Ch. XIII.] 



PHASES OF THE PRECESSION. 



281 



from it, so that here again an equable climate is produced. 

 In the fourth quarter, the autumnal equinox falling at, or 

 within 45° of aphelion, the same effects will be produced as 

 in the second quarter, and there will be no great exaggeration 



of heat or cold. . 



Let ns now consider how far extreme excentncity might 



circumstances 



were as 



_ « favourable as they are now to the bringing on of a 

 glacial period. Already the mere position of the polar lands 

 o-iyes rise to a great thickness of permanent ice at both poles, 



some 



at least, there was a luxurant vegetation. Let us suppose 

 that extreme excentricity would in the first of the four 

 quarters of precession before enumerated, add considerably, 

 for reasons already stated, to the quantity and extent of 

 perpetual snow in the arctic regions. 



Although the store of 

 accumulated ice might be somewhat reduced during the 

 second quarter, we have no right to infer from what now 



takes place at eithei 



temp 



which would characterise generally the four seasons of ^ the 

 year, would get rid of the whole excess of ice which obtained 



commencement 



imagined 



Mr 



com 



formed in the first quarter, and only partially 



volume 



persistent in the arctic regions, I cannot c0 ™P rehe ^_°* 

 what principle such a thaw could be effected. mi "" 



extremely 

 temperature of space which Mr. ' 

 js us even a rough approximation 



temperature 



summers 

 Herschel's estimate of 



of about 32° Fahr., 



and in Iceland, 12° below freezing, while the severity of 

 the climate between the arctic circle and the pole would be 

 proportionally great . No doubt, the winter, which would be 

 shorter than the summer by thirty-six days, would be very 

 warm ; but, if in the first quarter the 5,250 intensely hot sum- 

 mers could not prevent the accumulation of ice, how could the 

 same number of winters, with a long arctic night between 



