January 3, 1895] 



NA TURE 



227 



« times the conductivity outside ; if the specific heat inside is 

 s times the specific heat outside ; if the density inside is d times 

 the density outside ; then Kelvin's age of the earth is increased 

 Ki (/times. . . It is not likely that Dr. Weber's rate of increase 

 would be constant to such a temperature as 4000 C. ; but the 

 electric analogue allows us to imagine a greater and greater rate 

 of increase at higher temperatures ; therefore it is in Lord 

 Kelvin's interest to take Weber's rate, Now at 4000" C. the 

 conductivity would be [leaving out eurite, which seems abnor- 

 mal and too much in my favour], thirty times as great as it 

 is at the surface ; the specific heat would be \\\ times as great, 

 and taking the density as three times, we have, even for a pei- 

 ft'ctly solid earth an age 1300 times the age given by Lord 

 Kelvin. 



2. In answer to your second question. Lord Kelvin com- 

 pletely destroyed the uniformitarian geologists, and not one 

 now exists. It was an excellent thing to do. They are as 

 extinct as the dodo or the great auk. 



I have met many advanced geologists, and not one of them 

 demands more than 1,000,000,000 years. Probably Sir Archi- 

 bald Geikie is the most representative of the geologists who 

 have studied this question, and he never (in recent years) seems 

 to have desired even as much as 1,000,000,000 years. (See his 

 address as President of the liritish Association.) The biologists 

 have no independent scale of time ; they go by geological time. 

 According to Huxley, less than 1,000,000,000 years is enough as 

 the age of life on the earth. 



But surely the real question now is not so much what the 

 geologists care about, as — Had Lord Kelvin a right to fix 10* 

 years, or even 4 x 10' years, as the greatest possible age of the 

 earth ? Yours truly, 



John Perry. 



Dcicmber 6, 1S94. 



Dear Prof. Tait, — Prof. Fitzgerald has pointed out to 

 me that the five rocks given by Everett are not to be found in 

 his 1891 edition. I quoted from his 1886 edition. I therefore 

 wrote to Everett, asking why he had left them out — was there 

 a mistake ? He writes to say : " I copied Weber's data from a 

 copy of his paper which wa.s, and may be still, in my posses- 

 sion, having been sent me through the post, probably by the 

 author, or possibly by Dr. Stapff, the geologist of the St. 

 Gothard Tunnel, with whom I had much correspondence in 

 underground temperature. You seem to assume, in writing to 

 Tail, that I picked out samples of Weber's results ; but my 

 recollection is that I gave everything without reservation. 



" I did not reproduce his results in the 1891 edition, and I 

 cannot remember all my reasons lor dropping them. On com- 

 paring them with other people's, which I give, they appear to 

 be much too small. There is such a mass of conduction results 

 in my book, that I was on the look-out for something that might 

 be omitted. 



"I have just referred to the foreign translations of my book. The 

 German edition, published in 1888, gives only a page of conduc- 

 tivities of solids, and includes among them one of R. Weber's, 

 namely Glimmerschiefer 000733 + 'ooooio t. The Russian 

 edition, brought out by editors who took tremendous pains in 

 verifying and correcting references, gives my list of Weber's 

 results exactly as it stands in my book, the sign of the 

 temperature coefficient being positive in every case. I do not 

 know of any direct evidence as to the variation of rock con- 

 ductivity with temperature except R. Weber's, but there is 

 something approaching to direct evidence in the comparison of 

 George Forbes' results with Herschel and Dunn's (see my 1891 

 edition, pp. 126, 129). Forbes found at - 10° C. the con- 

 ductivity of white marble to be '00115, black marble '00177. 

 Dunn and Herschel found at the temperature of hot water, 

 marbles, &c., 0047 to 0056 (see Forbes' remark, quoted at p. 

 129). 



" You have built a very lofty edifice on the basis of Weber's 

 results, and extrapolation is proverbially a risky process, but I 

 consider you have established a strong presumption in favour 

 of the increase of rock conductivity with temperature." 



I did not know, when writing to you on November 26, that 

 the Rev. M. H. Close, M.A., had (K. Dublin Soc, Feb. 1878) 

 put forward in great detail the reasons which I gave you 

 shortly, against the tidal retardation argument. I thought they 

 were my own. I notice that this gentleman assumes that in- 

 creased conductivity inside would help Lord Kelvin, and indeed 

 1 cannot help thinking that, without mathematics, almost any- 



NO. 1314, VOL. 51] 



body would be of the same opinion — in spite of whit you say in 

 your first letter. I know that Lord Kelvin himself did not seem 

 to think me right when — after I had sent him the documents— 

 I talked to him at Cambridge. 



I remain, yours truly, 



I"HN Perrv. 



Copy of a Letter from Lord Kelvin. 

 The University, Glasgo-M, December 13, 1894. 



Dear Perry,— Many thanks for sending me the printed 

 copy of your letter to Larmor and the other pipers, which I 

 found waiting my arrival here on Saturday evening. I have 

 been much interested in them and in the whole question that 

 you raise, as to the effect of greater conductivity and greater 

 thermal capacity in the interior. Your «- ^ m theorem is 

 clearly right, and not limited to the case of the upper stratum 

 being infinitely thin. Twenty or thirty kilometres tnay be as 

 good as infinitely thin for our purposes. But your solution on the 

 supposition of an upper stratum of constant thickness, having 

 smaller conductivity and smaller thermal capacity than the 

 strata below it, is very far from being applicable to the true case 

 in which the qualities depend on the temperature. This is 

 a subject for mathematical investigation which is exceedingly 

 interesting in itself, quite irrespectively of its application to the 

 natural problem of underground heat. 



For the natural problem, we must try and find how far Robert 

 Weber's results can be accepted as trustworthy, and I have 

 written to Everett to ask him if he can send me the separate 

 copy of Weber's paper, which it seems was sent to him some 

 time before 18S6; but in any case it will be worth while to 

 make farther experiments on the subject, and I see quite a 

 simple way, which I think I must try, to find what deviation 

 from uniformity of conductivity there is in slate, or granite, or 

 marble between ordinary temperatures and a red heat. 



For all we know at present, however, I feel that we cannot 

 assume as in any way probable the enormous differences of con- 

 ductivity and thermal capacity at different depths which you 

 take for your calculations. If you look at Section 11 of 

 " Secular Cooling " (" Math, and Phys. Papers," vol. iii. p. 

 300), you will see that I refer to the question of thermal con- 

 ductivities and specific heats at high temperatures. I thought 

 my range from 20 millions to 400 millions was probably wide 

 enough, but it is quite possible that I should have put the 

 superior limit a good deal higher, perhaps 4000 instead of 400. 



The subject is intensely interesting ; in fact, I would rather 

 know the da'e of the Consistentior Status than of the Norman 

 Conquest ; but it can bring no comfort in respect to demand for 

 time in Palaeontological Geology. Ilelmholtz, Newcomb, and 

 another, are inexorable in refusing sunlight for more than a score 

 or a very few scores of million years of past time (see " Popular 

 Lectures and Addresses," vol. i. p. 397). 



So far as underground heat alone is concerned you are quite 

 right that my estimate was 100 millions, and please remark 

 (" P. L. and A.," vol. ii. p. 87) that that is all Geikie wants ; but 

 I should be exceedingly frightened to meet him now with only 

 20 million in my mouth. 



And, lastly, don't despise secular diminution of the earth's 

 moment of momentum. The thing is too obvious to every one 

 who understands dynamics. 



Yours always truly, 



Kelvin. 



JUPITER. 



JUPITER being now near opposition, and having an 

 apparent diameter of 47 ■/, is displayed as a very 

 brilliant object in the heavens, and his northerly 

 declination of 23 degrees enables him to remain above 

 the horizon for a period of i6j hours. 



During the few ensuing months, the observation of his 

 belts and spots will enlist a large amount of attention, 

 for there is probably no other planetary object which 

 exhibits a more diversified and variable aspect. One 

 feature of the present observations will be important as 

 enabling comparisons to be made as to the rates of 

 motion of the various white and dark spots in this and 

 preceding oppositions. No doubt many of the surface 



