440 



NATURE 



[March lo, 1892 



Sir R. Ball's " Cause of an Ice Age." 



Some books appear under such authoritative sanction that, 

 apart altogether from their arguments and their facts, they 

 naturally influence opinion. This must be said of a book re- 

 cently reviewed in your pages (January 28, p. 289) ; namely, 

 " The Cause of an Ice Age," by Sir Robert Ball, the first of a 

 series on modern science, edited by Sir John Lubbock. 



The position taken up in this work is so much at issue with 

 the views of many prominent geologists, and its general tend- 

 ency seems so retrograde, that I am a little surprised it has not 

 been adversely criticized. 



I do not propose in this letter to enter into the general question 

 as to the astronomical causes of an Ice Age, or whether an Ice Age 

 can be shown to be a consequence of a vavying eccentricity, upon 

 which Croll and others have spoken very emphatically. I would 

 rather limit myself to the particular new factor which Sir R. 

 Ball has added to the problem. He claims that he has shown, 

 and I do not contest the matter in any way, that, " of the total 

 amount of heat received from the sun on a hemisphere of the earth 

 in the course of a year, d}, per cent, is received during the sum- 

 mer, and 37 per cent, is received during the winter." This law he 

 claims as " the fundamental truth which is the cardinal feature 

 of his book, ... the one central feature by which it is to be 

 judged." His chief object, he says, " is to emphasize the rela- 

 tion of these figures to the astronomical theory, which will be 

 entirely misunderstood unless the facts signified by these 

 numbers are borne in mind." 



What I wish to point out is that, although I have read the 

 book more than once, I cannot find how this law is in any way 

 connected with the general conclusions of the book. 



"The cause of an Ice Age " must surely be something which 

 is not always present and always equally efficient, but which 

 works differently at different times, which, if operating at one 

 time to produce an Ice Age, must either lose its effectiveness or 

 be otherwise modified so as to permit of the existence of a 

 temperate climate at another time. 



Sir Robert Ball admits without doubt that the factor he relies 

 upon, instead of being a variable one, is constant. He says : 

 _" The datum in our system on which the distribution depends, 

 is the obliquity of the ecliptic " ; and he goes on to say that, 

 •'amid so much that is changeable in the planetary system, it 

 is fortunate that the obliquity of the ecliptic may for our present 

 purpose be regarded as practically constant" {op. cit., 87). 

 He then goes on to compare the conditions which follow a 

 small and a large eccentricity, and says: "Notwithstanding 

 the wide difference between such a movement and that pre- 

 viously considered " {i.e. between movement in a very oblate and 

 one in a more prolate ellipse), "it still remains true that 63 per 

 cent, of the sun's heat is received by each hemisphere in 

 summer, leaving only 37 percent, for the winter" {ib., 92) He 

 again tells us that the figures 63 and 37 are independent 

 both of the eccentricity of the orbit and of the position 

 of the line of equinoxes ; and that while the varying eccen- 

 tricity created a distinction between a possible winter of 

 199 days and a summer of 166 days in one hemisphere, and 

 the reversal of these same proportions in the other, that in each 

 case the figures 63 and 37 represent the proportional quan- 

 tities of heat which that hemisphere receives in summer and 

 winter respectively" (z;^., 99). Lastly, speaking of the same 

 figures, he says " they derive their importance from their con- 

 stancy ; they would remain the same however the dimensions 

 of the orbit be altered, however its eccentricity be altered, or in 

 whatever direction the plane of the earth's equator may intersect 

 the plane of the earth's revolution around the sun." "These 

 numbers are both functions of but a single element, which is 

 the obliquity of the ecliptic. As this fluctuates but little, at 

 least within the periods that are required for recent Ice Ages, 

 the numbers we have given are regarded as sensibly constant 

 throughout every phase through which the earth's orbit has 

 passed within Glacial times" {ib., 121): 



These statements are explicit enough, and they show that the 

 factor upon which Sir R. Ball relies is a constant factor, and 

 being constant under all circumstances it cannot be the cause 

 of an Ice Age. Whatever potency it has is being exerted 

 now as much as it would be then. If it were an efficient 

 cause of an Ice Age, we ought to be passing through one 

 now. This argument seems to me to be complete and 

 conclusive, and, if so, I cannot see how Sir R. Ball has 

 done anything at all to solve the problem ; for, putting this 

 factor aside, we are remitted back to the conditions present to 



NO. I 167, VOL. 45] 



Croll and others, which have been so completely shown to be 

 inadequate to produce an Ice Age. As I am writing a big book 

 in which I am attacking what I deem to be the extravagant and 

 fantastic views of an influential school of geologists in regard ta 

 the so-called Ice Age, I naturally looked forward to Sir R, 

 Ball's book with interest, and have read it with care, but I cannot 

 see how it advances the solution of the problem, or how its^ 

 position can be maintained. 



Henry H. Howorth. 

 House of Commons, February 13. 



The University of London. 



Mr. Thiselton-Dyer, in his recent discussion ot the 

 London University question (p. 392), makes one statement 

 which seems to me open to criticism : — 



The statement is that the Colleges of the English Universities 

 have "intrusted the business of sampling their goods to those 

 who had nothing to do with their manufacture." Of the in- 

 ternal mechanism of the University of Oxford I know nothing ; 

 but I do know that in Cambridge the tendency is, and has been 

 for the last ten years, in the direction of the reconstruction of 

 that " teacher-examiner system " which Mr. Thiselton-Dyer 

 believes to have been given up. The higher teaching in 

 Cambridge is falling more and more completely into the hands 

 of three classes of men, namely : — 



(i) Professors, appointed by the University, and imposed by 

 the University upon the Colleges, so that in each College there 

 is at least one person who is a member of the body simply by 

 virtue of his University office. In this way at least one subject 

 is represented in every College by a University officer. 



(2) University Readers and Lecturers, who give systematic 

 instruction to all members of the University, without distinction 

 of College. As these men are on the one hand appointed by 

 the University, and are on the other hand, as a rule, members of 

 various Colleges, they establish a further bond of union between 

 the Colleges and the University. 



(3)- College Lecturers, who are now in the habit of throwing 

 open their lectures to members of Colleges other than their own, 

 and who are frequently members of the University Boards of 

 Studies. 



In this way the higher teaching is being thrown more and 

 more completely into the hands of men who are under the direct 

 control of the University itself; and a study of the current 

 Calendar shows that the task of examining students is intrusted 

 largely to these very men. Of the examiners for the various 

 Triposes (of whom there are about eighty), at least fifty-six 

 belong to one of the three categories above mentioned. Those 

 examiners who are non-resident, or who are not engaged in 

 teaching, act as a rule in conjunction with colleagues who are 

 actual teachers, so that there is no single Tripos in which a 

 student is not fairly certain to be "sampled" (to use Mr. 

 Thiselton-Dyer's phrase) by a man who has had a great deal to 

 do with his " manufacture." 



This is almost precisely the "teacher-examiner system" to 

 which Mr. Thiselton-Dyer refers ; and the steady growth of this 

 system in Cambridge is a certain proof that it is not incom- 

 patible with the development of the highest type of University 

 in England. W. F. R. Weldon. 



University College, London, February 27. 



The Aneroid in Hypsometry. 



From a review under this title in Nature of the nth ult. 

 (p. 339) it appears that Mr. Whymper has done good service to 

 those who use the aneroid in measuring heights, by pointing out 

 a very serious source of error in this instrument. According to 

 the reviewer : — " All who have had any experience in testing 

 aneroids in the usual way, viz. by subjecting them to gradually 

 reduced pressures under the air-pump, and comparing their read- 

 ings with the concomitant indications of the manometer, are 

 aware that the variations of the two instruments with falling and 

 then with increasing pressures are by no means concordant ; but 

 it will be probably new to most that, when the aneroid is allowed 

 to remain for some weeks under the reduced pressure, its in- 

 dications continue falling, and to such an extent that its final 

 error in certain cases is five or six times as great as when the ex- 

 haustion was first completed. On the other hand, aneroids that 

 have been kept for some weeks at a low pressure when restored 

 to the full pressure of the atmosphere take many weeks to regain 



