224 KANSAS ACADEMY OF SCIENCE. 



duction of summer temijerature in Kansas would reduce the summer temper- 

 ature from 78 degrees to 73 degrees, and the annual mean temperature from 

 55 degrees to 53 degrees. A reduction of 23 per cent, around the north pole 

 would move the isothermal line of 32 degrees annual mean temperature 10 

 degrees to the south, as its heat is about all received in the six months of sum- 

 mer; and, inasmuch as the reduction would be greater over the land than over 

 the ocean, we might safely look for the limit of permanent ice to be removed 

 to the south of James bay, and even to approach Lake Superior. However, 

 this cause would be insufficient to bring permanent ice into Minnesota. But 

 it would become an important factor in aid of other causes. 



Second. Variation of ellipticity of the earth's orbit. It has been calcu- 

 lated that the ellipticity of the earth 100,000 years ago was two and one-half 

 times what it is now. If that were true, the difference between summer and 

 winter would be 15 days instead of six days; and the amount of heat re- 

 ceived by the northern hemisphere each summer during the platonic winter 

 would be 25 per cent, less instead of 10 per cent, less, as at present. This at 

 the north pole would be 58 per cent, less than the amount of heat received 

 there during the summer season at the present time. This is a very important 

 amount, and might or might not be sufficient to push the ice from British 

 America down into Minnesota. 



These two variations, having different periodic times, would sometimes 

 neutralize each other to some extent, and at other times reinforce each other. 

 It may be possible to calculate the effects at each particular period; but I have 

 not the data necessary to attempt it. 



But there is a third astronomical cause more important than either of 

 the others, namely: Changes of obliquity of the earth's axis. It has been 

 calculated by Laplace that the variation in obliquity amounts to 1 degree 33 

 minutes 45 seconds. Drayson, of England, as clearly shown by Gen. J. C. 

 Cowell (see Science, December 22, 1893), has demonstrated that the change 

 of obliquity of the earth's axis is caused, not by a nutation of the earth's axis, 

 but by a revolution of the pole of the heavens around a point six degrees re- 

 moved from the pole of the ecliptic. This, if true, is a very important point. 

 It would cause the obliquity of the earth's axis (since that always points to the 

 pole of the ecliptic) to vary from 23 degrees 25 minutes 47 seconds to 35 de- 

 grees 25 minutes 47 seconds. The length of this period is said to be 31,682 

 years. The time of least obliquity is placed at 400 years hence, and the 

 period of greatest obliquity is placed 13,544 years ago. Thirty-five degrees 

 obliquity would carry the tropic of Cancer up into Oklahoma, Arkansas, and 

 Tennessee; and bring the arctic circle down into Saskatchewan and south of 

 Hudson bay. This cause would be amply sufficient to account for all the 

 glacial epochs that have ever existed on the earth; and they seem to have 

 existed in varying degrees for all past time, even down to the Huronian. 



INFLUENCE OF THE OCEANS. 

 Another thing to be considered in connection with this is the form of the 

 oceans. It is well known that great currents of warm water are flowing 

 northward through the Atlantic ocean near the Atlantic coast, and through 

 the Pacific near the Asiatic coast. After the lapse of a long period of 

 time, when the increasing cold of the platonic winter causes the ice of the 

 Arctic ocean to spread so as to cover a large part of the northen Atlantic 

 ocean and close Bering strait, the warmer currents are entirely shut off from 

 reaching the polar seas, and there is nothing to prevent an increased accu- 

 mulation of the ice around the pole. The ice naturally spreads farthest in 



