=;iS 



XATL'Rn 



[SErXEMBER 26, 1S95 



a mighty conqueror, would not have made that kind of 

 attack. 



Without attempting to condense further this already 

 condensed and ver\- readable little volume written by a 

 distinguished inquirer, I will conclude by saying that it 

 well deser\-es a place in any general librar\\ 



Francis Gai.ton. 



SA TELLITE E I VL LTIOX. 

 Satellite Evolution. By James Xolan. Pp. 114. v Mel- 

 bourne, &c : George Robertson and Co., 1S95.) 

 IX this book Mr. Nolan discusses the part played by 

 tidal friction in the evolution of satellites, .\lthough 

 the subject is one of much scientific interest, his work is 

 hardly likely to attract the attention it deserves, because 

 the unmathematical reader will find the reasoning hard 

 to follow, whilst the mathematician will be repelled by 

 prolixity, due to the author's treatment of the problem by 

 means of general reasoning.' The first fifty pages of the 

 book appear to be virtually contained in the single equa- 

 tion which states the effect of tidal friction in increasing 

 the mean distance of a satellite. It might perhaps be in- 

 teresting to some to discuss the various elements of the 

 problem in detail, but those who are able tn comprehend 

 an analytical formula are not very likely to have the 

 patience to follow such a discussion. 



I shall not accordingly follow Mr. Nolan in detail, but 

 will pass at once to the conclusion to which he tends. 

 On p. 9 he says : — 



"Though Mr. Dar\vin made elaborate calculations to 

 support his theory respecting the part played by tidal 

 friction on the evolution of the earth and moon, he seems 

 to have dismissed the Jovian and .Saturnian systems with 

 the conclusion that their satellites, unlike our moon, could 

 not be traced much further in than the present distances of 

 their respective planets ; and that as the relation between 

 the mass of the planet and satellite, or relation of 

 rotational to orbital momentum is ver\- different in the 

 case of the earth and moon to that for other planets and 

 satellites, their modes of evolution may have differed con- 

 siderably. He seems to have gone something further 

 into the possible effects of solar tidal friction on the 

 planets revolving round the great central body, or at 

 least has come to the correct conclusion that the eflficiency 

 of such tides would be too small to effect any appreciable 

 change during the natural lifetime of a solar system. ' 



He then proceeds to show that, if the earth and Jupiter 

 rotate under the influence of tides subject to the same 

 frictional resistance, the proportionate rate of increase of 

 the moon's mean distance is much smaller than that of 

 all of Jupiter's satellites, save one. In other words, four 

 out of five of Jupiter's satellites would have their mean 

 distances increased by, say, one per cent, in a much 

 shorter time than would the moon. He then pursues the 

 same train of reasoning with respect to Saturn and 

 Mars. 



It appears to me that Mr. Nolan is correct in these 

 conclusions, and we are thus led to suppose that tidal 

 friction may have played a much more important part in 



>l I.1X, and il \\ not alwny« city in autirc 

 'rain ofruMOninK ; but where the conclusion 

 -.M> i» »o alio. 



' The nl.r 

 oneself <A \h' 

 wcorretl, the i 



NO. 1352, VOL. 52] 



the evolution of satellites than I was disposed to allow 

 it.' He points out (p. 70) that the satellites of Jupiter 

 are probably much younger than the moon ; " when the 

 moon was younger, her relative rate of recession was 

 faster, as now is the case for some satellites in other 

 systems." He finally concludes i^p. "S) that the majority 

 of satellites in each system may be traced to a positiort 

 corresponding with that of the rings of Saturn. 



But before arri\ing at this result, the author has treated 

 another problem, in which, in my opinion, his conclusion 

 is incorrect. On p. 45, he considers the effects of tidal 

 friction on such a ring as that of Saturn. He says : — 



Tidal friction "could have no effect if the ring were 

 perfectly even all round. When compmsed of individual 

 bodies it could not be or remain so. Each individual 

 would be unaffected by the tides of the others, and would 

 recede at the same rate as if it were the only body in the 

 ring. The moon recedes at exactly the same rale as she 

 would were there no solar tides ; and if there were a 

 second moon there would be no interference with the 

 recession of the first . . . Then if the bodies composing 

 the rings are 'as the sand on the seashore for multitude' 

 tidal friction must still effect the usual progressive change, 

 unless each individual body be small enough to be un- 

 affected at the distance, whether composing a ring or not. 

 This must have a dissolving effect on the ring, or tend to 

 shape certain sections of it into so many bodies, which, 

 having increased their mass at the expense of the ring, 

 finally recede therefrom, either to circle round at a great 

 distance or be precipitated into the planet increasing its 

 rotation speed." 



It would seem that the process here sketched is an 

 essential part of Mr. Nolans theory of the evolution of 

 satellites, but I believe it to be founded on erroneous pre- 

 mises. He omits in fact to notice the necessary condi- 

 tion for neglecting the effects of the tides raised by one 

 satellite on the mean distance of another ; this is, that the 

 periodic times of the two shall not be equal to one another. 

 If the periodic times of two satellites are unequal, we 

 need not invoke tidal friction to bring the two bodies near 

 : to one another. On the other hand, if four or eight satel- 

 lites be equally spaced round a planet and revolve with 

 the same periodic time, tidal friction would only influence 

 their motions to a very small extent. 1 am therefore un- 

 able to follow Mr. Nolan in this part of his work. 



.Several other points in the early history of satellites 

 are considered by Mr. Nolan, but I am unable to touch 

 on them within the limits of a review. 



Notwithstanding all that has been written by him 

 and others, we are still far from a consistent theory of 

 the formation of a satellite. In my own papers I have 

 ventured to throw out suggestions (which have but too 

 often been quoted as positive theories), and it still seems 

 to me at least, that neither the present contribution of the 

 author nor the theories of others .are adequate. 



This work touches on subjects of interest, and although 

 it seems o])en to much criticism, I for my part welcome 

 the extension given by Mr. Nolan to the ijart played by 

 tidal friction in evolutionary astronomy. 



G. H. Darwin. 



1 The arffumcnts by which I was led to an erroneous conclusion on thi» 

 point, will be found in t'hii. Tratu., part ii., 1861, p. 534. 



