256 Dr. J. Croll on some Controverted Points 
exactly as much more be melted in the other half. The colder 
winter and the warmer summer would exactly neutralize each 
other's effects, and on the average of years no accumulation 
could begin. Prima facie, therefore, high eccentricity will 
not account for glacial periods "* . In the language of Prof. 
Newcomb, it is as follows : — " During this perihelion summer 
the amount of heat received from the sun by every part of 
the northern hemisphere would suffice to melt from four to 
six inches of ice per day over its entire surface ; that is, it 
would suffice to melt the whole probable accumulation in 
three or four days. The reader can easily make a computa- 
tion of the incredible reflecting power of the snow and of 
the unexampled transparency of the air required to keep the 
snow unmelted for three or four months." 
It is assumed in this objection that because the heat received 
from the sun by an area is more than sufficient to melt all the 
snow that falls on it, no permanent accumulation of snow 
and ice can take place. It is assumed that the quantity of 
snow and ice melted must be proportional to the heat received. 
Suppose that on a certain area a given amount of snow falls 
annually. The amount of heat received from the sun per 
annum is computed ; and after the usual deduction for that 
cut off by the atmosphere has been made, if it be found that 
the quantity remaining is far more than sufficient to melt the 
snow, it is then assumed that the snow must be melted, and 
that no accumulation of snow and ice year by year in this 
area is possible. To one approaching this perplexing subject 
for the first time such an assumption looks very plausible ; 
but a little reflection will show that it is most superficial. 
The assumption is at the very outset totally opposed to known 
facts. Take the lofty peaks of the Himalayas and Andes as 
an example. Few, I suppose, would admit that at these great 
elevations as much as 50 per cent, of the sun's heat could be 
cut off. But if 50 per cent, reaches the snow, this would be 
sufficient to melt fifty feet of ice ; and this, no doubt, is more 
than ten times the quantity which actually requires to be 
melted. Notwithstanding all this the snow is never melted, 
but remains permanent. Take, as another example, South 
Georgia, in the latitude of England. Suppose we assume 
that one half of the sun's heat is cut off by the clouds and 
fogs which prevail to such an extent in that place, still the 
remaining half would be sufficient to melt upwards of thirty 
feet of ice, which is certainly more than the equivalent of all 
the snow which falls ; yet this island is covered with snow 
and ice down almost to the seashore during the whole year. 
* Geological Magazine, January 1880, p. 12. 
