J. Croll— Cause of Mild Polar Climates. 21 
from the sun only 54, of this amount of temperature, or 133°, 
this will’ give merely 186° as the difference which ought to 
econd, it was necessary to know at what rate the tempera- 
ture increased or decreased with a given inerease or decrease 
in the amount of heat received. It was well known that 
Newton’s law—that the change of temperature was directly 
proportionate to the change in the quantity of heat received— 
was far from being correct. The formula of Dulong and Petit . 
present case was not essential. All that really was required 
temperature could not be less (although probably more) than 
200°. But the present’ actual difference does not probably 
exceed 80°. We have no means of ascertaining with certainty 
what the mean annual temperature of the poles is; but as the 
temperature of latitude 80° N. is 4°°5, that of the poles is 
place depend alone on the heat received directly from the sun. 
This great reduction from about 200° to 80° can, of course, be 
due to no other cause than to a transference of heat from the 
+ €quator to the poles. The question then arose, by what means 
Was this transference effected? There were only two agencies 
available—the transference must be effected either by aerial or 
Y ocean currents. It was shown at considerable length (Cli- 
Mate and Time, pp. 27-30, and other places) that the amount 
of heat that can be conveyed from the equator to the poles by 
. ; 
