262 Dr. J. Croll on some Controverted Points 
rate of accumulation increases, because the quantity of snow 
and ice melted becomes thus annually less and less. In addi- 
tion, the loss of the rays cut off by the fogs lowers the tem- 
perature of the air and leads to more snow being formed, 
while, again, the snow thus formed chills the air still more and 
increases the fogs. Again, during the winters of a glacial 
epoch, the earth would be radiating its heat into space. Had 
this loss of heat simply lowered the temperature, the lowering 
of the temperature would have tended to diminish the rate of 
loss ; but the result is the formation of snow rather than the 
lowering of the temperature. 
Further, as snow and ice accumulate on the one hemisphere 
they diminish on the other. This increases the strength of 
the trade-winds on the cold hemisphere and weakens those on 
the warm. The effect of this is to impel the warm water of 
the tropics more to the warm hemisphere than to the cold. 
Suppose the northern hemisphere to be the cold one ; then, 
as the snow and ice begin gradually to accumulate, the ocean- 
currents of that hemisphere, more particularly the Gulf- 
stream, begin to decrease in volume, while those on the 
southern or warm hemisphere begin pari passu to increase *. 
This withdrawal of heat from the northern hemisphere favours 
the accumulation of snow and ice ; and as the snow and ice 
accumulate the ocean-currents decrease. On the other hand, 
as the ocean-currents diminish the snow and ice still more 
accumulate. Thus the two effects, in so far as the accumu- 
lation of snow and ice is concerned, mutually strengthen each 
other. 
The same process of mutual action and reaction takes place 
among the agencies in operation on the warm hemisphere; 
only the result produced is diametrically opposite to that pro- 
duced in the cold hemisphere. On this warm hemisphere 
action and reaction tend to raise the mean temperature and 
diminish the quantity of snow and ice existing in temperate 
and polar regions. 
* Prof. Dana has shown that in North America those areas which at 
present have the greatest rainfall are, as a rule, the areas which were 
most glaciated during the glacial epoch. Mr. Searles V. Wood (Geol. 
Mag., July) maintains that this fact is inconsistent with the theory that 
the glacial period was due to the cause to which I attribute it. I am 
totally unable to comprehend how he arrives at this conclusion. Sup- 
posing the Gulf-stream, as I have maintained, were greatly diminished 
during the glacial period, still I think it would follow, other things being 
equal, that the areas which now have the greatest rainfall would during 
that period probably have the greatest snowfall, and consequently the 
greatest accumulation of ice. The amount of precipitation might be less 
than at present ; but this would not prevent the areas which had the 
greatest snowfall from being most covered with ice. 
