CHAPTER IX. 



THE PHYSICAL CAUSE OF MILD POLAR CLIMATES. 



The Probably True Explanation. — Sir William Thomson on Mild 

 Arctic Climates. — Mr. Alfred R. Wallace on Mild Arctic 

 Climates. — Influence of Eccentricity during the Tertiary 

 Period. 



There are few facts within the domain of geology 

 better established than that at frequent periods in the 

 past the polar regions enjoyed a comparatively mild 

 and equable climate, and that places now buried under 

 permanent snow and ice were then covered with a 

 rich and luxuriant vegetation. Various theories have 

 been advanced to account for this remarkable state of 

 things, such as a different distribution of sea and land, 

 a change in the obliquity of the ecliptic, a displace- 

 ment in the position of the earth's axis of rotation, 

 and so forth. The true explanation will, I feel per- 

 suaded, be found to be the one I gave many years ago. 

 The steps by which my conclusions were reached were 

 as follows : — 



The annual quantity of heat received from the sun 

 at the equator is to that at the poles as 12 to 4*98, or, 

 say, as 12 to 5. This is on the supposition that the 

 same percentage of rays is cut off by the atmosphere 

 at the equator as at the poles, which, of course, is not 

 the case. More is cut off at the poles than at the 

 equator, and consequently the difference in the 

 amount of heat received at the two places is actually 



