Physical Theory of Secular Changes of Climate. 191 



strong undercurrent of air from the north implies an equally 

 strong upper current to the north. Now if the effect of the 

 undercurrent would be to impel the warm water at the equator 

 to the south, the effect of the upper current would be to carry 

 the aqueous vapour formed at the equator to the north ; the 

 upper current, on reaching the snow and ice of temperate re- 

 gions, would deposit its moisture in the form of snow ; so that 

 it is probable that, notwithstanding the great cold of the glacial 

 epoch, the quantity of snow falling in the northern regions would 

 be enormous. This would be particularly the case during sum- 

 mer, when the earth would be in the perihelion and the heat at 

 the equator great. The equator would be the furnace where eva- 

 poration would take place, and the snow and ice of temperate 

 regions would act as a condenser. 



The direct effect of eccentricity is to produce on one of the 

 hemispheres a long and cold winter. This alone would not lead 

 to a condition of things so severe as that which we know pre- 

 vailed during the glacial epoch. But the snow and ice thus 

 produced would bring into operation, as we have seen, a host of 

 physical agencies whose combined efforts would be perfectly suffi- 

 cient to do this. 



A remarkable Circumstance regarding those Causes which lead to 

 Secular Changes of Climate. — There is one remarkable circum- 

 stance connected with those physical causes which deserves 

 special notice. They not only all lead to one result, viz. an ac- 

 cumulation of snow and ice, but they mutually react on one 

 another. It is quite a common thing in physics for the effect 

 to react on the cause. In electricity and magnetism, for example, 

 cause and effect in almost every case mutually act and react upon 

 each other. But it is usually, if not universally, the case that 

 the reaction of the effect tends to weaken the cause. The weak- 

 ening influences of this reaction tend to impose a limit on the 

 efficiency of the cause. But, strange to say, in regard to the 

 physical causes concerned in the bringing about of the glacial 

 condition of climate, cause and effect mutually reacted so as to 

 strengthen each other. And this circumstance had a great 

 deal to do with the extraordinary results produced. 



We have seen that the accumulation of snow and ice on the 

 ground resulting from the long and cold winters tended to cool 

 the air and produce fogs which cut off the sun's rays. The rays 

 thus cut off diminished the melting-power of the sun, and so 

 increased the accumulation. As the snow and ice continued to 

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

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

 mulation increased, because the quantity of snow and ice melted 

 became thus annually less and less. 



