Physical Theory of Secular Changes in Climate. 441 
ice would have to be melted off the ground before the warmth 
of the interglacial period would commence. So long asa single 
inch of ice covered the surface of the country, the cold would 
‘continue. Ice, as we have seen, by chilling the air induces 
fresh snow to fall; and of course it is only when the amount of 
ice annually melted exceeds that being formed from the falling 
Snow, that a diminution in the thickness of the sheet would 
begin to take place. .A real melting of the ice, and consequent 
decrease in.the thickness of the sheet, would probably not com- 
mence till the astronomical and physical agencies in operation 
during the glacial period began to act in an opposite direction. 
In short, it would be the favorable conditions of the interglacial 
period that would effectually remove the ice; and it would be 
then, and only then, that the warmth would begin. While, 
again, at the close of the period, when the first inch of ice made 
its appearance on the surface of the country, the interglacial 
condition of climate would come to an end. The time require 
present. This, as we have seen, is a conclusion which is fully 
borne out by geological and paleontological facts. 
‘T’be question as to the probable cause of warm polar climates 
will next be considered. 
