Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/461

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 remoteness 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 diversities 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 promotes 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, determines 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 temperatures 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.