वेदों की प्राचीनता एवं काल-निर्णय / ओरायन (लोकमान्य बाल गंगाधर तिलक - खगोलीय वैदिक शोध)

26 THE ARCTIC HOME IN THE VEDAS. axis means a change in the equator, and as the earth owing to its diurnal rotation causes the equatorial regions to bulge out, a change in the axis would give rise to a second equatorial protu- berance, which, however, is not observable, and the theory cannot, therefore, be accepted. A gradual cooling of the earth would make the Polar regions habitable before the other parts of the globe; but a succession of Glacial epochs cannot be accounted for on this theory. Thus out of the various theories advanced to account for the vicissitudes of climate in the Pleistocene period only two have now remained in the field, the first that of Lyell which explains the changes by assuming different distribution of land and water combined with sudden elevation and submergence of large landed areas, and the second that of Croll which traces the glaciation to the precession of the equinoxes combined with the high value of the eccentricity of the earth's orbit. Lyell's theory has been worked out by Wallace who shows that such geographical changes are by themselves sufficient to produce heat and cold required to bring on the Glacial and Inter-Glacial periods. We have seen that in earlier geological ages a pleasant and equable climate prevailed over the whole surface of the globe owing mainly to different distribution of land and water, and the theory advanced by Lyell to account for the Glacial epoch is practically the same. Great elevation and depression of extensive areas can be effected only in thousands of years, and those who support Lyell's theory are of opinion that the duration of the Glacial epoch must be taken to be about 200,000 years in order to account for all the geo- graphical and geological changes, which, according to them, were the principal causes of the Glacial period. But there are other geologists, of the same school, who hold that the Glacial period may not have lasted longer than about 20 to 25 thousand years. The difference between the two estimates