

376 Physical Theory of Secular Changes of Climate. 



longer than the warmth of the interglacial will, I think, be 

 apparent from the following considerations. As long as a 

 country remains permanently covered with snow and ice, the 

 climate, as has been repeatedly shown, must continue cold, no 

 matter what the direct heat of the sun may be. Astronomi- 

 cally considered the interglacial periods are, of course, of the 

 same length as the glacial, — the mean length of which, during 

 the Glacial Epoch, was about 11,600 years ; but the cold of 

 a glacial period would not, as we shall presently see, actu- 

 ally terminate at the end of the period, but would be continued 

 on probably for centuries into the succeeding interglacial 

 period. Suppose that during a glacial period the country is 

 covered with a sheet of ice, which daring the continuance of 

 the period had accumulated to the thickness of 2000 or 3000 

 feet. All this enormous quantity of ice would have to be 

 melted off the ground before the warmth of the interglacial 

 period would commence. So long as a 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 commence 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 favourable 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 inter- 

 glacial condition of climate would come to an end. The time 

 required to remove the ice does not prevent an interglacial 

 condition of climate ; it only somewhat shortens its duration. 



There is another circumstance worthy of notice here. It is 

 this : as the mild and equable character of the climate during 

 interglacial periods resulted to a large extent from the enor- 

 mous transference of equatoreal heat, and its distribution over 

 temperate and polar regions, the difference of climatic condi- 

 tions between the subtropical and the temperate and polar 

 regions would be less marked than at present ; in other words, 

 the temperature would not differ so much with latitude as it 

 does at present. This, as we have seen, is a conclusion which 

 is fully borne out by geological and pala3ontological facts. 



The question as to the probable cause of warm polar climates 

 will next be considered. 



