Schaw. — On the Last Glacial Epoch. 521 



able part of North America was submerged ; and he conceives 

 that the greater extent of sea and the absence of the warming 

 influence of the Gulf Stream would account for the change of 

 cfimate. We have seen, however, that there is the strongest 

 ground for believing that the warm Gulf Stream exercised the 

 same influence on the north-western shores of Europe in the 

 ice age as it does now ; and the conditions of the Southern 

 Hemisphere are at the present time even more favourable for 

 the production of an ice age than those he conceives to have 

 existed in North America during the Glacial period in regard 

 to the distribution of land and water, yet we certainly have no 

 approach to an ice age in the Southern Hemisphere now. We 

 may therefore conclude, I think, that no redistribution of land 

 and water can have been the main factor in the phenomenon. 



The question, then, naturally arises, Can there have been 

 any astronomical causes which would suffice to explain the 

 occurrence of an ice age terminating from 7.000 to 10,000 

 years ago after having endured for from 12,000 to 25,000 

 years ? Dr. Croll has attempted to give an astronomical 

 reason, and Sir Eobert Ball has lately perfected his suggestion. 



I will not attempt to explain the astronomical theory as 

 put forward by Sir Eobert Ball in his most charming and 

 interesting little book, " The Cause of an Ice Age," but merely 

 give his conclusions, which are as follow : He shows, first, 

 that, with the present obliquity, or approximately the same, 

 " of the total amount of heat received from the sun on a 

 hemisphere of the earth in the course of a year, 63 per cent, 

 is received during the summer and 37 per cent, is received 

 during the winter." He then goes on to show that the ellip- 

 ticity of the earth's orbit round the sun varies within certain 

 limits by disturbances caused by the other planets, and that 

 at certain extremely long intervals this ellipticity attains its 

 maximum. This maximum endures for a very long period, 

 and then gradually diminishes to its minimum again. He 

 also shows that, owing to the precession of the equinoxes, 

 combined with a very slow motion of the major axis of the 

 earth's orbit in an opposite direction, the line of equinoxes 

 travels all round the ellipse in about 21,000 years. This 

 period appears to be incorrect, and should be about 32,000 

 years, but in any case it is a short period compared with 

 that occupied by the change in the ellipticity of the orbit, 

 so that the line of equinoxes may travel round several 

 times during a period of high eccentricity. He then explains 

 that when during this period of high eccentricity of the orbit 

 the line of equinoxes cuts the major axis at right angles, one 

 hemisphere will have a very long summer (199 days) and a 

 very short winter (166 days), while the other hemisphere will 

 have a very short summer (166 days) and a very long winter 



