430 Mr. J. Croll on the Change in the Obliquity of the Ecliptic, 



counted for ? We require a cause not only of wide influence but 

 of great intensity of action — in short, a cause which can at one 

 time confer on Greenland, Siberia, and the entire arctic regions 

 the mild climate of England or Madeira, and at another time 

 bury England and nearly the entire temperate regions under per- 

 petual snow and ice. And it must be able not only to do this, 

 but to do it during the continuance of one epoch. There are 

 few, I presume, who can seriously entertain the opinion that 

 these changes can be satisfactorily accounted for without having 

 reference to cosmical agency. When we view the matter from a 

 purely physical and cosmical standpoint, we at once perceive that 

 those changes, extreme as they no doubt appear, are in reality 

 what we ought a priori to conclude must have occurred. I feel 

 persuaded that those very conclusions to which geologists have 

 been led regarding the changes of climate during past ages, 

 would ultimately have all been arrived at through purely cosmi- 

 cal and physical considerations, even although geology as a 

 science had not existed ; they follow so obviously from theory. 



It follows as a necessary conclusion, that when the excentri- 

 city of the earth's orbit reaches a very high value, the hemisphere 

 which has its winter occurring in aphelion will be under a gla- 

 cial condition, while the other hemisphere having its winter in 

 perihelion will be enjoying a warm and equable climate. And 

 the occurrence at times of cold and warm conditions of climate 

 during the same geological epoch is in fact a necessary result of 

 the precession of the equinoxes combined with that of excen- 

 tricity. 



There is still another cause which I feel convinced must to a 

 very considerable extent have affected climate duriug past geo- 

 logical ages. I refer to the change in the Obliquity of the Eclip- 

 tic. This cause has long engaged the attention of geologists and 

 physicists; and the conclusion generally arrived at, like that 

 which had been arrived at in regard to excentricity, is that no 

 great effect can be attributed to it. After giving special attention 

 to the matter, I have been led to the very opposite conclusion. 

 It is quite true, as has been urged, that the changes in the ob- 

 liquity of the ecliptic cannot sensibly affect the climate of tem- 

 perate regions. But it will produce a slight change on the cli- 

 mate of the tropical regions, and a very considerable effect on 

 that of the polar regions, especially at the poles themselves. We 

 shall now consider the matter briefly. 



It was found by Laplace that the obliquity of the ecliptic will 

 oscillate to the extent of 1° 22' 34" on each side of 23° 28', the 

 obliquity in the year 1801. This change will but slightly affect 

 the climate of the temperate regions, but it will exercise a very 

 considerable influence on the climate of the polar regions. Ac- 



