34 



NATURE 



[November 8, 1894 



loss of sun-heat would not alTect our winter temperature 

 very much, and the first argument is wrong ; if i: be all 

 due to sun-heat, then the first argument is right, and the 

 second wrong. 



Nor do we find much greater accuracy in Sir R. Ball's 

 exposition of the theory. He is, indeed, much fairer than 

 Croll in taking the winter temperature as proportional to 

 the inrraij-t' daily supply of winter sun-heat, instead of the 

 Midwinter Day sun-heat, for it is evident that the adjust- 

 ment of temperature to sun-heat could not take place 

 instantaneously. But in another particular he seems 

 greatly to undirstale the case for the theory. His method 

 of calculating the average daily sun-heat is to take the 

 winter heat over the whole northern liemisfthere, and 

 divide it by the number of days in winter, and similarly 

 for the daily summer sun-heat. He applies the average 

 thus obtained to calculate variations in temperature in the 

 latitude of the British Islands. But when we remember 

 that the theory of the Ice Age is the theory of the tem- 

 perature of the latitudes from about 45^ N. to lat. 

 70^ X.,or, if you like, to the pole, it appears quite mislead- 

 ing to use numbers obtained from the sun-heat received 

 by the whole hemisphere. For the proportion which the 

 total winter sun-heat we leceive in these Isles bears to 

 the total summer-heat is expressed, not by Ball's numbers 

 37 and 63, but by the very ditVerent numbers 25 and 75. 

 The great disparity between these numbers, contrasted 

 with the temperate character of our climate, enables us to 

 see how futile it is to appeal to our imagination, as Ball 

 does, to conceive what vast ditTerences of climate must 

 be produced by differences in the daily receipt of sun-heat. 



" If," he says, "a double supply of heat [63 measures] 

 be poured in like a torrent during the short season 

 [the 166 days of the short summer] while the single 

 supply [37 measures] is constrained to do duty over the 

 long season [the 199 days of the long winter], then an 

 intolerable climate is the result. The total quantity of 

 heat received on the hemisphere in the course of a year 

 is no doubt the same in each case, but its unsuitable 

 distribution bespeaks a climate of appalling severity — an 

 Ice Age, in fact." 



How untrustworthy this style of argument is, will appear 

 when It is pointed out that in order to get a latitude m 

 which as large a proportion as 37 per cent, of the annual 

 sun-heat is receivtfd in the coldest 199 days, we have to go 

 as far south as Madrid, Naples, Constantinople, New 

 York, or Pekm I Yet «e are asked to believe that this 

 distributior, appioximately two measures in 166 day?, and 

 one in 199 days, will produce '"a climate of appalling 

 severity— an Ice Age, in fact." ("Cause of an Ice Age," 

 P- 135) 



There is another form in which the numerical method 

 is applied by Ball, the result of which, so far from sup- 

 porting the astronomical theory, would, if true, appear 

 to me to be conclusive against it. The present mean 

 annual range of temperature in Great Britain is about 

 20 F., and this, according to Ball, is caused by the dis- 

 parity in the daily receipt of winter and sumtner sun- 

 heat, acting agamst the mitigating causes. In the epoch 

 of k;reat eccentricity the disparity will be much greater, 

 and instead of the range of 20' F. we shall have a range 

 of 28' F. (•' Cause of an Ice Age," p. 151.) Ball then 

 goes on to say that such proportionate changes "are 

 quite large enough to imply profound differences in the 

 climatic condition. It is to be observed that, generally 

 speaking, the coldest places are those of the greatest 

 mean annual range. We are therefore entitled to infer 

 that the effect of such a change in the eccentricity as wc 

 have supposed, would be to increase the range, lower the 

 temperature of the hemisphere, and thus induce the 

 glacial period." 



One would not consider such a statement out of place 

 in a popular series if it embodied the result of an inquirv 

 too complicated 10 be explained except in technical 



NO. 1306, VOL. 51 j 



language ; but that is not the case here, nor can the 

 conclusion be admitted as in the slightest degree probable. 

 In fact, so far from our being entitled to infer that such 

 an increase in the mean annual range would induce a. 

 glacial period, it appears to me that the mere fact that in 

 all continental climates north of Lat. 40", the present 

 range is greater than 28° F., entitles us to infer that such 

 a range would have no power whatever to induce an 

 Ice Age. 



The problem of ascertaining the elTect of different 

 astronomical conditions upon terrestrial temperatures is 

 too complicated for accurate solution. How far the tem- 

 perature at any place depends on the sun-heat falling on 

 the outer layers of the atmosphere at the place (which is 

 all that we can lind by calculation), and how far on the 

 transference of heat by ocean or air currents, must always 

 remain to some degree uncertain, but that the latter 

 exert a preponderating influence seems evident for two 

 reasons — first, that while the sun-heat in each season 

 remains the same from year to year, the seasons them- 

 selves vary enormously (we have cold summers and 

 hot summers, warm wmters and cold winters, all with 

 unchanged conditions of sun-heal) ; aiul second, the 

 difference between summer and winter temperatures is, 

 in northern latitudes, but slight when compared with the 

 difference between the quantities of winter and summer 

 sun-heat received. Hence it appeared to me that no 

 modification of CroU's method of calculating differences 

 of temperature due to dilTeiences of sun-heat could be 

 relied on, for our knowledge of the transference of heat 

 from one region to another is too slight to enable us to 

 allow for its effect in our equations. But there is another 

 method which seems very reliable, especially when applied 

 to the British Isles, or any region where warm ocean 

 currents flow from the south. Not, indeed, that the 

 method enables us to calculate the loweiing, if any, of 

 temperature in the epoch of great eccentricity, but it 

 appears to enable us to fix, with soine degree of certainty, 

 a limit to the direct effect of the diminished winter sun- 

 heat. 



The method depends on coinparing those regions 

 which now receive given allowances of summer and 

 winter sun-heat with the regions which, in the epoch of 

 great eccentricity, received the same allowances. If, fol- 

 lowing Croll, we suppose the temperature on Midwinter 

 Day to depend on the sun-heat received on that day, we 

 find that latitudes 90, 80', 70°, 61 , 52, and 43" no»v 

 receive the same sun-heat on Midwinter Day as latitude-* 

 90^, 80 , 70^, 60 , 50 , and 40 received on the Midwinter 

 Day of the most extreme eccentricity. In other words, 

 instead of Dr. Croll's fall of 45^ F. (I omit his decimal 

 point), the midwinter temperature of London would, in 

 the supposed glacial epoch, be lowered to that of Man- 

 chester at present, for Manchester is about r" north of 

 London. If, following Ball, we take the average daily 

 heat in winter as the basis of comparison, we should find 

 that lalitudesgo , Si ,71 -3,61 7, 52 4, and 43"-3 receive 

 in the present winter the same daily average of sun-heat 

 as latitudes 90", So , 70 , 60', 50 , and 40 received in the 

 long winters of greatest eccentricity. ( r, finally, if we 

 adopt the hypothesis, too favourable to the astronomical 

 theory, that the widwinier temperature depends on the 

 daily average through the 199 coldest days of the year, 

 we find that latitudes 84 5, 74% 63-5, 54', and 44°'2 

 ought now, so far as direct sun-heat is cimcerned, to 

 have the same midwinter temperature as 80', 70', 60', 50% 

 and 40 had in the supposed glacial epoch ; and when 

 it is observed that the summers in these latitudes were 

 then considerably hotierthan the summers in the former 

 latitudes, 84'-5, 74^63 5,54 , and 44 2 now aie, the utter 

 inadequacy of the astronomical theory to explain the 

 vast dilTeiences in temperature must surely be admitted 

 by any reasonable mind. 



But when we take account of the ocean currents, it 



