264 PROCEEDINGS OF SECTION A, 



much as two months, in fact it never can amount, as I have 

 already stated, to more than 33 days. I trust, however, I have 

 made it plain that there must be a wide distinction between 

 the climates of the two hemispheres when the difference 

 between the lengths of the seasons becomes considerable. 

 In the more fortunate hemisphere the tenij^erature would be 

 much more uniform than we have it at ju'esent, in the less 

 fortunate hemisphere the seasons of summer and winter 

 would be intensely contrasted. In the latter case we find 

 the conditions requisite for a glacial period, while the other 

 hemisphere will be exceptionally genial. 



It is an essential feature of the astronomical theory of the 

 ice age that the glacial condition in one hemisphere must be 

 accompanied by a genial condition in the othe]'. We are 

 thus able to explain not only those indications of glaciation 

 which the rocks afford, but we can also account for the com- 

 paratively mild periods which seem to have occurred occasion- 

 ally in the course of geological time. We can even exhibit 

 in some degree the law by which glacial conditions and genial 

 conditions succeed each other. I desire it to be particularly 

 understood that 1 do not now attempt to assign any date to 

 the last glacial jieriod. There is, however, one chronological 

 element in the subject of which we can feel sufficiently con- 

 fident, and which I now proceed to explain. 



Let us suppose that the earth's orbit has reached a 

 condition of maximum eccentricity. As the rate at which 

 the eccentricity changes is very slow we shall find that the 

 earth's orbit will preserve for a protracted period sufficient 

 eccentricity to produce a considerable difference between the 

 lengths of the seasons when other circumstances conspire. 

 Let us farther sujipose that the northern hemisphere is the 

 glaciated one, and that the southern is in a genial condition. 

 Quite inde[)endently of any change in the eccentricity, there 

 is a well-known cause in operation to prevent the conditions 

 we have supposed from being permanent. Owing to the 

 phenomenon of " precession " the line of equinoxes rotates 

 in the ecliptic with a period of 21,000 years relatively to the 

 axis of the earth's orbit. It follows that in 10,500 years 

 after the glaciation of the northern hemisphere is at its 

 maximum the glaciation is transferred to the southern hemi- 

 sphere, and the inter-exchange is complete, for a genial 

 period arrives to cheer the desolated North. Nor will this 

 be permanent, for the eccentricity declines so slowly that it 

 will often be considerable even after the lapse of another 



