[ 268 ] 



XXXIII. On the Cause of Mild Polar Climates, 

 By James Croll, LL.D. 9 F.R.S* 



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 vegeta- 

 tion. 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 

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

 and so forth. The true explanation will, I feel persuaded, be 

 found to be the one I gave many years ago. The steps by 

 which the results 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 greater than that 

 indicated by the ratio 12 to 5. But, assuming 12 to 5 to be 

 the ratio, the question arose what ought to be the difference 

 of temperature between the two places in question on the 

 supposition that the temperature was due solely to the direct 

 heat received from the sun ? This was a question difficult 

 to answer, for its answer mainly depended upon two things, 

 regarding both of which a very considerable amount of un- 

 certainty prevailed. 



First, it was necessary to know how much of the total 

 amount of heat received by the earth was derived from the 

 sun and how much from the stars and other sources, or, in 

 other words, from space. Absolute zero is considered to be 

 461° below zero Fahr. The temperature of the equator is 

 about 80°, This gives 541° as the absolute temperature of 

 the equator. Now were all the heat received by the earth 

 derived simply from the sun, and were the temperature of 

 each place proportionate to the amount directly received, then 

 the absolute temperature of the poles would be <fe of that 

 of the equator, or 225°. This would give a difference of 316° 

 between the temperature of the equator and that of the poles. 

 According to Pouillet and Herschel, space has a tempera- 

 ture of -^239°, or 222° of absolute temperature. If this be 



* Communicated "by the Author. 



