108 Dr. J. Croll on the Physical Theory 



the production of snow and ice on the cold hemisphere, or the 

 melting of them on the warm, — whether it be their production 

 when the winter solstice of a hemisphere is in aphelion, or 

 their melting when that solstice is in perihelion. 



We have, however, to take into account not merely the 

 action of the physical agents, but their Mutual Reactions on 

 one another. The effect of this mutual reaction is very stri- 

 king. Not only do the physical agents, in their actions, all 

 lead to one result, viz. an accumulation of snow and ice when 

 the winters are in aphelion, but their efficiency in bringing 

 about this result is actually strengthened by their mutual 

 reactions on one another. To illustrate this effect I quote the 

 following from a former article : — 



" To begin with, we have a high state of eccentricity. This 

 leads to long and cold winters. The cold leads to snow, and 

 although heat is given out in the formation of the snow, yet 

 the final result is that the snow intensifies the cold : it cools 

 the air and leads to still more snow. The cold and snow bring 

 a third agent into play — -fogs, which act still in the same 

 direction. The fogs intercept the sun's rays, this interception 

 of the rays diminishes the melting-power of the sun, and so 

 increases the accumulation. As the snow and ice continue to 

 accumulate, more and more of the rays are cut off; and on the 

 other hand, as the rays continue to be cut off, the rate of accu- 

 mulation increases, because the quantity of snow and ice 

 melted becomes thus annually less and less. In addition, the 

 loss of the rays cut off by the fogs lowers the temperature 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 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 hemi- 

 sphere 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. 

 Supposing 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 

 w r arm hemisphere begin pari passu to increase. This with- 

 drawal of heat from the northern hemisphere favours the 



