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round numbers, 21,000 years ; so that in 10,500 years (one half of that 

 time) the conditions will be reversed ; that is, we shall have a winter of 

 186 days, and a summer of 179 days. Then we shall have for winter the 

 37 per cent, of the annual heat spread over 186 days, whereas now it is 

 spread over only 179; and there must be, in consequence, a lowering of 

 temperature, which, when counted from absolute zero or the temperature 

 of intersellar space, must be quite an appreciable quantity, and 

 one not to be neglected. The maximum difference there can be 

 between winter and summer is about 25 days, and this occurs when the 

 orbit of the earth has its greatest eccentricity, and the line of the 

 equinoxes is perpendicular to the major axis of the earth's orbit 

 we have then the maximum astronomic cause for glaciation. 



Another important point which must be taken into consideration is 

 this, that the total amount of heat received upon a hemisphere at any 

 time, and in any geologic age, is practically constant, or nearly so. 



Now, as the total annual heat is constant, and the ratio of heat 

 received in summer and winter is also constant, but the lengths of 

 winter and summer vary, therefore the average amount of daily heat 

 received in winter and summer during different years varies. This 

 gives us a clue for one of the causes for an ice age. 



The condition favourable for glaciation is, naturally, one where the 

 winter is longer than the summer, and whenever that condition obtains 

 in one hemisphere of course the reverse obtains in the opposite one. 

 We can safely say, therefore, that 10,500 years ago the northern hemis- 

 phere, if not glaciated, was at all events, disposed towards glaciation, 

 and very likely, in part, was glaciated. From astronomic reasoning, 

 there would be a succession of periods tending towards glaciation, the 

 effect of which other causes may mitigate, or even obliterate, or on the 

 other hand, intensify. 



The distribution of land and water, atmospheric and oceanic 

 currents, of course, exert a powerful influence on the conditions brought 

 about through astronomic causes. The preponderance of land in the 

 northern hemisphere may possibly be due to the fact that during the 

 critical time of crust-forming that hemisphere had its winters for 

 thousands of years in aphelion. 



