192 Mr. J. Croll on Ocean-currents in relation to the 



Again, during the long and dreary winters of the glacial 

 epoch the earth would be radiating off" its heat into space. Had 

 the heat thus lost simply gone to lower the temperature, the 

 lowering of the temperature would have tended to diminish 

 the rate of loss ; but it was the formation of snow and ice 

 more than the lowering of temperature that was the result. 



And, again, the formation of snow and ice facilitated the rate 

 at which the earth lost its heat ; and on the other hand, the 

 more rapidly the earth parted with its heat, the more rapidly 

 were the snow and ice formed. 



Further, as the snow and ice accumulated on the one he- 

 misphere, they at the same time continued to diminish on 

 the other. This tended to increase the strength of the trade - 

 winds on the cold hemisphere, and to weaken those on the warm. 

 The effect of this on ocean-currents would be 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 began gradually to accumulate on it, 

 the ocean-currents of that hemisphere would begin to decrease 

 in volume, while those on the southern, or warm hemisphere, 

 would pari passu increase. This- withdrawal of heat from the 

 northern hemisphere would tend, of course, to lower the tempe- 

 rature of that hemisphere and thus favour the accumulation of 

 snow and ice. The more the snow and ice accumulated, the less, 

 on this account, would 'grow the ocean-currents ; and on the 

 other hand, the less the ocean-currents became, the greater 

 would become the accumulation of snow and ice, — the two effects 

 mutually strengthening each other. 



The same must have held true in regard to aerial currents. 

 The more the polar and temperate regions became covered 

 with snow and ice, the stronger would become the trades 

 and anti-trades of the hemisphere ; and the stronger those winds 

 became, the greater would be the amount of moisture transferred 

 from the tropical regions by the anti-trades to the temperate re- 

 gions ; and on the other hand, the more moisture those winds 

 brought to temperate regions, the greater would be the quan- 

 tity of snow produced. 



The same process of mutual action and reaction would take 

 place among the agencies in operation on the warm hemisphere, 

 only the result produced would be the diametrically opposite of 

 that produced in the cold hemisphere. On this warm hemi- 

 sphere action and reaction would tend to raise the mean tempe- 

 rature and diminish the quantity of snow and ice existing in 

 temperate and polar regions. 



Had it been possible for each of those various physical agents 

 which we have been considering to produce its direct effects 



