220 



NA TURE 



[January 9, 1896 



LETTERS TO THE EDITOR. 



f The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. '\ 



The Cause of an Ice Age, 



Several letters from Sir Henry Howorth, Dr. Hobson, Mr. 

 •Culverwell, and Prof. Darwin, having appeared in Nature 

 relating to my little book on the " Cause of an Ice Age," I shall 

 be glad if you will allow me to make a few remarks on the 

 matter. In his first letter, Sir Henry Howorth thinks I have 

 omitted to give Wiener the credit which was justly his due. 

 Subsequent letters by Dr. Hobson and Sir Henry Howorth may 

 be held to have cleared up this matter ; still there is a point 

 which has escaped Sir Henry Howorth's attention, and I there- 

 fore refer to it again. 



The facts are as follows. When I first began to work at the 

 Ice Age I arrived independently, as any mathematician might 

 easily have done, at a theorem by no means difficult, which 

 seemed to me of importance in connection with the subject of 

 geological climates. I had never seen this theorem before ; had 

 I done so I should, of course, have properly acknowledged its 

 prior discovery. 



Soon after the publication of my book, Prof. Darwin kindly 

 pointed out to me that the mathematical theorem in question 

 had been already given by Wiener. Thereupon I did all that it 

 seemed possible to do. I called attention to Wiener's priority 

 -at once by a letter to Nature, which appeared on February i8, 

 1892, arid I also mentioned his priority both in the preface and 

 the text of the second edition of the " Cause of an Ice Age," 

 which was published in 1892. Sir Henry Howorth, when he 

 wrote his recent letters in which he thought I had not rendered 

 justice to Wiener, could not, I am sure, have known all the 

 facts as above stated. 



Mr. Culverwell thinks that I was wrong in attributing a certain 

 opinion to Croll, and I quite admit that this charge might once 

 have been correct. The fact is, I had been mistaken in the 

 meaning I read into a passage in Croll's "Climate and Time," 

 p. 56. But I think if Mr. Culverwell had known the circum- 

 stances, he would hardly have considered it necessary to raise 

 this question again. On the appearance of the first edition 

 of my book, the mistake I had made was kindly pointed 

 ■out by Mr. Monck, as well as by Mr. Noble, and I think 

 by others ; and I accordingly amended the second edition. In 

 the Geological Magazine for February 1895, P 5^, Mr. Culver- 

 well appeals to me to correct certain passages relating to this 

 point which he puts into italics from the first edition. My 

 excellent friend had not the slightest notion that these passages 

 liad been already corrected in the second edition, published two 

 •years before his paper. 



I must, however, say that on looking over my book again in 

 ■connection with this correspondence, I consider that some of 

 -the references I have made to this particular point might be 

 further amended. If, however, Sir Henry Howorth still thinks 

 'that I have at any time regarded Croll's work otherwise than 

 with due respect, I would like to remind him of the words in 

 "both editions, p. 112, in which I said : — 



"I was greatly struck by this work {'Climate and Time') 

 when I first read it many years ago. Subsequent acquaintance 

 with this volume, and also with his second work ( ' Climate and 

 Cosmology'), has only increased my respect for the author's 

 scientific sagacity, and my admiration for the patience and the 

 skill with which he has collected and marshalled the evidence 

 for the theory that he has urged so forcibly." 



I have studied with, much interest and profit the investigations 

 made by Mr. Culverwell in connection with the astronomical 

 theory of the Ice Age, and I may be permitted to say how glad 

 I am that so excellent a mathematician and physicist should 

 have had his attention drawn to this subject. I may, however, 

 take this opportunity to explain why I have had to remain 

 -unconverted by certain of his arguments, notwithstanding that 

 they have carried conviction to Sir Henry Howorth and Prof. 

 Darwin. 



In his earlier paper in the Geological Magazine for January 

 1895, P- ?> Mr. Culverwell has demonstrated that the direct sun- 

 heat received on any parallel at the time of greatest eccentricity, 

 as the same as that now received on the parallel not more than 



Na 1367, VOL. 53] 



three or four degrees north. This seems to me not only a novel, 

 but also a very instructive result, and is in any case a valuable 

 contribution to the theory. Mr. Culverwell, however, goes on 

 to deduce from this that the climatic change in England between 

 the present time and the time of the greatest eccentricity, would 

 be no greater than the present climatic difference between 

 Yorkshire and Cornwall, and hence he concludes that the astro- 

 nomical theory is incompetent to account for the Ice Age. 

 Prof. Darwin seems to think that this argument is unanswer- 

 able ; I hope he will forgive me if I say that here my dissent 

 begins. I think the facts cited do not warrant the inference 

 which Mr. Culverwell would draw from them. With due 

 respect to Mr. Culverwell, I would say that he seems at this 

 point to have quite forgotten that the actual temperature in a 

 region depends not merely upon the sun-heat there receivetl, but 

 also upon the transference of heat across the boundaries of that 

 region. He takes the actual temperatures of Yorkshire and 

 Cornwall ; but what his argument would really require is a 

 totally different thing. It would be the temperatures of those 

 counties if each of them were perennially surrounded by a wall 

 extending to the top of the atmosphere, and adiabatic to all heat 

 except direct solar radiation. 



This point is so important that I must put it in a somewhat 

 different manner. It is certain that the actual climatic gradient 

 from the equator to the pole is very different from what that 

 gradient would have been if each parallel of latitude had marked 

 the course of an adiabatic barricade such that no heat transfer- 

 ence via earth, air or water could take place from zone to zone. 

 In the latter case I quite admit that the.mean temperature due to 

 the sun-heat received on any zone would be actually the mean 

 temperature of that zone, but the same is not true of the actual 

 climatic gradient as we have it in nature. For, on account ot 

 heat transference, the mean temperature of a zone is by no means 

 the same thing as the mean temperature due to the sunbeams 

 received by that zone. 



May I say that I think the fallacy throughout this part of Mr. 

 Culverwell's argument arises from his overlooking the distinc- 

 tion between the actual gradient and the adiabatic gradient. 

 There may be but little difference between the mean tempera- 

 tures of a zone through Yorkshire, and a zone through Cornwall ; 

 but this does not prove, as Mr. Culverwell's theory requires, 

 that there would be but little difference between a mean tem- 

 perature due solely to the direct sun-heat falling on the zone 

 through Yorkshire, and a mean temperature due solely to the 

 direct sun-heat falling on the zone through Cornwall. This 

 inference would only be sound if all parallels were adiabatic. 

 This they certainly are not. 



I do not question that the difference between present 

 temperatures and the temperatures at the time of highest eccen- 

 tricity might be fairly represented by the difference between the 

 temperature due to the sun-heat received in the latitude of 

 Yorkshire, and the sun-heat received in the latitude of Cornwall. 

 What I do question are the grounds on which Mr. Culverwell 

 maintains that this latter difference (and therefore the former 

 one) is so insignificant as to discredit the astronomical theory of 

 the Ice Age. 



I have thus explained in what respect Mr. Culverwell's invest- 

 igation involves assumptions which are in my opinion unsound. 

 I am accordingly to this extent unable to accept the conclusions 

 at which he has arrived. Robert S. Ball. 



Observatory, Cambridge, January 2. 



The letter of Prof. G. H. Darwin in your last issue states 

 very clearly the argument on which Mr. Culverwell and himself 

 rely as affording a demonstration of the inadequacy of the 

 astronomical theory. It now seems opportune, therefore, to lay be- 

 fore your readers the general considerations which lead me to the 

 conclusion that the whole argument they rest upon is unsound ; 

 and, further, that Sir Robert Ball's ratio of 63 to 37, represent- 

 ing the ratios of sun-heat received by each hemisphere in 

 summer and winter respectively, is (contrary to Prof. Darwin's 

 view) an important factor in any adequate discussion of the 

 problem. 



Accepting Prof. Darwin's estimate that the difference in the 

 amount of sun-heat received in our latitudes during high and 

 low eccentricity, would only give to Yorkshire the amount 

 received by London or vice versa, I entirely demur to his 

 statement that this would be also a measure of the amount of 

 change in the climates of these places. To do so is to assume 

 that the climate of a place, as regards the amount and distribution 



