America to be as recent as in the north, and lastly, from there being some evidence of the former lower descent of glaciers all along the Cordilleras, I inferred that the whole world was at this period cooler. It did not appear to me justifiable without distinct evidence to suppose that the N. and S. glacial deposits belonged to distinct epochs, though it would have been an immense relief to my mind if I could have assumed that this had been the case. Secondly, do you believe that during the Glacial period in one hemisphere the opposite hemisphere actually becomes warmer, or does it merely retain the same temperature as before? I do not ask these questions out of mere curiosity; but I have to prepare a new edition of my "Origin of Species," and am anxious to say a few words on this subject on your authority. I hope that you will excuse my troubling you.
LETTER 510. TO J. CROLL. Down, January 31st, 1869.
To-morrow I will return registered your book, which I have kept so long. I am most sincerely obliged for its loan, and especially for the MS., without which I should have been afraid of making mistakes. If you require it, the MS. shall be returned. Your results have been of more use to me than, I think, any other set of papers which I can remember. Sir C. Lyell, who is staying here, is very unwilling to admit the greater warmth of the S. hemisphere during the Glacial period in the N.; but, as I have told him, this conclusion which you have arrived at from physical considerations, explains so well whole classes of facts in distribution, that I must joyfully accept it; indeed, I go so far as to think that your conclusion is strengthened by the facts in distribution. Your discussion on the flowing of the great ice-cap southward is most interesting. I suppose that you have read Mr. Moseley's recent discussion on the force of gravity being quite insufficient to account for the downward movement of glaciers (510/1. Canon Henry Moseley, "On the Mechanical Impossibility of the Descent of Glaciers by their Weight only." "Proc. R. Soc." Volume XVII., page 202, 1869; "Phil. Mag." Volume XXXVII., page 229, 1869.): if he is right, do you not think that the unknown force may make more intelligible the extension of the great northern ice-cap? Notwithstanding your excellent remarks on the work which can be effected within the million years (510/2. In his paper "On Geological Time, and the probable Date of the Glacial and the Upper Miocene Period" ("Phil. Mag." Volume XXXV., page 363, 1868), Croll endeavours to convey to the mind some idea of what a million years really is: "Take a narrow strip of paper, an inch broad or more, and 83 feet 4 inches in length, and stretch it along the wall of a large hall, or round the walls of an apartment somewhat over 20 feet square. Recall to memory the days of your boyhood, so as to get some adequate conception of what a period of a hundred years is. Then mark off from one of the ends of the strip one-tenth of an inch. The one-tenth of an inch will then represent a hundred years, and the entire length of the strip a million of years" (loc. cit., page 375).), I am greatly troubled at the short duration of the world according to Sir W. Thomson (510/3. In a paper communicated to the Royal Society of Edinburgh, Lord Kelvin (then Sir William Thomson) stated his belief that the age of our planet must be more than twenty millions of years, but not more than four hundred millions of years ("Trans. R. Soc. Edinb." Volume XXIII., page 157, 1861, "On the Secular Cooling of the Earth."). This subject has been recently dealt with by Sir Archibald Geikie in his address as President of the Geological Section of the British Association, 1899 ("Brit. Assoc. Report," Dover Meeting, 1899, page 718).), for I require for my theoretical views a very long period BEFORE the Cambrian formation. If it would not trouble you, I should like to hear what you think of Lyell's remark on the magnetic force which comes from the sun to the earth: might not this penetrate the crust of the earth and then be converted into heat? This would give a somewhat longer time during which the crust might have been solid; and this is the argument on which Sir W.