Date of the Glacial and the Upper' Miocene Period. 3G5 



unless the extent of that increase was very great*. Arago, for 

 example, states that so little is the climate of our globe affected 

 by the excentricity of its orbit, that even were the orbit to become 

 as excentric as that of the planet Pallas (that is, as great as 0*24), 

 "still this would not alter in any appreciable manner the mean 

 therrnometrical state of the globe." 



This idea, supported by these great authorities, got possession 

 of the public mind ; and ever since it has been almost universally 

 regarded as settled that the great changes of climate indicated 

 by geological phenomena could not have resulted from any 

 change in the relation of the earth to the sun. 



There is, however, one effect that was not regarded as com- 

 pensated. The total amount of heat received by the earth is 

 inversely proportional to the minor axis of its orbit ; and it fol- 

 lows, therefore, that the greater the excentricity, the greater is 

 the total amount of heat received by the earth. On this account 

 it was concluded that an increase of excentricity would tend to 

 a certain extent to produce a warmer climate f. 



All those conclusions to which I refer, arrived at by astrono- 

 mers, are perfectly legitimate so far as the direct effects of ex- 

 centricity are concerned ; and it was quite natural and, in fact, 

 proper to conclude that there was nothing in the mere increase 

 of excentricity that could produce a glacial epoch. How unna- 

 tural and even absurd would it have been to have concluded that 

 an increase in the quantity of heat received from the sun should 

 lower the temperature and cover the country with snow and ice. 

 Neither would excessively cold winters followed by excessively 

 hot summers produce a glacial epoch. If any person had asserted 

 that the purely astronomical causes could produce such an effect, 

 he would certainly and deservedly have been regarded as a fool. 



The important fact, however, was overlooked that, although 

 the glacial epoch could not result directly from an increase of 

 excentricity, it might nevertheless do so indirectly. As was 

 stated on a former occasion, the glacial epoch was not due 

 directly to an increase in the excentricity of the earth's orbit, but 

 to a number of physical agents that were brought into operation 



* Herschel in "Treatise of Astronomy," Cabinet Cyclopaedia, § 315; 

 Outlines of Astronomy, § 3(58. 



Arago in the Annuaire for 1834, p. 199. Edinb. New Phil. Journ. for 

 April 1834, p. 224. 



Poisson in Covnaissance des Temps for 1836, pp. 38-54. 



Humboldt in Cosmos, vol. iv. p. 459. Physical Description of the 

 Heavens, p. 33(5. 



t Herschel in " Discourse on the Study of Natural Philosophy," § 1 40, 

 Trans, of Geol. Soc. of London, vol. iii. p. 297 (2nd scries). 



Lyell in ' Principles of Geology/ p. 12(5, /th edit. 



Professor Haughton in Phil. Mag. for May 1866. 



