CHAP, VIII.] 



•THE CAUSES OF GLACIAL EPOCHS. 



129 



form of rain, or as a solid in the form of snow, although the two 

 may not differ from each other more than two or three degrees 

 in temperature. The rain, however much of it may fall, runs off 

 rapidly into streams and rivers, and soon reaches the ocean. If 

 cold it cools the air and the earth somewhat while passing 

 through or over them, but produces no permanent effect on 

 temperature, because a few hours of sunshine restore to the air 

 or the surface-soil all the heat they had lost. But if snow 

 falls for a long time, the effect, as we all know, is very different, 

 because it has no motility. It remains where it fell and becomes 

 compacted into a mass, and it then keeps the earth below it and 

 the air above, at or near the freezing-point till it is all melted. 

 If the quantity is great it may take days or weeks to melt ; and 

 if snow continues falling it goes on accumulating all over the 

 surface of a country (which water cannot do), and may thus 

 form such a mass that the warmth of the whole succeeding 

 summer may not be able to melt it. It then produces perpetual 

 snow, such as v/e find above a certain altitude on all the great 

 mountains of the globe ; and when this takes place cold is 

 rendered permanent, no amount of sun -heat warming the air or 

 the earth much above the freezing-point. This is illustrated by 

 the often-quoted fact that at 80° Lat., Captain Scoresby had 

 the pitch melted on one side of his ship by the heat of the sun, 

 while water was freezing on the other side owing to the coldness 

 of the air. 



The quantity of heat required to melt ice or snow is very 

 great, as we all know by experience of the long time masses of 

 snow will remain unmelted even in warm weather. We shall 

 however be better able to appreciate the great effect this has 

 upon climate, by a few figures showing what this amount really 

 is. In order to melt one cubic foot of ice, as much heat is 

 required as would heat a cubic foot of water from the freezing- 

 point to 176° F., or two cubic feet to 88° F. To melt a layer of 

 ice a foot thick will therefore use up as much heat as would 

 raise a layer of water two feet thick to the temperature of 

 88° F. ; and the effect becomes still more easily understood if 

 we estimate it as applied to air, for to melt a layer of ice only 

 1^ inches thick would require as much heat as would raise a 



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