186 SPARKS FROM A GEOLOGIST'S HAMMER. 



enough longer to compensate for its feebleness. M. Ad- 

 hemar subsequently subjected the question to a more 

 thorough investigation, and announced that increased ec- 

 centricity concurring with the precession of the equinoxes 

 would so modify the climate of the northern hemisphere 

 as to produce, once in 21,000 years, the geological winter 

 to which I before referred. Nevertheless, the general 

 opinion of physicists has been opposed to Adhemar's con- 

 clusion in reference to the amount of the modification. 



The subject has been more recently taken up by Mr. 

 Croll, of Glasgow ; and he has shown by an ingenious 

 course of reasoning that though the direct effect of an 

 increased ellipticity in the earth's orbit might be incon- 

 siderable, still the effect produced would so modify the 

 oceanic currents as to greatly increase the precipitation 

 of snow in the northern hemisphere, and diminish the 

 amount of snow and ice in the southern.* 



The calculations of astronomers have shown that when 

 the eccentricity is at a maximum the earth will be 14,212,- 

 700 miles farther from the sun in aphelion than in peri- 

 helion. As the periods of high eccentricity continue from 

 50,000 to 75,000 years, the precession of the equinoxes, 

 which completes its cycle in about 21,000 years, will 

 bring the winter solstice of either hemisphere to coincide 



*Jatnps Croll, Climate and Time. See a brief statement of the theory by 

 Mr. Croll in the Geological Magazine, Sept. 1878, extracted in Amer. Jour. Sci., 

 Ill, xvi, 389, and a fuller statement by the present writer in International 

 Review, July-August 1876- See criticisms of Croll's work, by S. Newcomb, 

 Amer. Jour. Sci., Ill, xi, 263; J. J. Murphy, Quar. Jour. Geol. Soc., xxv, 350, 

 1869, abstract Amer. Jour. Sci., II, xlix, 115-18; Ch. Martins, Revue des Deux 

 Mondes, 1867; W. J. McGee, Popular Science Monthly, xvi, 810; C. B. Warring, 

 Penn Monthly, 1880. Further on this subject the reader may consult Le Hon, 

 L'llomme Fossile, pt. ii; Col. Dray son, Phil. Mag., 1871, abstracted in Amer. 

 Jour. Sci., Ill, ii, 304; Sir W. Thomson, Geological Climate, Trans. Geol. Soc., 

 Glasgow, February 1877, vol. v, pt. ii; James Geikie, Prehistoric Europe, 1880; 

 G. Pilar, Ueber die Ursache der Eiszeiten. 



