Page 240 - Lokmanya Tilak Samagra (khand 2)

THE GLACIAL PERIOD 27

colder winters, and ice and snow accumulated in the long winter
will not be melted or removed by the heat of the sun in the short
summer, giving rise, thereby, to what is known as the Glacial period
in the northern hemisphere. From what has been stated above,
it may be seen that the southern hemisphere during this period will
have long and cool summers and short and warm winters, a condi-
tion precisely reverse to that in the northern hemisphere. In short
the Glacial and Inter-Glacial periods in the two hemispheres will
alternate with each other very 10,500 years, if the eccentricity of
the earth be sufficiently great to make a perceptibly large difference
between the winters and the summers in each hemisphere.
If Dr. Croll had gone only so far, his position would have
been unassailable, for the cause, enumerated above, is sufficiently
potent to produce the climatic changes attributed to it. At any rate,
if this was not the sole cause of a succession of Glacial and Inter-
Glacial periods, there could be no doubt that it must have been an
important contributory cause in bringing about these changes. But
taking the value of the eccentricity of the earth's orbit from the
tables of Leverrier, Dr. Croll calculated that during the last three
million years there were three periods of maximum eccentricity,
the first of 170,000, the second of 260,000, and the third of 160,000
years; and that 80,000 years have elapsed since the close of the
third or the last period. According to Dr. Croll the Glacial epochs
in the Pleistocene period must, therefore, have begun 240,000 years
ago, and ended, followed by the post-Glacial period, about 80,000
years ago. During this long period of 160,000 years, there must
have been several alternations of mild and severe climates, accord-
ing as the winter in a hemisphere occurred when the earth was at
perihelion or aphelion in its orbit, which happened every 10,500
years during the period. But as the cold epoch can be at its
maximum only during the early part of each period, according to
Dr. Croll's theory, the last epoch of maximum glaciation must be
placed 200,000 years ago, or a about 40,000 years, after the
commencement of the last period of maximum eccentricity.
The reliability of these elaborate calculations has, however,
been questioned by astronomers and geologists alike. Sir Robert
Ball, who supports Croll in every other repect, has himself re-
frained from making any astronomical calculations regarding the
maximum value of the eccentricity of the earth's orbit or the time
when the last Glacial epoch should have occurred, or when the next