110 On the Physical Theory of Secular Changes of Climate. 



to move towards the perihelion, the mutual reactions of these 

 physical causes will be reversed and will go on with increasing 

 intensity till the perihelion is reached, melting the very ice 

 which they had previously produced. 



We have already seen that the greater the extent of the 

 eccentricity, the greater is the effect resulting from the actions 

 of the physical causes, whether this effect be the production of 

 ice on the cold hemisphere, or its removal from the warm. It 

 is evident that the same thing must necessarily hold true in 

 regard to the mutual reactions of the physical causes. Conse- 

 quently if the mutual actions and reactions of the physical 

 causes, brought into operation during a high state of eccen- 

 tricity, led at the glacial epoch to the great accumulation of 

 iue when the winters were in aphelion, they must have led to 

 an equally great melting and dispersal of that ice w T hen pre- 

 cession brought the winters round to perihelion. These causes 

 would be as efficient in the removal of the ice as they were in 

 its production. In so far as the physical and astronomical 

 causes were concerned, the greater the amount of ice formed 

 during the cold periods the greater would be the amount 

 melted during the warm intergiacial periods. 



Another Reason assigned ivhy the Ice does not Melt. — Mr. 

 Wallace assigns the following as an additional reason why the 

 ice does not disappear during the intergiacial periods when 

 the eccentricity is high: — 



" When a country is largely covered with ice, we may look 

 upon it as possessing the accumulated or stored-up cold of a 

 long series of preceding winters ; and however much heat is 

 poured upon it, its temperature cannot be raised above the 

 freezing-point till that store of cold is got rid of — that is, till 

 the ice is all melted. But the ice itself, when extensive, 

 tends to its own preservation, even under the influence of 

 heat ; for the chilled atmosphere becomes filled with fog, and 

 this keeps off the sun-heat, and then snow falls even during 

 summer, and the stored-up cold does not diminish during the 

 year. When, however, only a small portion of the surface is 

 covered with ice, the exposed earth becomes heated by the hot 

 sun, this warms the air, and the warm air melts the adjacent 

 ice. It follows that, towards the equatorial limits of a gla- 

 ciated country alternations of climate may occur during a 

 period of high eccentricity, while nearer the pole, where the 

 whole country is completely ice-clad, no amelioration may 

 take place."— P. 154. 



For the past nineteen years I have been maintaining that, 

 when a country is covered with ice, it becomes a permanent 

 source of cold; and however much heat may be received from 



