276 Dr. J. Croll's Remarks on 



but may be allowed briefly to refer to those points on which I 

 have been so thoroughly misunderstood. 



Of course I fully concur in Professor NewcomVs opening 

 remarks as to the desirability of "a purely mathematical 

 investigation of the subject."" Such an investigation, how- 

 ever, is, I think, impossible at present. In a question so 

 complex and difficult as that of the cause of the Glacial Epoch, 

 depending as it does on the consideration of so many different 

 elements, some of which are but little understood, logical ana- 

 lysis rather than mathematics will require to be our instrument 

 in the mean time. The question must first assume a clear, 

 definite, and logical form before mathematics can possibly be 

 applied to it. 



Prof. Newcomb objects that my language is wanting in 

 quantitative precision — that I use such terms as "great," "very 

 great," "small," " comparatively small," and so forth without 

 any statement of the units of comparison relatively to which these 

 expressions are employed. No one reasoning on the combined 

 influence of a multitude of physical causes could well avoid 

 the almost continual use of such terms. Besides, my critic 

 forgets that in almost every case in which I use these terms 

 numerical exactness is not attainable ; and even if it were, it 

 would, as a rule, be of little service, seeing that the conclusion 

 generally depends on the simple fact that one quantity is less 

 or greater than another; not on how much less or how much 

 greater the one may be than the other. Although my argu- 

 ments are logical, few writers, I venture to say, have done 

 more than myself to introduce definite quantitative exactness 

 into the questions I have discussed. 



Prof. Newcomb gives his readers to understand that I 

 assume Newton's law of cooling to be correct ; and that I ap- 

 parently nowhere adduce the more correct law of Dulong and 

 Petit — viz. that if we take a series of temperatures in arith- 

 metical progression, the corresponding rates of radiation of 

 heat will not be in arithmetical progression, but in a series of 

 which the differences continually increase. If he will refer 

 to the ' Header, 7 Dec. 9, 1865, Phil. Mag. Feb. 1870, < Nature/ 

 April 1, 1880, and • Climate and Time ' (the book he reviewed), 

 p. 37, he will there see the question discussed at considerable 

 length. He will also find reference made to a remarkable 

 circumstance connected with radiation which perhaps may be 

 new to him. It is this: the law of Dulong and Petit (that as 

 the temperature of a body rises the radiation of the body 

 increases in a much higher ratio) holds true only of the body 

 considered as a mass. The probability is, as has been shown 

 by Prof. Balfour Stewart, that the individual particles com- 



