NATURE 



121 



THURSDAY, JUNE 17, 1875 



CROLLS ''CLIMATE AND TIME" 



Climate and Time in their Geological Relations j a theory 

 of Secular Chanties of the Earth^s Climate. By James 

 Croll, of H.M. Geological Survey of Scotland. (London : 

 Daldy, Isbister, and Co., 1875.) 



MR. CROLL is well known as an original thinker of 

 considerable power, who has turned his attention 

 to the physics of geology, and has produced a series of 

 remarkable papers on questions of the highest interest in 

 that subject. His views are opposed in many respects to 

 those accepted by other influential thinkers, and have given 

 rise to a considerable amount of controversy. Hitherto 

 they have been scattered in papers to various periodicals, 

 and it has been difficult to obtain a consecutive view of 

 them. The work which is now issued, while not an 

 actual reprint of previous papers, is a complete exposi- 

 tion of their contents, or at least of that part of their 

 contents that Mr. Croll is prepared to stand by, some of 

 the arguments that occur in his papers being omitted in 

 his book. We are now therefore able to judge fairly what 

 truth there is in Mr. CroU's ideas, and to compare them 

 with those of his opponents. Even were all his ideas un- 

 tenable, we should still have to thank him for his 

 vigorous discussion of these interesting questions, but 

 there can be no doubt that in many instances he proves 

 his point. 



Mr. Croll does not possess the happy faculty which 

 some authors have of carrying his readers with him : on 

 the contrary, his style is so controversial, that to agree 

 with him is to have the feeling of being vanquished, 

 and the reader is throughout set on his metal to find 

 out some flaw in the argument. This, as in most cases 

 of controversy, it would not be difficult to do ; but we 

 must confine ourselves to the discussion of his main 

 results. 



One peculiarity of Mr. Croll's arguments must here be 

 noticed. After having assumed certain figures and 

 arrived by their means at definite results, he proceeds to 

 show that these figures are unreliable, and then to state 

 that their unreliableness will not affect his results ; or else, 

 in order to bring his results more into accordance with 

 received opinions and probable facts, he generously halves 

 them or diminishes them still more, apparently unaware 

 that had his arguments been correct and his first results 

 the true ones, he would have proved too much and 

 refuted himself. Examples of this peculiarity will be 

 seen in the sequel. 



The first question discussed is the heating influence of 

 the Gulf Stream. To estimate this Mr. Croll uses the 

 method of heat units, and prides himself on doing so. 

 The method is an undeniable one, and is perhaps the 

 only one by which the influence of the high specific heat 

 of water can be made manifest. Mr. Croll compares the 

 number of foot-pounds conveyed by the Gulf Stream into 

 temperate regions with the number due to the heat of the 

 sun shining directly on those regions. The relative value 

 of these depends on the absolute value of each. The 

 volume, velocity, and temperature of the Gulf Stream 

 have been very variously estimated ; and as to the sun's 

 Vol. xii.— No. 294 



heat, when we remember how much the diathermancy of 

 the air depends on its condition, we may not be able to 

 accept with such confidence as Mr. Croll, the estimates 

 of Pouillet ; yet with every possible allowance, when the 

 influence of a vast body of heated water is calculated, it 

 will undoubtedly be much greater than would have been 

 previously supposed, and actually amounts to a very con- 

 siderable fraction, say f\ of the whole of the sun's direct 

 heat on the North Atlantic. Dr. Carpenter* brings 

 objections against this method which render, in his 

 opinion, the " figures " " utterly valueless." The first of 

 these is that Mr. Croll does not give a correct account of 

 the diff"erence in temperature between the northern and 

 southern hemispheres in assigning it to the transport of 

 heated water by ocean currents ; but it is obvious that 

 the question as to where the Gulf Stream obtains its heat 

 is entirely distinct from that as to its actual amount. 

 The second objection, that since the temperature of the 

 ocean is seldom more than 82° — 86'', while the " direct 

 heat of radiation" may amount to 215°; and therefore 

 that " the heat lost by evaporation from the sea must be 

 far greater than that lost by radiation from the land," is 

 just one that shows the value of Mr. CroU's method. For, 

 when treated in this way, the above figures show that the 

 sea contains more heat units in its heated surface stratum 

 than the layer of land that is influenced by the variations 

 of surface- temperature, and that therefore the water at 

 the equator is, as Mr. Croll states, the best adapted for 

 retaining the heat of the sun, which is in reality no more 

 than an elementary result of its high specific heat. Mr. 

 Croll considers that the influence of the Gulf Stream is 

 indirect, being manifested by the warming of the S.W. 

 winds ; and to the extent that he proves that the Gulf 

 Stream raises the general temperature of the Atlantic he 

 cannot be wrong. Were he to confine himself to the 

 statement that the Gulf Stream and other ocean currents 

 have a very sensible influence on the climate of the tem- 

 perate regions, his position would appear to be impreg- 

 nable against any who should represent its thermal effect 

 " as very insignificant ; " but when he adds that " ocean 

 currents are the great agents employed " (to the exclusion 

 of others) " in the distribution of heat over the globe," and 

 estimates that the Gulf Stream alone raises the mean tem- 

 perature of London 40'', he stands upon less certain ground. 

 For these results depend on the following arguments : — 

 (i) There is no ocean circulation but that by sensible 

 currents ; (2) The internal heat of the earth has no 

 influence on climate ; (3) The temperature of space is 

 - 239° F. ; and (4), the Gulf Stream supplies \ as much 

 heat to the Atlantic as the direct rays of the sun. Of 

 these arguments we will below discuss the first at length. 

 The second is founded on a statement of Sir Wm. 

 Thomson's, that an increase of temperature as great as 

 2° F. per foot in descending into the earth would not have 

 an influence of more than 1° on the climate of the surface. 

 This, however, means 1° over the present mean tempera- 

 ture, and in no way disproves that the internal heat of the 

 earth does nothing in raising the temperature of its sur- 

 face over that of space, an effect which it most certainly 

 would have in a large degree. The third argument, as to 

 the temperature of space, is therefore nothing to the 

 point, and is moreover, as Mr. Croll himself admits, 



» Proceedings of the Ryal Society, June 13, 1872. 



H 



