﻿THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE II. VOL. IV. 



No. III.— MAECH, 1877. 



I. — Evidence Afforded by the Planet Mars on the Subject 



OF Glacial Periods. 



By Edward Carpentbr, M.A., late Fellow of Trinity Hall, Cambridge. 



HAVING read with much interest Mr. Murphy's papers in the 

 Journal of the Geological Society for 1869 (p. 350) and for 

 1876 (p. 400), in which he maintains that at the time of maximum 

 excentricity of the Earth's orbit that hemisphere would be glaciated 

 which had its lointer in aphelion, as against Mr. Croll, whose theory 

 is that the glaciated hemisphere would be that which had its summer 

 in aphelion : it has occurred to me that 1 have never seen, in this 

 discussion, any reference to the case of the planet Mars. 



It may be worth while, therefore, in case no one has done so 

 before, to point out how remarkable a parallel may be drawn 

 between the state of affairs on that planet and on the earth, and how 

 remarkable a confirmation the relative dimensions of the snow-caps 

 on that planet appear to give of Mr. Murphy's view of the matter. 



The excentricity of the orbit of the planet Mars is, as is well 

 known, considerably greater than that of the earth's orbit. It is 

 given by Sir John Herschel, in his Outlines of Astronomy, as 

 U'0931125. Leverrier's estimate of the maximum excentricity of the 

 earth's orbit, as quoted by Mr. Croll, is 0-07075. The excentricity 

 of Mars' orbit is therefore somewhat greater than the maximum 

 excentricity of that of the earth. Again, the inclination of the axis 

 of Mars to the perpendicular to the plane of its orbit is 25° 51', 

 which in a similar way is somewhat greater than the inclination of 

 the earth's axis to the plane of the ecliptic, which is 23° 27' 24". So 

 that in both respects Mars offers a slight exaggeration of the 

 conditions supposed to prevail in the case of the earth at the time 

 under consideration. But these coincidences would be of no service 

 to us, were they not supplemented by a third, most fortunate, 

 coincidence. The axis of Mars, namely, is inclined at the present 

 time towards one extremity of the axis of its orbit, and indeed 

 towards the perihelion point ; so tliat as on the earth at the present 

 time the winter solstice of the Northern hemisphere coincides with 

 perihelion, its summer solstice with aphelion, and the winter solstice 

 of the Southern hemisphere coincides with aphelion, the summer 

 solstice with perihelion. To show how nearly this is the case it 

 will be sufficient to quote the lengths of the seasons. They are 

 given as follows (from elements of the planet obtained by Maedler 



DECADE II. — YOL. IV, — NO. Ill 7 



