150 



KNOWLEDGE 



[July, 1903. 



TCirkwood, in 1864,* had recourse to solar tidal friction 

 for the purpose of chanrjinw planetary axial movement 

 from an anomalous into the conventional direction. And 

 he was followed, doubtless independently, by Eoche, and 

 by Eoche's interpreter, M. C. Wolf, of Paris. Objections 

 to anv particular mode of planetary formation, oa the 

 sjroiind that its outcome must have been retrograde rota- 

 tion, lost their validity, they remarked, through the con- 

 sideration that solar tidal friction would have availed to 

 redress the incongruity. For its retarding action would 

 have ceased only when synchronism with the revolutionary 

 period was attained ; that is, when the planet wheeled in 

 its orbit, as Mercury seems to do, turning always the same 

 face inward ; and then already direct rotation would have 

 set in, and, becoming accelerated by contraction, should 

 permanently retain the direction impressed upon it by the 

 friction of sun-raised tides. A certain air of plausibility 

 is given to this view by the fact that the only two retro- 

 grade planetary systems are situated entirely beyond the 

 possible range of any such manner of influence, and may 

 accordingly be supposed to have preserved unaltered their 

 primitive fashion of gyration. 



The late M. Faye was less loyal to tradition than the 

 savant of Montpellier. The appearance, in 1884-, of his 

 work " Sur I'Origine du Monde " gave the signal for 

 renewed activity, and a larger license in cosmological 

 speculation. Conservative opinions on the subject are 

 now rarely held ; the old groove has been, by most, 

 definitively quitted ; enquiry becomes continually more 

 individual, and less constrained by prescription. Faye's 

 reform, however, was not avowedly of a revolutionary 

 character. He did not make a clean sweep of the work of 

 his great predecessor, by way of preliminary to setting 

 forth his own more perfect plan. Yet his emendations of 

 it went very deep. Laplace's nebula was of a gaseous 

 consistence, and it stood in a genuine atmospheric relation 

 to the central condensation. That is to say, its strata 

 gravitated one upon the other ; they were subject to 

 hydrostiitic pressure. Faye ruled things otherwise. The 

 nebulous matrix which he postulated was a vast congeries 

 of independently moving particles, forming a system 

 governed by a single period, in which both gravity and 

 velocity increased in the direct ratio of the distance from 

 the centre. Now globes formed by the method of annula- 

 tion (admitting its ])racticaliility; out of materials thus 

 conditioned should have j'^i^sessed, ab initio, a direct 

 rotation ; their axial spinning would have been in the same 

 sense as their orbital circulation. And this it was which 

 recommended to Faye the adoption of a meteoric structure 

 for the inchoate solar system. But the simple law of force 

 regulating it at first would, by degrees, have become 

 essentially modified. That of inverse squares, familiarised 

 to ourselves by long habits of thought, woidd have begun 

 to supersede it so soon as a sun, properly so-called, could 

 be said to exist. The retrograde planets Uranus and 

 Neptune must, however, by Faye's supposition, have taken 

 shape under the modern regimen ; they were then formed 

 subsequently to the earth and all the rest of her sister 

 orbs. This unexpected inversion of the recognised order 

 of planetary age involved the further consequence that the 

 ante-natal offspring of the sun — thus paradoxically to 

 designate them — must have drawn closer to him as his 

 attractive power developed, Uranus and Neptune alone 

 among the entire cortege preserving the original span of 

 their orbits. 



Faye's scheme, if it did not meet all the arduous 

 requirements of the problem it confronted, served at any 

 rate to illustrate very forcibly the devious variety of tracks 



American Jour, of Science, Vol. XXXVIII., p. 3. 



by which nebular evolution might advance towards its 

 goal. The particular one chosen was certiinly not clear 

 of impediments. In his pre-occupation with the removal 

 from Laplace's Hypothesis of the flaw relating to p'anetary 

 rotation, M. Faye had discarded its cardinal merit of 

 explaining secessions of material by the growth of centri- 

 fugal force ; for no sufficient reason could be alleged why 

 the remodelled nebula should have separated into rings.* 

 The process implies definite and special conditions; it 

 testifies to a symmetrically acting cause. Laplace brought 

 such a cause into play. Faye abolished it, and his aunuli, 

 accordingly, wear a fictitious aspect. It is indeed true 

 that an annular structure is commonly visible in nebute ; 

 but it is begging a most arduous question to assume that 

 nebular spires have anything in common with planet- 

 forming rings. 



These would probably never have been heard of save for 

 the Saturnian example. A pattern is easily copied ; an 

 idea palpably feasible is tempting to adopt ; a demon- 

 stration on the soivitur amhuJando principle cannot but 

 prove convincing. But how, if the rings cannot be made 

 to coalesce into globes ':' And the difficulty of the trans- 

 formation becomes more apparent the more clearly its 

 details are sought to be realised. Eeversed in direction, 

 it might better find a place in the order of nature. 

 " Analysis seems to indicate," Kirkwood wrote in 1884,t 

 " that planets and comets have not been formed from 

 rings, but rings from planets and comets." 



Faye's theory was disfigured b}' a still more glaring 

 incongruity. Nothing in the planetary economy seems 

 more evident than that the zone of asteroids marks a 

 division bet ween two strongly dissimilar states of the solar 

 nebula. It is a visible halting-place. One series of events 

 came to an &nd, and there was an interlude before the 

 nest began. Dui-ing that interlude, during the partial 

 suspension of activity which ensued upon the production 

 of the Ajax among the planets, the crowd of planetoids 

 were launched to fill the blank space. Here, if anywhere, 

 nature changed her hand, and tried a fresh method. 

 Faye's shifting of the scene of change to trans-Saturniau 

 regions is then, as M. Wolf justly perceived, non-natural, 

 and undermines the credit of a plan to which the device 

 is essential. 



On the other hand, it had the merit of being elastic 

 enough to include the great cometary family. Kant had 

 also, although in an unsatisfactory manner, made room for 

 them ; but Laplace had no choice save to regard them as 

 casual intruders from space, the admission of which as 

 natives of his well-ordered domain would have led to the 

 subversion of all its harmonious regulations. Modern 

 enquiries, however, prove comets decisively to be no such 

 stray visitors as Laplace supposed, but to be of the same 

 lineage —however remotely traceable — with the planets, 

 and to own the same allegiance. Drifting with the sun, 

 they form part of its escort on its long, irrevocable voyage, 

 and cannot, save by accidents of perturbation, be driven 

 finally to part from its company. The problems of 

 planetarv and cometary origin are then inseparable ; the 

 two classes of body are fellow-citizens of one kingdom. 

 Comets become only by compulsion cosmopolitan wanderers 

 from star to star. 



There was yet another motive, and semblance of justifi- 

 cation, for Faye's reform of the Nebular Hypothesis. The 

 discovery of the conservation of energy supplemented, as 

 we have seen, very happily the mechanics of a condensing 

 nebula by satisfactorily solving the enigma of solar 

 radiation. Helmholtz was thus able, in 1871, to sketch 



* G. H. Darwin, Nature, Vol. XXXI., p. 506. 

 t Proc. Amer. Phil. Society, Vol. XXII., p. 109. 



