28 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



While the climate of the globe is, no doubt, much 

 affected by geographical conditions, the cold of the 

 glacial period was, I believe, mainly due to the eccen- 

 tricity of the earth's orbit combined with the oblique 

 effects of precession of the ecliptic. The result of the 

 latter condition is a period of 21,000 years, during one 

 half of which the northern hemisphere is warmer than 

 the southern, while during the other 10,500 years the 

 reverse is the case. At present we are in the former 

 phase, and there is, we know, a vast accumulation of 

 ice at the south pole. But when the earth's orbit is 

 nearly circular, as it is at present, the difference between 

 the two hemispheres is not very great ; while on the 

 contrary, as the eccentricity of the orbit increases, the 

 contrast between them increases also. This eccentricity 

 is continually oscillating within certain limits which 

 Croll and subsequently Stone have calculated for the last 

 million years. At present the eccentricity is *016 and 

 the mean temperature of the coldest month in London 

 is about 40. Such has been the state of things for 

 nearly 100,000 years ; but before that there was a 

 period, beginning 300,000 years ago, when the eccen- 

 tricity of the orbit varied from *26 to *57. The result 

 of this would be greatly to increase the effect due to the 

 obliquity of the orbit ; at certain periods the climate 

 would be much warmer than at present, while at others 

 the number of days in winter would be twenty more, 

 and of summer twenty less, than now. while the mean 

 temperature of the coldest month would be lowered 

 20. We thus get something like a date for the last 

 glacial epoch, and we see that it was not simply a 

 period of cold, but rather one of extremes, each beat of 



