280 



CLIMATES OF THE SUCCESSIVE 



[Ch. XIII. 



the two opposite 



ernes 



greater than 



of aphelion and perihelion be 



is generally assumed. Under normal geo- 



circumstances 



of sea in high latitudes is in excess, there would be scarcely 

 if any permanent ice at either pole, and a freer circulation of 

 oceanic currents from equatorial to polar regions than now • 



om 



is always equal, there would be no tendency in variations of 



3 of temperature. 

 ^cession. — But there is 



Climates of the successive phases of precei 

 another circumstance tending to the equalisation of the heat 

 which must be borne in mind, lest we should exaggerate the 

 effects of excentricity on climate, even when intensified by 

 such abnormal conditions of the earth's geography as now 

 prevail. We cannot divide the 21,000 years before spoken of 

 as constituting the cycle caused by precession and the revo- 

 lution of the apsides into two equal parts, as M. Adhemar 

 and others have proposed, one of which in a given hemisphere 

 should be a cold period when the winters coincide with 



period when the winters 

 coincide with perihelion. We must rather divide the cycle of 

 precession into four quarters, in the first of which there 

 is an accumulation of ice, because of the coincidence of the 

 long polar night and the short days of all high latitudes 

 with the greatest distance of the planet from the sun, or, in 

 other words, because winter happens when the earth is at or 

 within 45° of the aphelion, and because, in accordance with 

 Mr. CrolFs hypothesis, the more intense heat of 5,250 sum- 

 mers in periheliofi is so intercepted by fogs and clouds as to 



m 



be unequal to 

 number 



sk of melting the snows of an equal 

 Then follows the next quarter, when 

 the vernal equinox occurs at the least distance from the sun, 

 and an equable climate is produced ; for the 5,250 winters and 



of nearly equal duration, and the summer 



the sun also ecraaL both of which 



summers 



om 



mean 



I combine to make these seasons vary but little from 

 In the third quarter the cold of all the winters 

 is neutralised by proximity to the sun, while the heat of the 



summers is in like manner morlprn.Wl hv+,lip ph^Ii's distance 



I 



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