1869.] MURPHY GLACIAL CLIMATE. 351 



The following propositions are self-evident when stated : — 



When the excentricity of the earth's orbit is very considerable, 

 and the winter of either hemisphere occurs in perihelio and its 

 summer in aphelio, the nearness of the sun in winter will cause a 

 mild winter, and his remoteness in summer will cause a cool 

 summer. 



Conversely, in the opposite hemisphere at the same time, the 

 winter will occur in aphelio and the summer in perihelio : the re- 

 moteness of the sun in winter will cause a cold winter, and his 

 nearness in summer will cause a hot summer. 



Suppose, for instance, that when the excentricity of the earth's 

 orbit is much greater than at present, the midwinter of the Northern 

 hemisphere occurs in perihelio : then 



the Northern hemisphere will have a mild winter and cool summer , 

 the Southern hemisphere will have a cold winter and hot summer. 



So far (granting Mr. Croll's astronomical data, for which he cites 

 Leverrier, and which I believe are indisputable) there is no room for 

 doubt. I have now to discuss the question, what effect these diver- 

 sities of climate will have in producing glaciation. 



Mr. Croll thinks the hemisphere of cold winter will be the glaciated 

 one. I think, on the contrary, the hemisphere of cool summer will 

 be the glaciated one. 



On this subject it is needless to attempt to make any deductions 

 from theory. We have plenty of observed data ; and I think I can 

 show that they all go to prove a cool summer to be what most pro- 

 motes glaciation, while a cold winter has, usually, no effect on it 

 whatever. 



Forbes, in his work on Norway and its Glaciers, p. 206, quotes 

 " the excellent generalization of von Buch, that it is the temperature of 

 the summer months which determines the plane of perpetual snow." 

 This indeed is almost an identical proposition ; for perpetual snow is 

 snow that lies through the heats of summer ; and it would appear 

 obvious enough, had it not been frequently overlooked, that it must 

 be the temperature of summer which, other things being equal, de- 

 termines the level of summer snow. 



But, according to the same authority (Forbes's ' Norway and its 

 Glaciers,' p. 206), " another cause affecting exceedingly the level of 

 the snow-line is the amount of snow which falls." 



These laws are illustrated in detail by the following table. In 

 constructing it I have assumed, what is tolerably near the truth, 

 that the temperature of the hottest month of the year decreases in 

 ascending at the rate of 1° F. for every 300 feet. The tem- 

 peratures are taken, as accurately as I have been able to do it, 

 from Dove's map. My authorities for the heights of the snow- 

 line are, for the first four, Durocher as quoted by Mr. Hopkins 

 in the ' Proceedings of the Geological Society ' for Dec. 17, 1851, 

 for the rest, Mrs. Somerville's ' Physical Geography,' p. 314. The 

 temperatures are in degrees of Fahrenheit. The heights are in 

 feet. 



