Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/367

 account for it, the author brought forward the following considerations : —

Of the three principal movements of the earth, namely its daily rotation on its axis, its revolution round the sun, and the slow conical movement of its axis of rotation round the poles of the ecliptic, the third is, in the author's opinion, the cause of glacial epochs. The angular distance of the pole of the ecliptic and the pole of the heavens is at all times the exact measure of the obliquity of the ecliptic, or the extent of the Arctic circle upon the earth. There is evidence that during the last 2000 years these two poles have gradually decreased their angular distance, so that, while the pole of the heavens moves round some curve at the rate of about 1° in seventy- two years, it approaches the pole of the ecliptic at a variable rate, at present about 46" in a century. The author had calculated from the recorded positions of the pole of the heavens during the last 2000 years, the curve traced by this pole with relation to the pole of the ecliptic. He found it to be a circle, the centre of which is 6° from the pole of the ecliptic, and 29° 25' 47" from that of the heavens.

Taking this curve as a guide, the author finds that 2000 years ago the angular distance of the two poles was about 24°, giving a climate scarcely different from that now prevailing. 4000 years ago the angular distance would be about 29°, when the climates of high northern and southern latitudes would present much greater seasonal changes than at present ; and these changes would go on increasing back to the year 13,000 B.C., at which period the distance between the two poles would be greatest, namely 35° 25' 47". This would bring the arctic circle down to latitude 54° 35' N., that is to a line passing south of Moscow, just north of Berlin, north of Amsterdam, nearly through the middle of Great Britain, and then through Labrador and British Columbia. All the countries north of these places would be subjected to arctic conditions ; and these would prevail with even more severity than at present in the winter, whilst from the greater elevation attained by the sun in summer that season would be much hotter than at present, the summer of the lower arctic latitudes being even tropical in its intensity. These extreme changes of climate would prevail, according to the author, during a period of about 15,000 years ; that is to say, commencing about 21,000 years ago, the climate would become more and more extreme up to about 15,000 years ago, and then gradually more and more equable to about 6000 years ago.

Discussion.

Prof. Ramsay inquired whether the author's theory involved the recurrence of glacial epochs, and whether he considered the course of the described phenomena to be constant in early astronomical epochs.

Rev. Osmond Fisher inquired whether the curve was founded on observed facts, or whether it was obtained from physical considera-