CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 145 



portion of the reflected heat would no doubt warm the 

 vapour in the atmosphere, but this heat would be carried 

 off to other parts of the earth, while a considerable portion 

 of the whole would be lost in space. It must also be 

 remembered that an enormous quantity of heat is used up 

 in- melting snow and ice, without raising its temperature ; 

 each cubic foot of ice requiring as much heat to melt it as 

 would raise nearly six cubic feet of water 30° F. It has, 

 however, been argued that because when water is frozen 

 it evolves just as much heat as it requires to melt it again, 

 there is no loss of heat on the whole ; and as this is ad- 

 duced over and over again as a valid argument in every 

 criticism of Dr. Croll's theory, it may be well to consider it 

 a little more closely. In the act of freezing no doubt 

 water gives up some of its heat to the surrounding air ; but 

 that air still o'cmains heloio the freezing ^96»m?J or freezino- 

 would not take place. The heat liberated by freezing is. 

 therefore, what may be termed low-grade heat — heat 

 incapable of melting snow or ice ; while the heat absorbed 

 while ice or snow is melting is high-grade heat, such as is 

 capable of melting snow and supporting vegetable growth. 

 Moreover, the low-grade heat liberated in the formation of 

 snow is usually liberated high up in the atmosphere, where 

 it may be carried off by winds to more southern latitudes, 

 while the heat absorbed in melting the surface of snow and 

 ice is absorbed close to the earth and is thus prevented 

 from warming the lower atmosphere, which is in contact 

 with vegetation. The two phenomena, therefore, by no 

 means counterbalance or counteract each other, as it is so 

 constantly and superficially asserted that they do. 



Uffcd of Clouds and Fog in cutting off the Smis Heat. — 

 Another very important cause of diminution of heat during 

 summer in a glaciated country would be the intervention 

 of clouds and fogs, which would reflect or absorb a large 

 proportion of the sun-heat and prevent it reaching the 

 surface of the earth ; and such a cloudy atmosphere would 

 be a necessary result of large areas of high land covered 

 with snoAV and ice. That such a prevalence of fogs and 

 cloud is an actual fact in all ice-clad countries has been 

 shown by Dr. Croll most conclusively, and he has further 



