56 DISCUSSIONS IN CLIMATOLOGY. 



winters of a glacial epoch, the earth would be radiat- 

 ing its heat into space. Had this loss of heat simply 

 lowered the temperature, the lowering of the tempera- 

 ture 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 in- 

 creases 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 hemi- 

 sphere begin pari passu to increase .* This withdrawal 

 of heat from the northern hemisphere favours the ac- 

 cumulation 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 



* 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, 1883) 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. Supposing 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. 



