322 



NATURE 



{August 4, 1887 



system, we are asked to concentrate our attention. The 

 manner of its development is, however, a widely different 

 one from that traced by the great French geometer. 

 Laplace assumed the needful rotation, and left the rest to 

 work itself out spontaneously. He permitted no external 

 interference with the tranquil processes which he dis- 

 cerned as progressing through the ages. Dr. Braun, on 

 the other hand, assumes less to begin with, but invokes 

 adventitious aid in emergencies. No single nebula 

 thrown on its own resources sufficed, he finds, for the 

 production of the solar system. The complicated pheno- 

 mena which it presents demand a complex origin to 

 explain them. The mass of cosmical matter in which 

 they first began to unfold themselves was accordingly but 

 the nucleus of what it afterwards became. It not only 

 grew by the accretion of similar masses falling towards 

 it from space, but acquired its gyratory movement by 

 eccentric collisions with them. The consequences of 

 such events elsewhere are visibly pictured to us in the 

 spiral lines of light of certain nebulae. The great whirl- 

 pool in the Canes Venatici, for instance, betrays and 

 records the fall of a comet, on the gigantic primitive scale, 

 into an embryo sun. Only thus, in our author's opinion, 

 can the strange peculiarities of its structure be accounted 

 for ; and only thus can the first impulse to axial rotation 

 in our own system have been given. 



The visits of comets, as we now see them, feebly repre- 

 sent, we are told, the colossal in-rushes from interstellar 

 regions by which the machinery of planetary production 

 was set going and modified. But it is difficult to allow 

 to such bodies the independent origin implied in the 

 claim for them of such illustrative significance. Comets 

 can no longer be set down as mere casual intruders upon 

 the solar system. They certainly share its translatory 

 motion, since, if they were either overtaken or en- 

 countered, they should seem to come most numerously 

 from near the apex of the sun's path. But they approach 

 him indifferently from all parts of the sky. A further 

 proof of the absence of relative motion is afforded by the 

 shape of the tracks they pursue. M. Faye has remarked 

 that, of 364 cometary orbits calculated, not one is a de- 

 cided hyperbola (" Origine du Monde," p. 146) ; and 

 Laplace's view that they are hyperbolic by nature, and 

 elliptical only through perturbations, is thus seen to be 

 the exact reverse of the truth. 



A fundamental objection to Laplace's cosmogony is 

 that it implies a far swifter axial movement in the central 

 bodies of our system than they actually possess. For in 

 the theory of annular separation, the rotation of the 

 generating mass is strictly correlated with the revolution 

 of its offspring by the principle of the conservation of 

 areas, which requires that a rotating homogeneous globe 

 should spin quicker, as it contracts, in the proportion of 

 the square of its radius. Thus, if the solar nebula, when 

 it filled the orbit of Neptune, rotated (as on the hypothesis 

 in question it must have done) in Neptune's period of 

 165 years, the period of the sun shrunken to its present 

 dimensions, should have shortened in the ratio of the 

 square of 2,780,000,000 (the mean distance of Neptune) 

 to the square of 434,000 miles (the solar radius). In other 

 words, the rotation of the sun should be accomplished 

 in 127 seconds, in lieu of 25 days. Similarly, the ter- 

 restrial rotation-period corresponding to the lunar revolu- 

 tion in 27 J days, is no more than io| minutes ! It is true 

 that in both these estimates (the latter taken from Dr. 

 Braun's pages), the effects of central condensation are 

 neglected, although it must inevitably have rhade some 

 progress before annulation began ; but no allowance on 

 this score, however liberal, can possibly reconcile, though 

 it contributes to lessen, the discrepancy. 



Dr. Braun adjusts the balance in this way. The solar 

 nebula had never at any time, in his view, a uniform axial 

 movement. He even ventures to consider the present 

 unequal rotation of the sun as a survival of the primitive 



state of things to which the central deficiency of rotational 

 momentum is due. For the entire mass was, in the 

 beginning, set gyrating by external impacts. Movement 

 was hence generated predominantly in its outer regions, 

 and was only by degrees and imperfectly communicated 

 to the nuclear parts. 



The device is marked by considerable ingenuity, and 

 is at any rate preferable to the eddying movements 

 by which M. Faye evades the same embarrassment. 

 It is, however, scarcely needed by Dr. Braun, since 

 the "ring-theory" of planetary formation is almost, 

 and logically ought to be completely, abandoned by 

 him. Difficulties have of late been thickening round 

 it ; they reached a climax when the conviction was 

 attained that, apart from the neutralizing effects of tidal 

 friction, it could only result in the retrograde motion of 

 all secondary systems. The plan of centres of condensa- 

 tion is accordingly substituted by our author. This has 

 the advantage of allowing planets to begin to form any- 

 where in the nebula. It emancipates them from that 

 strict conformity to the equatorial level which was an 

 inconvenient feature of Laplace's hypothesis ; and though 

 they necessarily tended, in the course of their growth, to 

 descend towards it, enough may perhaps remain of their 

 primitive divergence to explain the observed slight devia- 

 tions from the fundamental plane. Yet Dr. Braun's con- 

 fidence in this rationale of the inclinations of the planet- 

 ary orbits is so far from being unlimited that he holds in 

 reserve, in case of its failure, other means for bringing 

 about the same end. 



Each planet is roughly estimated to have started on its 

 career at about five times its present distance from the 

 sun. In condensing, it descended towards it, sweeping 

 up materials as it went, until finally almost the whole of 

 the diffused gaseous stuff was concentrated in sun and 

 planets, and the intervening spaces were void. By that 

 time, too, tangential velocity had come to balance gravity, 

 and the slow inward approach ceased. But the resistance 

 met with in the earlier stages of its history by the grow- 

 ing and falhng planet had had one result of vital import- 

 ance to its future. It had imparted to it a movement of 

 rotation. As it settled down in close spirals towards its 

 present orbit, its velocity must everywhere have exceeded, 

 by a small amount, the velocity in the same direction 

 of the medium in which it moved. The density of that 

 medium must, however, have increased towards the sun ; 

 and the embryo-planet consequently experienced a slight 

 excess of resistance on its inner side, resulting in a direct 

 whirling movement. Dr. Braun endeavours to show that 

 the rotation thus set on foot must have belonged chiefly 

 to the external layers of the planetary nebula. His motive 

 is that of conciliating the swfft ciiculation of satellites yet 

 to be born from it with the comparatively sluggish spinning 

 of the parent mass. 



Tidal friction he rejects as an agent of planetary 

 development, attributing to it barely the power to have 

 rendered absolute an already approximate coincidence 

 between the periods of rotation and revolution of satel- 

 lites. Perhaps he might here be induced to reconsider 

 his position. At least in the case of the lunar-terrestrial 

 system, the evidence is overwhelming that tidal friction 

 was largely concerned in bringing about its present con- 

 dition. We may further assure him that Prof. G. H. 

 Darwin (whom he evidently identifies with the late Charles 

 Darwin, his father) has not committed the blunder he 

 imputes to him of ascribing to the moon a sJiorier period 

 of revolution than that of the earth's rotation, at the timfi 

 when it began, under the reactive influence of the tida 

 wave, to travel slowly outward from near its surface. Ofl 

 the contrary, a shghtly inferior angular velocity in the 

 satellite is the assumed starting-point of all his subsequenlfl 

 reasoning on the subject. 



For the completion of the solar system in its mino*;. 

 details. Dr. Braun is driven to the expedient of coUisions 



