20 J. Croll— Cause of Mild Polar Climates. 
Art. VI.—On the Cause of Mild Polar Climates ; by 
James Crou1, LL.D., F.R.S.* 
HERE 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 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 
eceived from the sun is 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. 
irst, it was necessary to know how much of the total: 
amount of heat received by the earth was derived from the sun’ 
and bow 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. 
ow were all the heat received by the earth derived’ simply 
from the sun, and were the temperature of each place propor: 
tionate to the amount directly received, then the absolute tem- 
perature of the poles would be 3% of that of the equator, or 
225°. .This would give a difference of 316° between the tem-, 
perature of the equator and that of the poles. According to 
Pouillet and Herschel space has a temperature of —239°, or 
222° of absolute temperature. If this be the temperature of 
space, then only 319° of the absolute temperature of the equa- 
tor are derived from the sun; consequently as the poles receive © 
* From the Phil. Mag, for October, 1884. 
