July 26, 1894] 



NA TURE 



291 



Or, if the earth be supported like a terrestrial globe 

 by a polar axis loo miles in diameter, that axle must 

 be clamped and subjected to a tangential drag of 320 

 million tons in order to represent the energy dissipated 

 by tidal friction. 



Given this gigantic retarding " couple," it is surprising 

 to remember how slow the consequent lengthening of the 

 day actually is : that the earth, in fact, lags behind a 

 perfect time-keeper only an accumulated few seconds — 

 the estimate in this book is the rather high one of 

 twenty-two seconds — ^in the course of a century, i.e. in 

 the course of 36,500 rotations. 



To test or detect this retardation by direct observa- 

 tion is, as is well known, very difficult, because the only 

 other time-keeper available for purposes of comparison 

 is the moon, and she is a very bad and complicated one. 

 Only by efforts of great genius has the astronomical 

 discovery of the diminishing speed of the earth's rota- 

 tion been made ; but if astronomical clocks were as per- 

 fect as Lord Kelvin thinks they ought to be, the observa- 

 tion would be comparatively easy. He looks forward to 

 a time when clocks will not be set by the stars, as at 

 present, but when the earth's motion will be rated by a 

 standard clock of extraordinary perfection. At present, 

 or at least in 1 858, "astronomical clocks are just as 

 great a disgrace to the mechanical genius of Europe 

 and America as chronometer watches are a credit." 



(Incidentally a curious statement is made as to the 

 feasibility of working coal at enormous depths, in spite of 

 the presumably high temperature there — " .Suppose there 

 was coal, or rather charcoal, where the strata were 

 red hot, it might be gone into, and that with perfect 

 ease. All that is necessary is plenty of ventilation" : the 

 ventilation being conducted on the freezing-machine 

 principle of adiabatic expansion of previously com- 

 pressed air.) 



Other causesaffecting the rate of the earth's rotation are 

 likewise considered, such as the deposition of meteoric 

 dust, the redistribution of polar ice and e(|uatorial water, 

 the shrinking of the earth by cooling. If meteoric dust, 

 without initial moment of momentum, were deposited at 

 the rate of one foot in 4000 years it would produce 

 the observed retardation, and likewise the acceleration 

 of the moon's mean motion, without aid from the tides. 

 On the other hand, any redistribution or accumulation 

 and dissipation of polar ice must be a periodic pheno- 

 menon, and therefore, though it may exert a distinct 

 effect for some cycles of years, disturbing calculations of 

 eclipses and such like, yet in the long run it must inte- 

 grate out and be inoperative. As to the accelerative 

 influence of thermal contraction, it is believed by the 

 author to be extremely small, probably not the i 6000th 

 part of that due to tidal friction. 



Concerning the probable antecedent condition of the 

 matter which has fallen together to make the earth, it is 

 interesting to note (p. 121) that "any great degree of 

 relative motion of different portions of matter through 

 space renders the chance of their hitting one another very 

 small" ; it is probable, therefore, that the heat developed 

 by the falling together of the earth's materials arose 

 simply from their gravitative potential energy, which is 

 fairly calculable, and not from vague stores of unknown 

 initial motions. 



NO. I 29 I, VOL, 50] 



On p. 185 a statement is made with respect to Helm- 

 holtz' theory of solar heat, to the effect that "this con- 

 densation can only follow from cooling." It is rash to 

 question a statement allowed to stand by this author 

 after revision, but perhaps he would look at it again. 

 Surely condensation can, under some conditions, not 

 only generate heat but also elevate temperature ? 



Towards the end of a Presidential address delivered 

 to the British Association at Edinburgh in 1871, the 

 author, while quoting with " cordial sympathy " a 

 couple of sentences from " The Origin of Species," 

 on the subject of biological evolution, omits an inter- 

 vening sentence " describing briefly the hypothesis 

 of 'the origin of species by natural selection' because 

 [he] had always felt that this hypothesis does not 

 contain the true {i.e. doubtless the complete] theory 

 of evolution ... in biology. Sir John Herschel, in 

 expressing a favourable judgment on the hypothesis 

 of zoological evolution . . . objected to the doctrine 

 of natural selection, that it was too like the Laputan 

 method of making books [which it may be recol- 

 lected was something like this : haphazardly composing 

 all the type available, and hoping that of all the random 

 statements thus made the fittest might survive] and that 

 it did not sufficiently take into account a continually 

 guiding and controlling intelligence. This seems to me 

 a most valuable and instructive criticism.'' 



Eliminating the slightly anthropomorphic mode of ex- 

 pression from one sentence of this quotation, it illustrates, 

 what is certainly the truth, that to the interested on- 

 lookers from other sciences there already seemed cogent 

 need of a supplement to the fraction of truth contained 

 in Natural Selection, a supplement involving some such 

 treatment of the Origin of Variations as is now attempted 

 in Mr. Bateson's recent work. 



In an address to Section A, at Glasgow, 1876, the 

 author goes back to geology and considers the question 

 of a possible shift of the earth's polar axis and of possible 

 temporary alterations in the length of the day, while he . 

 entirely repudiates and demolishes the view that the 

 earth's interior can be mainly or even largely liquid. 

 His conclusion from the whole of tidal phenomena is 

 that the earth is now extremely rigid and must be practi- 

 cally solid all through. 



He nevertheless contemplates with equanimity New- 

 comb's bold hypothesis, based on the lunar theory and 

 on apparent irregularities in the moon's motions, that 

 the earth actually went slow and lost seven seconds 

 between 1S50 and 1S62, and then went fast and gained 

 eight seconds from 1862 to 1S72. Lord Kelvin tentatively 

 explains the conceivable possibility of this acceleration 

 by possible changes in the earth's shape, as detectable by 

 changes in sea-level. 



"A settlement of 14 centimetres in the equatorial 

 regions, with corresponding rise of 28 centimetres at 

 the poles (which is so slight as to be absolutely undis- 

 coverable in astronomical observatories, and which would 

 involve no change of sea-level absolutely disproved by 

 reduction of tidal observations hitherto made), would 

 suffice. Such settlements must occur from time to time ; 

 and a settlement of the amount suggested might result 

 from the diminution of centrifugal torce due to 150 or 

 200 centuries' tidal retardation of the earth's rotational 

 speed." 



