The yielding of plastic bodies to the strain of unequal attractions is a phenomenon of far-reaching consequence. We know that the sun as well as the moon causes tides in our oceans. There must, then, be solar, no less than lunar, tidal friction. The question at once arises: What part has it played in the development of the solar system? Has it ever been one of leading importance, or has its influence always been, as it now is, subordinate, almost negligible? To this, too, Professor Darwin supplies an answer.

It can be stated without hesitation that the sun did _not_ give birth to the planets, as the earth has been supposed to have given birth to the moon, by the disruption of its already condensed, though viscous and glowing ma.s.s, pushing them then gradually backward from its surface into their present places. For the utmost possible increase in the length of the year through tidal friction is one hour; and five minutes is a more probable estimate.[1181] So far as the pull of tide-waves raised on the sun by the planets is concerned, then, the distances of the latter have never been notably different from what they now are; though that cause may have converted the paths traversed by them from circles into ellipses.

Over their _physical_ history, however, it was probably in a large measure influential. The first vital issue for each of them was--satellites or no satellites? Were they to be governors as well as governed, or should they revolve in sterile isolation throughout the aeons of their future existence? Here there is strong reason to believe that solar tidal friction was the overruling power. It is remarkable that planetary fecundity increases--at least so far outward as Saturn--with distance from the sun. Can these two facts be in any way related? In other words, is there any conceivable way by which tidal influence could prevent or impede the throwingoff of secondary bodies?

We have only to think for a moment in order to see that this is precisely one of its direct results.[1182]

Tidal friction, whether solar or lunar, tends to reduce the axial movement of the body it acts upon. But the separation of satellites depends--according to the received view--upon the attainment of a disruptive rate of rotation. Hence, if solar tidal friction were strong enough to keep down the pace below this critical point, the contracting ma.s.s would remain intact--there would be no satellite-production. This, in all probability, actually occurred in the case both of Mercury and Venus. They cooled without dividing, because the solar friction-brake applied to them was too strong to permit acceleration to pa.s.s the limit of equilibrium. The complete destruction of their relative axial movement has been rendered probable by recent observations; and that the process went on rapidly is a reasonable further inference. The earth barely escaped the fate of loneliness incurred by her neighbours. Her first and only epoch of instability was r.e.t.a.r.ded until she had nearly reached maturity. The late appearance of the moon accounts for its large relative size--through the increased cohesion of an already strongly condensed parent ma.s.s--and for the distinctive peculiarities of its history and influence on the producing globe.

Solar tidal friction, although it did not hinder the formation of two minute dependents of Mars, has been invoked to explain the anomalously rapid revolution of one of them. Phobos, we have seen, completes more than three revolutions while Mars rotates once. But this was probably not always so. The two periods were originally nearly equal. The difference, it is alleged, was brought about by tidal waves raised by the sun on the semi-fluid spheroid of Mars. Rotatory velocity was thereby destroyed, the Martian day slowly lengthened, and, as a secondary consequence, the period of the inner satellite, become shorter than the augmented day, began progressively to diminish. So that Phobos, unlike our moon, was in the beginning farther from its primary than now.

But here again Mr. Nolan entered a _caveat_. Applying the simple test of numerical evaluation, he showed that before solar tidal friction could lengthen the rotation-period of Mars by so much as one minute, Phobos should have been precipitated upon its surface.[1183] For the enormous disparity of ma.s.s between it and the sun is so far neutralised by the enormous disparity in their respective distances from Mars that solar tidal force there is only fifty times that of the little satellite. But the tidal effects of a satellite circulating quicker than its primary rotates exactly reverse those of one moving, like our moon, comparatively slowly, so that the tides raised by Phobos tend to _shorten_ both periods. Its...o...b..tal momentum, however, is so extremely small in proportion to the rotational momentum of Mars, that any perceptible inroad upon the latter is attended by a lavish and ruinous expenditure of the former. It is as if a man owning a single five-pound note were to play for equal stakes with a man possessing a million. The bankruptcy sure to ensue is typified by the coming fate of the Martian inner satellite. The catastrophe of its fall needs to bring it about only a very feeble reactive pull compared with the friction which the sun should apply in order to protract the Martian day by one minute. And from the proportionate strength of the forces at work, it is quite certain that one result cannot take place without the other. Nor can things have been materially different in the past; hence the idea must be abandoned that the primitive time of rotation of Mars survives in the period of its inner satellite.

The anomalous shortness of the latter may, however, in M. Wolf's opinion,[1184] be explained by the "trainees elliptiques" with which Roche supplemented nebular annulation.[1185] These are traced back to the descent of separating strata from the _shoulders_ of the great nebulous spheroid towards its equatorial plane. Their rotational velocity being thus relatively small, they formed "inner rings," very much nearer to the centre of condensation than would have been possible on the unmodified theory of Laplace. Phobos might, in this view, be called a polar offset of Mars; and the rings of Saturn are thought to own a similar origin.

Outside the orbit of Mars, solar tidal friction can scarcely be said to possess at present any sensible power. But it is far from certain that this was always so. It seems not unlikely that its influence was the overruling one in determining the direction of planetary rotation. M.

Faye, as we have seen, objected to Laplace's scheme that only retrograde secondary systems could be produced by it. In this he was antic.i.p.ated by Kirkwood, who, however, supplied an answer to his own objection.[1186]

Sun-raised tides must have acted with great power on the diffused ma.s.ses of the embryo planets. By their means they doubtless very soon came to turn (in lunar fashion) the same hemisphere always towards their centre of motion. This amounts to saying that even if they started with retrograde rotation, it was, by solar tidal friction, quickly rendered direct.[1187] For it is scarcely necessary to point out that a planet turning an invariable face to the sun rotates in the same direction in which it revolves, and in the same period. As, with the progress of condensation, tides became feebler and rotation more rapid, the accelerated spinning necessarily proceeded in the sense thus prescribed for it. Hence the backward axial movements of Ura.n.u.s and Neptune may very well be a survival, due to the inefficiency of solar tides at their great distance, of a state of things originally prevailing universally throughout the system.

The general outcome of Mr. Darwin's researches has been to leave Laplace's cosmogony untouched. He concludes nothing against it, and, what perhaps tells with more weight in the long run, has nothing to subst.i.tute for it. In one form or the other, if we speculate at all on the development of the planetary system, our speculations are driven into conformity with the broad lines of the Nebular Hypothesis--to the extent, at least, of admitting an original material unity and motive uniformity. But we can see now, better than formerly, that these supply a bare and imperfect sketch of the truth. We should err gravely were we to suppose it possible to reconstruct, with the help of any knowledge our race is ever likely to possess, the real and complete history of our admirable system. "The subtlety of nature," Bacon says, "transcends in many ways the subtlety of the intellect and senses of man." By no mere barren formula of evolution, indiscriminately applied all round, the results we marvel at, and by a fragment of which our life is conditioned, were brought forth; but by the manifold play of interacting forces, variously modified and variously prevailing, according to the local requirements of the design they were appointed to execute.

FOOTNOTES:

[Footnote 1150: _Exposition du Systeme du Monde_, t. ii., p. 295.]

[Footnote 1151: In later editions a retrospective clause was added admitting a prior condition of all but evanescent nebulosity.]

[Footnote 1152: _Mec. Cel._, lib. xiv., ch. iii.]

[Footnote 1153: _Beitrage zur Dynamik des Himmels_, p. 12.]

[Footnote 1154: _Trans. Roy. Soc. of Edinburgh_, vol. xxi., p. 66.]

[Footnote 1155: Newcomb, _Pop. Astr._, p. 521 (2nd ed.).]

[Footnote 1156: M. Williams, _Nature_, vol. iii., p. 26.]

[Footnote 1157: _Comp. Brit. Almanac_, p. 94.]

[Footnote 1158: Radau, _Bull. Astr._, t. ii., p. 316.]

[Footnote 1159: Newcomb, _Pop. Astr._, pp. 521-525.]

[Footnote 1160: _Proc. Roy. Soc._, vol. x.x.xiii., p. 393.]

[Footnote 1161: To this hostile argument, as urged by Mr. E. Douglas Archibald, Sir W. Siemens opposed the increase of rotative velocity through contraction (_Nature_, vol. xxv., p. 505). But contraction cannot restore lost momentum.]

[Footnote 1162: _Stellar Evolution, and its Relations to Geological Time_, 1889.]

[Footnote 1163: _Comptes Rendus_, t. lii., p. 481. See also Kirkwood, _Observatory_, vol. iii., p. 409.]

[Footnote 1164: Fouche, _Comptes Rendus_, t. xcix., p. 903.]

[Footnote 1165: _Astroph. Jour._, vol. xiii., p. 338.]

[Footnote 1166: _Month. Not._, vol. xxix., p. 96.]

[Footnote 1167: _Pop. Astr._, p. 257.]

[Footnote 1168: _Sur l'Origine du Monde_, 1884.]

[Footnote 1169: Kirkwood adverted to it in 1864, _Am. Jour._, vol.

x.x.xviii., p. 1.]

[Footnote 1170: _Bull. Astr._, t. ii.]

[Footnote 1171: _Nature_, vol. x.x.xi., p. 506.]

[Footnote 1172: _Formation Mecanique du Systeme du Monde; Bull. Astr._, t. xiv., p. 313 (O. Callandreau). See also, _Le Probleme Solaire_, by l'Abbe Th. Moreux, 1900.]

[Footnote 1173: _Phil. Trans._, vol. clxxi., p. 713.]

[Footnote 1174: Mr. J. Nolan has pointed out (_Nature_, vol. x.x.xiv., p.

287) that the length of the equal day and month will be reduced to about 1,240 hours by the effects of _solar_ tidal friction.]

[Footnote 1175: _Phil. Trans._, vol. clxxi., p. 835.]

[Footnote 1176: _Nature_, vol. x.x.xiii., p. 368; see also Nolan, _Ibid._, vol. x.x.xiv., p. 286.]

[Footnote 1177: _Phil. Trans._, vol. clxxviii., p. 422.]

[Footnote 1178: _Ibid._, vol. clxxii., p. 491.]

[Footnote 1179: _Ibid._, p. 530.]

[Footnote 1180: _Satellite Evolution_, Melbourne, 1895; _Knowledge_, vol. xviii., p. 205.]

[Footnote 1181: _Phil. Trans._, vol. clxxii., p. 533.]

[Footnote 1182: This was perceived by M. Ed. Roche in 1872. _Mem. de l'Acad. des Sciences de Montpellier_, t. viii., p. 247.]

[Footnote 1183: _Nature_, vol. x.x.xiv., p. 287.]

[Footnote 1184: _Bull. Astr._, t. ii., p. 223.]

[Footnote 1185: _Montpellier Mems._, t. viii., p. 242.]

[Footnote 1186: _Amer. Jour._, vol. x.x.xviii. (1864), p. 1.]