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 mobility. 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 we 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° 1ST. 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 J inches thick would require as much heat as would raise a 
K 
