404 JOSEPH JOHN MURPHY ON THE GLACIAL 



winter cold ; and such was probably the climate of Greenland when 

 it was clothed with forests. 



It is true that a long and cold winter will be favourable to the 

 formation of ice, by the freezing of water. But no great accumula- 

 tion of ice, like that of the glacial period, can have been due to this 

 cause, because the freezing of water is such a slow process that 

 the thickest ice thus formed does not approach the thickness of a 

 moderately thick glacier. 



Thus the great heat of the perihelion summer will rapidly melt 

 away the snow which has fallen during the aphelion winter. The 

 question of the effect of summer heat on glaciation is, practically, to 

 what height the temperature of the hottest month is able to clear 

 the mountains of snow. This height is the height of the snow- 

 line ; and I have shown in my former paper, by an appeal to the 

 facts of physical geography, that this depends chiefly on the tem- 

 perature of the hottest month. 



Humboldt, in his ' Cosmos/ makes the interesting remark, that if 

 the mountains of the world were high enough, we should see an 

 upper as- well as a lower limit to the region of perpetual snow. That 

 is to say, at a very great height the snowfall would be so small that 

 the snow would disappear by evaporation under the summer sun. 

 It seems not unlikely that this actually took place during the peri- 

 helion summer at maximum eccentricity, when the amount of heat 

 received by the earth in perihelion exceeded what it is now in the 

 ratio of 1-154 to 1-035, equal to 1-115 to 1-000, or about 10 to 9. 



There is one fact of physical geography, which seems at first sight 

 to support Mr. Croll's theory that the glacial climate was due to 

 an aphelion winter, and not, as I maintain, to an aphelion summer. 

 In the Antarctic regions there is a glacial climate now ; the entire 

 Antarctic continent is covered with perpetual snow down to the 

 water's edge ; yet the Antarctic summer is in perihelion. Mr. Croll 

 thinks this is the normal state of things, and in support of his theory 

 he states (p. 77) that 



"1. The mean temperature of the southern hemisphere is less 

 than that of the northern. 



" 2. The winters of the southern hemisphere are colder than those 

 of the northern. 



" 3. The summers, though occurring in perihelion, are also 

 comparatively cold. 



" 4. The mean temperature of the whole earth is greater in June, 

 when it is in aphelion, than in December, when it is in perihelion." 



I believe it may be confidently asserted that the first two of 

 these statements are erroneous. The mean temperatures of the 

 two hemispheres appear to be very nearly the same : what makes 

 the great difference in their climates is difference of range of tem- 

 perature. As compared with the northern hemisphere, the range in 

 the southern is less ; the summers are cooler, and the winters warmer. 

 The following tabular statement of the climates of the two hemi- 

 spheres is from Mr. Hopkins's paper on " Changes of Climate " in 

 the « Quarterly Journal ' of this Society, vol. viii. (p. 72) : — 



