Ch. XIII.] 



THE OBLIQUITY OF THE ECLIPTIC 



283 



secular changes of the 



each side of the mean* The questions entered into by 

 Laplace and Leverrier respecting 

 ecliptic relative to a fixed plane, and possible changes in the 

 position of the earth's equator, must be the subject of labo- 

 rious computations before astronomers will have decided what 

 may be the extreme range of obliquity, but they are agreed 



must 



The re- 



suit of this movement, whether we adopt the higher or lower 



limit 



move 



inent gives rise. Whenever 



mor 



a long nirfit in winter, and consequently the cold 



at that 

 circum- 



The bearing of this 



season would be greater, and, under ■ 



stances, the reverse would take place. 



cause on geological phenomena would be twofold. So often 



as the extreme of possible obliquity happened to 



-w • i 1 



combine 



maximu 



cumstances 



those now pre- 



vailing in high latitudes, a greater intensity of cold would 

 be produced than could exist without such a combination, 

 and so far this would favour a glacial epoch. But when, on 



minimum 



would be 

 promote 



b 



may, however, be observed that the moi 



astronomical 



such as precession, change 

 of excentricity, and variation in the obliquity of the ecliptic, 

 each of them being capable of acting independently of the 

 others and of intensifying or extenuating the cold or heat— 



so much the gr< 

 interval of time) 

 all of them con* 

 unless the 



and the same direction. And 

 tinna. the most important of 



all, should happen to be in that exceptional state in which 

 they exert a refrigerating influence, there would be no ten- 



mi 



climate. On the other hand, it may be 



* Letter to the Author, Oct. 1866. 



