124 Mr. J. Croll on the Excentricity of the Earth's Orbit, 



were the sun's distance the only element that determined the 

 temperature, the summers would have been at least 45° warmer 

 than at present. But had such been the case, there could 

 have been no glacial epoch. For a summer so warm, notwith- 

 standing its shortness, would have been sufficient to melt the 

 snows of winter. In this case, the theory which attributes the 

 glacial epoch to an extreme condition of excentricity must be 

 abandoned. It was, no doubt, this which had led to the gene- 

 ral, if not universal, belief that the glacial epoch could not have 

 resulted from an augmentation in the excentricity of the earth's 

 orbit. And it is perfectly true that were it not for physical 

 agencies which are brought into operation by the change of ex- 

 centricity, and thus to a great extent alter the condition of 

 things, the astronomical causes would wholly fail in producing 

 a glacial epoch ; for the purely astronomical effects of excentri- 

 city, as has been clearly shown by Sir John Herschel*, Aragof, 

 Humboldt J, and others, are compensated by others of an oppo- 

 site character; so that excentricity, viewed from an astronomical 

 stand-point, does not appear capable of accounting for the gla- 

 cial epoch. In short, without having taken into consideration 

 the physical causes to which we refer, it was impossible that any 

 relation could ever have been perceived between a high condi- 

 tion of excentricity and the glacial epoch. 



There is one astronomical effect however, which is, not com- 

 pensated by an astronomical effect of an opposite character. 

 The total quantity of heat received from the sun per annum is 

 inversely proportional to the minor axis of the earth's orbit. 

 And this has been stated by Sir John Herschel to be an astro- 

 nomical vera causa of change of climate during geological epochs§. 

 But as the excentricity increases, the total quantity of heat re- 

 ceived from the sun increases also. Hence astronomy would 

 lead us to conclude with Prof. Haughton||, that a glacial epoch 

 ought to occur, not when the excentricity is at the superior, but 

 at the inferior limit. In a former paper I endeavoured to show 

 that this astronomical effect, which is but trifling^, is far more 

 than neutralized by causes of a physical nature, and that, instead of 

 an increase of excentricity producing an increase of temperature, 

 as had been generally considered, the very reverse is the case. 



One of the physical causes to which I refer is the presence of 



* Outlines of Astronomy, article 368. 



t Edinburgh New Philosophical Journal for April 1834, p. 224. An- 

 nuaire for 1834, pp. 199, 201. 



X Cosmos, vol. iv. p. 459. 



§ Discourse on the Study of Natural Philosophy, article 140. 



I| Phil. Mag. for May 1866. 



II The variation in the quantity of heat received from the sun per annum 

 can never exceed one three -hundredth part of the total amount. 



