282 



CLIMATE AFFECTED BY VARIATION IN 



[Ch. XIII. 





lat. 70° and the pole, and short days with the sun at a mode- 

 rate height above the horizon in all temperate latitudes, exert 

 a great melting power ? We must also recollect what would 

 now be going on at the opposite, or south pole, where the 

 shortest day would coincide with the greatest distance of the 



om 



accumulated 



m 



remaimn 



upon the northern h< 



undiminished), and under-currents of water far colder than 

 any which belong to the ocean in our time 

 towards the equator. It is hardly necessary to say how little 

 the equable climate of the fourth quarter could overcome all 

 the excess of snow and ice which would thus have been 

 formed at both poles; and it seems inevitable, that, whatever 

 refrigerating effect extreme excentricity can produce in exag- 

 gerating the cold, that effect must be cumulative during each 

 successive cycle of precession, until the orbit becomes again 

 more circular. The lowering of the temperature would be 

 cosmical ; there would be two antipodal ice-caps, one of them 

 always greater in volume than the other, but neither of them 

 of as moderate dimensions as those of the present epoch. 



Variation in the obliquity of the ecliptic. — Hitherto we have 

 been considering the effect on climate of changes in the ex- 

 centricity of the orbit, as if the earth's axis of rotation were 

 always inclined, as now, at an angle of 23° 28' to the plane of 

 the ecliptic ; but it is well known that this angle is made to 

 vary by about forty-eight seconds per century by the action 

 of the planets on the earth, by which the plane of the 

 ecliptic is now becoming more nearly coincident from year to 

 year with the equator. This diminution of the obliquity 

 will go on for ages, after which ' it will again increase, and 

 thus oscillate backwards and forwards about a mean position, 

 the extent of its deviation to one side and the other being 

 less than 1° 21' * But Sir John Herschel informs me that 

 although this limit as calculated by Laplace is true as re- 

 gards the last 100,000 years, yet if millions of years are 

 taken into account, he thinks it conceivable that the devi- 

 ation may possibly be sometimes greater, and may even 

 be found to extend as much as three or even four degrees on 







* Herschel's Astronomy, art. 640. 



