovery of 
traces of the occurrence of another Ice Age previous to the 
last one. 
In examination of this point Croll has carried back his 
calculations of the changes in the eccentricity of the earth’s 
orbit for a period of as much as three millions of years, and 
has found that there were two other periods in that time when 
an Ice Age must have occurred. The result of his cal¬ 
culations shows that besides the Ice Age of 200,000 years 
ago, a previous Ice Age nearly a million years ago, for which 
he has suggested the name of the Miocene Ice Age, and 
another before that at two and a half million years ago, which 
he calls the Eocene Ice Age. In both these the eccentricity of 
the earth’s orbit is calculated to have been still greater than 
in the most recent Ice Age, and to have amounted to as much 
as one-thirteenth part of the earth’s distance from the sun, 
and the average mid-winter temperature is calculated to have 
fallen as much as 44° below that of the present winters. 
The calculations were also carried forward by Croll for a 
