410 PROBABLE EFFECT OF PRECESSION. 



Mr. Croll* does not indeed consider that an increase in the 

 excentricity would directly alter the relative temperature of 

 the two hemispheres, though it would bring about a condition 

 of things that would have this effect. The mid-winter tem- 

 perature of one hemisphere would be greatly lowered, the 

 consequence of which would be that all the moisture would 

 take the form of snow instead of rain, which would be the 

 more important because the winter would be longer. The 

 heat of the summer would be insufficient to melt the snow, 

 which consequently would accumulate year by year. On the 

 opposite hemisphere the reverse would be the case, and com- 

 paratively little snow would fall. The difference of tempera- 

 ture thus produced would cause the aerial currents, and espe- 

 cially the trade winds on the colder hemisphere, to be much 

 stronger than those on the other ; they would, therefore, blow 

 across the Equator, and, by impelling the equatorial waters 

 towards the hemisphere which was already the warmer of the 

 two, would raise its temperature still further. 



This table shows that there are four periods, marked A, B, 

 C, and D, in which there has been a large excentricity and 

 an extreme climate. The periods marked A and B, says Sir 

 Charles Lyell, " would not, I conceive, be sufficiently distant 

 from our era to afford time for that series of glacial and post- 

 glacial events which we can prove to have happened since 

 the epoch of the greatest cold. These events relate to changes 

 in the level of the land in opposite directions, as well as the 

 excavation of valleys, and variations in the range and distri- 

 bution of aquatic and terrestrial animals, all of which take 

 place at so slow a rate that 200,000 years would not be suffi- 

 cient to allow of the series of changes with which we are 

 acquainted. I cannot but think, therefore, that if the date of 

 the most intense glacial cold can be arrived at by aid of a 

 very large excentricity, it would be a more probable conjecture 



* Climate and Time, p. 228. 



