CHANGES OF CLIMATE DURING GEOLOGICAL PERIODS. 109 



first, by radiation from the water; and, second, by contaot with the 

 water. The south-west winds, to which we owe our heat, derive their 

 temperature from the south-east portion, which flows away in the 

 direction of the Azores. The north-east portion of the stream, which 

 flows past our shores up into the Arctic seas, protects us from the ice of 

 Greenland by warming the north-west winds which come to us thence. 

 The strong under-current of air from the north implies an equally 

 strong upper-current to the north. This would carry northwards the 

 aqueous vapour formed at the Equator, which would be deposited in the 

 form of snow, an enormous quantity of which would fall in the 

 northern regions. 



These various agencies re-act on one another, and so as to strengthen 

 each other. The accumulation of snow and ice tends to cool the air, and 

 produce fogs. This diminishes the melting power of the sun, and 

 increases the accumulation, the rate of which continually increases. As 

 the snow and ice accumulate on the one hemisphere, they diminish on 

 the other. This tends to increase the strength of the trade winds on the 

 cold hemisphere, and to weaken those on the other. The effect of this 

 is to impel the warm wind of the tropics more to the warm hemisphere 

 than to the cold. As the snow and ice accumulate, the ocean currents 

 decrease; and, on the other hand, as the ocean currents diminish, snow 

 and ice accumulate, the two effects mutually strengthening each other. 

 As the eccentricity increases century by century, the temperate regions 

 become more and more covered with snow and ice, and this state of 

 things goes on increasing until the solstice point arrives at the aphelion. 

 Then, a contrary process commences, which continues for ten or twelve 

 thousand years, until the winter solstice reaches the perihelion. The 

 transference of the ice from one hemisphere to the other continues as 

 long as the eccentricity remains at a high value. 



To meet a possible objection, it may be stated that this theory not 

 only comports with the fact that the mean temperature of the whole 

 earth is somewhat greater when it is in aphelion than in perihelion, but 

 requires it. It is greater, because then the sun is over the hemisphere 

 which is comparatively free from ice, while in perihelion it is over the 

 hemisphere nearly covered with snow and ice, and the heat is spent in 

 melting these. 



The argument, then, is this: There is ample evidence to prove 

 changes in the earth's relation to the sun, which, to some extent directly, 

 by increasing or decreasing the summer and winter temperature, and to 

 a much larger extent indirectly, by altering the direction of the winds 

 and currents, would effect, at longer or shorter intervals, alterations of 

 climate, inducing, in succession, periods of great cold and of great 

 heat in the two hemispheres alternately. Corresponding with these 

 changes of eccentricity, there is very ample evidence of the existence of 

 a glacial age in the northern hemisphere, and of a temperate or sub- 

 tropical age in the southern, while there is scantier, but still sufficient 

 evidence of interglacial periods — alternating with temperate and tropi'-al 

 ones — the dates of which, as far as these can be estimated, are in 

 accord with the known dates of these astronomical changes. 



