400 JOSEPH JOHN MURPHY ON THE GLACIAL 



42. Tlie Glacial Climate and the Polar Ice-cap. By Joseph John 

 Murphy, Esq., F.G.S. (Read June 21, 1876.) 



In a paper " On the Nature and Cause of the Glacial Climate," in 

 the Journal of the Society for 1869 (p. 350), I gave my reasons for 

 thinking that the glacial climate was not one of intense cold, but of 

 snowy winters and cold summers, with a small range of temperature 

 — in four words, not Siberian but Faegian. I agree with Mr. Croll 

 that a glacial epoch is one of maximum eccentricity of the earth's 

 orbit, and that the northern and southern hemispheres, during such 

 an epoch, are glaciated alternately. Where I differ from him 

 is, that while he thinks the glaciated hemisphere has its winter in 

 aphelion, I maintain, on the contrary, that the glaciated hemisphere 

 is that which has its summer in aphelion. Mr. Croll, in his work 

 on ' Climate and Time,' has replied to me ; and I propose in this 

 paper to supplement my former one and give a fuller exposition of 

 my views, showing also where I think he has fallen into error. 



I quote the following from ' Climate and Time ' (p. 54) : — 



" According to the calculations of Leverrier, the superior limit of 

 the earth's eccentricity is 0*07075. Lagrange's determination makes 

 the superior limit 0*07641. Recently the laborious task of reinves- 

 tigating the whole subject has been undertaken by Mr. Stockwell, of 

 the United States. He has taken into aeeount the disturbing in- 

 fluence of the planet Neptune, the existence of which was not known 

 when Leverrier's computations were made ; and he finds that the 

 eccentricity of the earth's orbit will always be included within 

 the limits of and 0-0693888." 



In order not to take too high a value, and for facility of calcula- 

 tion where minntely accurate results as to the effect on climate 

 are unattainable, I take the maximum eccentricity at 0*069. The 

 following tabular statement shows the sun's mean, perihelion, and 

 aphelion distances, at present, and at maximum eccentricity, the 

 ratios of the same, and the ratios of heat received by the earth 

 at each distance : — 



At present. At maximum eccentricity. 



Sun's distance in miles. 



Mean 91,400,000 91,400,000 



Perihelion .... 89,864,000 85,093,400 



Aphelion .... 92,936,000 97,706,600 



Ratios of distance. 



Mean 1-000 1000 



Perihelion 0*983 0*931 



Aphelion 1*017 1*069 



Ratios of heat received. 



Mean 1-000 1-000 



Perihelion 1-035 1*154 



Aphelion 0*967 0*875 



