270 GENERALIZATIONS. 



the present level. The hypothesis of Dr. Blandet does not ex- 

 plain these circumstances. 



A change in the relative position of the earth and the sun has 

 been suggested by Mr. James Croll in a series of memoirs, 

 dwelling principally on the periodic changes in the excentricity 

 of the earth's orbit. This orbit, as is well known, is an ellipse, 

 produced by the attractive influence of the great planets upon 

 the movement of the earth ; and this ellipse moves within deter- 

 minate limits in a cycle embracing thousands of years. At 

 present the orbit is constantly approaching the circular form, and 

 in 23,900 years its excentricity will attain its minimum, and the 

 orbit its maximum tendency towards a circle. From that time 

 it will gradually depart from the circular form. The mean dis- 

 tance of the earth from the sun is 91,430,000 English miles. 

 The greatest excentricity amounts to ^ of this distance ; its 

 smallest excentricity is only -g-J-o-. At the period of the greatest 

 excentricity the earth would be distant from the sun about 

 14,500,000 English miles more than at the period when the 

 orbit most nearly approaches to the circular form ; this difference 

 is at present 3,000,000 miles. 



It must be borne in mind that in our time the earth is in 

 perihelion during the winter of the northern hemisphere, and 

 that in summer it is in aphelion. This condition is also subject 

 to a periodical change the cycle of which embraces about 21,000 

 years. In about 10,000 years the summer of the northern 

 hemisphere will coincide with the time when the earth is in 

 perihelion, and the winter when it is in aphelion, whilst in the 

 southern hemisphere these conditions will of course be reversed. 

 It has, accordingly, been assumed that at periods when the orbit 

 of the earth attained the maximum of its excentricity, and was 

 at the same time in perihelion, one of the hemispheres had a 

 shorter and warmer winter, and on the other hand a longer and 

 cooler summer, whilst the reverse was the case in the other 

 hemisphere namely, a long and cold winter and a short and hot 

 summer, because the greatest distance from the sun would then 

 coincide with the winter. From this Mr. Croll concludes that, 

 during this long and cold winter there was formed so large a 

 quantity of ice that the short summer, although hot, did not 

 suffice to melt it entirely, that this ice, increasing and extending 



