1869.] MURPHY GLACIAL CLIMATE. 353 



of descent. This might not have much effect on the climate of 

 Central Europe, but it would have a very great effect in those high 

 latitudes where the glaciers would reach the sea and give origin to 

 icebergs ; for we know that icebergs have great influence as trans- 

 porters of cold. 



In particular cases the effect of a comparatively slight fall of 

 summer temperature would be very great. I quote from Forbes's 

 * Norway and its Glaciers,' p. 215 : — 



*' Though the surface actually covered by perpetual snow in Nor- 

 way be small, yet the mountainous districts and tablelands every- 

 where approach it so nearly that the snow-plane may be said to 

 hover over the peninsula, and any cause which should lower it even 

 a little would plunge a great part of the country under a mantle of 

 frost." 



And again, p. 243 : — 



" It is exceedingly probable that a diminution of the temperature 

 of the summer months by 4° only would at once place one-fourth of 

 the surface of Norway within the snow-line ; and so vast a mass of 

 snow would refrigerate the climate, especially the summer tempera- 

 ture, to such a degree as would unquestionably pour glaciers into 

 the head of every fiord in western Norway. . . . The lowering 

 of the snow-line over so large a surface would deteriorate the chmate 

 and lower the mean temperature, which would lower the snow-line 

 still further." 



The change in the eccentricity of the earth's orbit is in all proba- 

 bility amply sufficient to account for this or a much greater change 

 in summer temperature. 



I take the following data from Mr. Croll's paper. The recently 

 ascertained error in the old determinations of the sun's distance affects 

 both distances ahke, and consequently does not affect their ratio. 

 Along with the maximum distances of the sun at present and at 

 greatest excentricity, I state the proportionate quantities of heat the 

 earth wiU receive under those two different conditions : — 



Sun's maximuin Ratio of heat 



distance. received. 



At present 96,473,205 miles 100 



At greatest excentricity. . 102,256,873 „ 90 



So that in the one case the earth receives about one-tenth less heat 

 than in the other. 



The sun's maximum distance occurs at present a little after the 

 midsummer of the northern hemisphere. When it occurred at the 

 same time of the year during the period of greatest excentricity, 

 the earth at our midsummer was receiving only nine-tenths of the 

 quantity of heat which it now receives at that time of the year. I 

 cannot calculate the effect on climate ; but it must have been very 

 great, not only directly, by depressing the snow-line, but, as Forbes 

 remarks in the place cited above, indirectly by chilling the air — and, 

 I wiU add, by ffUing the North Sea with the icebergs which must 

 have broken off from the glaciers that filled the Norwegian fiords, as 



