[ W2 ] 





XVIII. On some Points in Climatology. A Rejoinder to 

 Mr. Croll. By Simon Newcomb*. 



IN the Philosophical Magazine for October 1883, page 241, 

 Mr. Croll publishes a reply to certain criticisms of mine 

 urged seven years ago against his theory of the cause of glacial 

 epochs (American Journal of Science, vol. xi. p. 263, May 

 1876). The pleasure and interest with which I have read 

 Mr. Croll's paper induce me to reply to it, notwithstanding 

 a want of confidence on my part in the value of anything 

 short of a purely mathematical investigation of the subject. 

 It will be well to begin by examining the nature of the ques- 

 tion, and stating in a broad way what seems to me unsatisfac- 

 tory in the foundation of Mr. CrolPs method. 



What we are concerned with is the inference that at some 

 former epoch in geological history the mean temperature of 

 the northern hemisphere was much lower than it is now. 

 Assuming this as the basis of discussion, the question is, what 

 was the cause of this "glacial epoch"? To speak more accu- 

 rately,, since we can only take the causes relatively, Why was 

 the northern hemisphere any colder then than it is now ? This 

 question Mr. Croll endeavours to answer from purely astro- 

 nomical causes, combined with elementary considerations 

 respecting the motion of heat and its relation to meteorolo- 

 gical phenomena. His conclusion is that a great eccentricity 

 of the earth's orbit, combined with a position of the perihelion 

 near the northern solstice, will cause a great annual fall of 

 temperature in the northern hemisphere, which in such a case 

 would have a short perihelion summer and a long aphelion 

 winter. 



To this my reply is, that too little is known of the laws of 

 terrestrial radiation of heat through the atmosphere to justify 

 the establishment of any theory of the glacial epoch, and that, 

 taking the case up exactly as Mr. Croll does, he fails to show 

 sound reason why the mean temperature should be different at 

 the supposed periods. At the same time my verdict would be, 

 not that Mr. Croll's thesis was false, but that it was not proven. 

 I do not deny the possibility that, when the laws of climate 

 become thoroughly known, it may be found that epochs of 

 great eccentricity are always glacial epochs. All I claim is 

 that if such should be proven to be the case, it will be through 

 the action of causes different from those adduced by Mr. Croll. I 



In fact, without going any further, we have at hand a vera 

 causa acting in this direction which has not been considered 



* Communicated by the Author. 



