198 



KNOWLEDGE. 



[September, 1903. 



already to spread ovit invitingly before the gaze of investi- 

 gators. 



But whether the moon emerged from the earth as a 

 protuberance, or was abandoned by it as an equatorial ring, 

 it was revolving, when our theoretical acquaintance with 

 it begins, in a jieriod of not less than two, and not more 

 than four hours, quite close to the earth's surface ; while 

 the nearly isochronous rotation of the earth was conducted 

 with all but disruptive rapidity. The situation is so 

 suggestive that it needs only a short and tolerably safe 

 leap in the dark to reach the conclusion that the two gloljes 

 had very recently been one. With their division, at an epoch 

 estimated to have been at least fifty-four million years 

 ago, the process began by which the moon was pushed 

 back along a widening spiral course to its present position, 

 the vanished rotational momentum of the earth cropping 

 up again in the augmented orbital momentum of the 

 moon. And the transformation is, at least in theory, still 

 gomg on. 



Tidal friction has further capabilities. The transfer- 

 ence of momentum from one part of a system to another 

 is only the most obvious among the crowd of its results. 

 Scarcely au element of movement escapes its influence. 

 It increases, as a rule, orbital eccentricity. The smallest 

 initial deviation from circularity develops, through the 

 inequality of accelerative action thence ensuing, into 

 pronounced ovalness. That of the moon's path can in 

 this way be accounted for. Moreover, its plane was, in all 

 probability, shifted simultaneously, and under com- 

 pulsion of the same power, from its original coincidence 

 with the earth's equatorial plane to the level now occupied 

 by it. The obliquity of the ecliptic, too, is partially 

 explicable on the same principle. '• The present motion of 

 the two bodies " (to quote Professor Darwin's words) are 

 " completely co-ordinated by the theory that tidal friction 

 was the ruling power in their evolution." Holding this 

 clue, we are enabled to trace them back to the start of their 

 dual existence, and to follow the insensible modifications 

 by which theii' state was moulded to its actual form. 



In no other satellite- system is this possible. No 

 moon besides our own possesses a stock of orbital 

 momentum large enough to intimate for it an analogous 

 history. The planetary attendants travel nearly in their 

 original tracks ; the fluid ripples raised by them on the 

 surfaces of their primaries lacked power to displace them. 

 Their own rotation, indeed, seems to have been completely 

 destroyed. Destroyed, that is, relatively to the destroying 

 body. There is a certainty that some, there is the strongest 

 likelihood that all of the Jovian and Saturnian satellites 

 turn unchangingly the same face inward. They rotate in 

 the periods of their several revolutions, just as our moon 

 does, and as a consequence of the same cause. Tidal 

 friction, however, appears to have been otherwise of sub- 

 ordinate importance in shaping their dynamical relations. 



The agency will not, then, serve in all cases for a deus ex 

 machind. It is not indiscriminately efiicacious. The modes 

 of its action have, in each of the systems considered, to be 

 delicately distinguished. The stage of development arrived 

 at by the globes affected, their degree of viscosity, their 

 comparative mass and bulk, their modes of motion, all 

 avail profoundly, and it may be incalculably, to modify 

 the outcome. The facility of error in estimates of the 

 kind is illustrated by Professor Darwin's remark that the 

 magnitude of the tide-raising force is only one factor of 

 the product.* The other is relative movement. Now, in 

 the case of the moon, the former continually augmented 

 retrospectively, while the latter fell off. Tidal generative 

 power varies inversely as the cube of the distance ; in 



Phil. Trant., Vol. CLXXI., p. 876. 



antique times, then, when the earth and moon revolved 

 contiguously, the bodily distortions they mutually pro- 

 duced must have been on an extremely large scale. Yet, 

 because of the near coincidence of the periods of the 

 globes, they must have been almost inoperative for fric- 

 tional purposes. The travelling of the piled-up matter 

 over their surfaces was too slow to lend it much efficiency 

 as a frictiou-brake. The insignificant waves raised by the 

 sun were, we are led to believe, because of their swift 

 relative motion, more influential at that early epoch in 

 checking terrestrial rotation than the colossal, but nearly 

 stationary, waves due to the moon. 



Numerical calculations, where they are practicable, 

 afford the only safe guide to this intricate field of enquiry. 

 It does not suffice to show that tidal action would have 

 been of the kind required — would have taken the right 

 direction — for bringing about some apparently anomalous 

 result. Proof must besides be forthcoming that the action 

 would have been of adequate power. Plausible guesses on 

 the subject may be entirely fallacious. The machine, even 

 if- properly constructed for the end in view, may work too 

 feebly for its attainment. We are, for instance, assured 

 that no difficulty connected with the sense of planetary 

 rotation need impede acceptance of the theory of planetary 

 origin from separated rings, since even if the embryo 

 globes gyrated the wrong way at the outset, solar tidal 

 friction would promptly have reduced them to conformity 

 with the general current of movement. This is true in 

 principle ; but will it l)ear quantitative investigation ? 

 Many promising hypotheses have broken down under the 

 weight of figures ; whether this particular one is strong 

 enough to survive their application remains to be seen. 

 We are, indeed, sure of its validity as regards Mercury, 

 but the efficacy of tidal friction decreases as the sixth 

 power of increasing distance, and the actual rotation of 

 Venus furnishes an enigma sufficiently perplexing to 

 discourage scrutiny of its dimly discerned antecedent 

 conditions. As regards the earth and the exterior planets, 

 the question could only be answered with the help of 

 information which is not forthcoming. 



Prof. Darwin's researches were fruitful just because 

 they were definite. They demonstrated, once for all, the 

 power of tidal friction as a cosmogonic agency, and 

 indicated clearly the departments of cosmogonic change in 

 which its competence lay. They availed, moreover, to 

 determine for the earth-moon system the amount of work 

 actually done by tidal friction in these several departments, 

 and to prove its large excess over the corresponding output 

 in any other sub-system falling within the sphere of 

 observation. This memorable result suggests that our 

 terrestrial home may be singular, not only in its evolutionary 

 history, but in the innumerable adjustments fitting it to 

 be the abode of life. 



The relations of the earth and moon adumbrate, and 

 scarcely more than adumbrate, the physical influences 

 mutually exerted upon each other by numerous twin- 

 globes in stellar space. Tidal friction is of maximum 

 power in systems formed of equal masses ; and those of 

 double stars are seldom widel_y disparate. Most, if not all 

 of them, were, besides, primitively very near neighbours ; 

 so that their symmetry must have been marred by con- 

 spicuous tidal deformations. The results upon their 

 development have been expounded in detail by Dr. See. 

 One of the most remarkable is the high average eccentricity 

 of their orbits. Visual binaries, with few exceptions, travel 

 in consideralily elongated ellipses, while spectroscopic 

 binaries usually pursue sensibly circular paths. Dr. See's 

 argument that their eccentricity was acquired during the 

 process of separation, under the influence of tidal friction, 

 is well-nigh irresistible. True, this line of explanation is 



