290 



NATURE 



[January 28, 1892 



The question we have to ask is, If the orbit becomes 

 eccentric, how will the contrast of daily supplies be 

 affected ? 



In order to answer this, let us go at once to the 

 extreme, when the eccentricity of orbit is a maximum. 

 We learn that if aphelion is at midsummer, summer will 

 be 199 days, and winter 166 days ; and the converse is 

 true when the perihelion is at midsummer. 



Since 199 is to 166 nearly as 6 to 5, we see that with 

 midsummer perihelion there are 5 days of summer to 6 

 of winter, and with midsummer aphelion there are 6 of 

 summer to 5 of winter. 



Hence, with midsummer perihelion, the short summer 

 daily supply of heat may be taken as proportional to 

 ^(i + fl), and the long winter daily supply as propor- 

 tional to \{\ - a). Hence the contrast between the 

 short summer and long winter daily supplies is repre- 



sented by 



6(1 +«) 



; that is to say, the standard contrast 



5(1 -«) 



is augmented in the ratio of 6 to 5. Next, with mid- 

 summer aphelion, the long summer daily supply of heat 

 may be taken as proportional to \(i + <«)} and the short 

 winter daily supply as proportional to \{\ — a). Hence 

 the contrast between the long summer and short winter 



daily supplies is represented by ^^ *" ^ ; that is to say, 



the standard contrast is diminished in the ratio of 5 to 6. 



In the first case, the heat supply is less evenly dis- 

 tributed through the year than at present, and we have a 

 much more severe climate ; in the second, it is more 

 evenly distributed, and we have a much milder one. It 

 follows also that, if we compare the two extreme cases 

 together instead of both with the mean case, the change 

 of contrast is represented by the ratio of 6^ to 5^, or of 

 36 to 25. 



I must refer the reader to the able discussion in the book 

 of the effects which we have reason to suppose would flow 

 from a change of contrast represented by the numbers 36 

 to 25 ; and it must suffice to say here that it seems enough 

 to explain on the one hand the occurrence of the glacia- 

 tion of England, and on the other hand the occurrence of 

 sub-tropical plants in Greenland. 



Now, the above seems to me to be substantially the 

 argument in the book, but I dissent from the stress laid 

 on the numerical determination of the quantity a. On 

 p. 90 Sir Robert says : — 



" This theory will be entirely misunderstood unless the 

 facts signified by these numbers (the evaluation of (i -\- a) 

 -7- (i — a)) are borne in mind. No one can discuss the 

 astronomical theory of the Ice Age unless the figures 63 

 and 37 (5 and 3 are more accurate) form a portion of his 

 consciousness, and the refrain of his every argument." 



It may be admitted that it might have been more difficult 

 to present the argument in a popular form without assigning 

 a numerical value to a, but Sir Robert Ball is fully equal 

 to such a task ; and I contend that the numerical value of 

 a is beside the mark, even if a value, appropriate to the 

 investigation in hand, were attainable. 



After presenting his own view of the question. Sir 

 Robert Ball says (p. 134) that Croll does not seem to 

 have been really aware of the full strength of the astro- 

 nomical theory, and in this I entirely agree. Croll, in 

 fact, rather weakens than strengthens his position when 

 NO. 1 161, VOL. 45] 



he tries to trace in detail the action and reaction of the 

 astronomical cause, for in doing so he is led to maintain 

 various theses which are not susceptible of proof, and are 

 even highly doubtful. He thus takes as the central point 

 of his position one at which it appears to me to be 

 weakest. In 1886 I wrote : — 



" Adequate criticism of Mr. Croll's views is a matter of 

 great difficulty, on account of the diversity of causes 

 which are said to co-operate in the glaciation. In the 

 case of an effect arising from a number of causes, each 

 of which contributes its share, it is obvious that if the 

 amount of each cause and of each effect is largely con- 

 jectural, the uncertainty of the total result is by no means 

 to be measured by the uncertainty of each item, but 

 is enormously augmented. Without going far into 

 details, it may be said that these various concurrent 

 causes result in one fundamental proposition with regard 

 to climate, which must be regarded as the keystone of 

 the whole argument. That proposition amounts to this — 

 that climate is unstable. 



" Mr. Croll holds that the various causes of change of 

 climate operate inter se in such a way as to augment their 

 several efficiencies. Thus, the trade-winds are driven by 

 the difference of temperature between the frigid and 

 torrid zones, and if from the astronomical cause the 

 northern hemisphere becomes cooler, the trade-winds on 

 that hemisphere encroach on those of the other, and the 

 part of the warm oceanic current, which formerly flowed 

 into the cold north zone, will be diverted into the southern 

 hemisphere.^ Thus the cold of the northern hemisphere 

 is augmented, and this in its turn displaces the trade- 

 winds further, and this again acts on the ocean currents, 

 and so on ; and this is neither more nor less than in- 

 stability. 



" But, if climate be unstable, and if from some of those 

 temporary causes, for which no reasons can as yet be 

 assigned, there occurs a short period of cold, then surely 

 some even infinitesimal portion of the second link in the 

 chain of causation must exist ; and this should proceed, 

 as in the first case, to augment the departure from the 

 original condition, and the climate must change."^ 



I see no reason to depart from what I said five years 

 ago, but I now learn from this book how it is that Croll 

 mistook the strong points of his own theory, and that a 

 more forcible proof of it may be contained in a short 

 work than in an elaborate volume. After expressing this 

 opinion, it is but fair to quote and indorse the following 

 passage (p. 112) on Croll's famous work on " Climate and 

 Time":— 



" I was greatly struck," says the author, "by this work 

 when I first read it many years ago. Subsequent ac- 

 quaintance with this volume . . . 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." 



There" are a few other points in the "Ice Age," not 

 involved in the main line of argument, on which I should 

 like to comment. 



The method adopted of stating the disturbing forces 

 of the planets on the earth appears to me unduly sensa- 

 tional. We learn (p. 74) that the disturbing force of 

 Venus is 130 million million tons, and it is impossible 

 not to be impressed with the magnitude of the force. 

 But if we had been told that the disturbing force on each 

 pound of the earth's mass was only 1/7000 of a grain, 



I Ball (p. 134) fails to see th'e force of this argument. 

 ^ Bnt. Assoc. Keport, i886, Address to Section A. 



