106 



KNOWLEDGE. 



[May, 1903. 



The first ascertaiued anomalv of the kiud was met with iu 

 the swift circulation of Phobos, the inner satellite of Mars, 

 which completes three revolutions, and enters upon a 

 fourth, while the planet attended by it wheels once i>n its 

 axis. The fact is most perplexing ; and the confident 

 persuasion that solar tidal friction would avail to remove 

 the difficulty has not proved well i,'rounded. Solar tidal 

 friction, it may be remarked, acts as an external force upon 

 subordinate systems submitted to its influence. Within 

 their precincts, moment of momentum may be destroyed by 

 it ; and it was hence a feasible supposition that tlie rotation 

 of Mars had, in the course of ages, greatly slackened 

 through the retarding effect of sun-raised tides. But the 

 agencv was demonstrably inadequate to the task assigned 

 to it. ' The reduction of the rotational moment of Mars to 

 about one twenty-fifth its primitive amoimt* would have 

 brought other consequences in its train, at least one of 

 which did clearly not ensue. At an early stage of the 

 process, Phobos should have been re-engulfed in the parent 

 body.f For the p>dl of the small tidal wave raised by it 

 on the surface of that body would have been backward 

 from the instant that the balance of periods became 

 inclined, through solar compulsion, in a direction contrary 

 to that it would naturally have taken ; and the loss of 

 velocity must have entailed its descent, along a spiral path, 

 towards an inevitable doom. The continued existence, 

 then, of the little satellite closes this way of escape from 

 the difficulty raised by the shortness of its period. M. Wolf 

 liad recourse to a different explanatory subterfuge. J He 

 believed that Phobos might have owed its origin to one 

 of Eoehe's "elliptic sheddings" of nebulous matter 

 dropped downward from near the polar regions of the 

 distended Martian spheroid, and rotating, owing to its 

 low rate of linear speed, in the immediate vicinity of the 

 cooling planet. The explanation, though ingenious, is too 

 recondite and evasive of mental grip to be satisfactory. 



The Saturnian system exhibits a case of the same kind, 

 but still more perplexing to speculative prepossessions. 

 Saturn's ring system has always appealed to thinkers as a 

 striking object-lesson in nebular development. It forcibly 

 arrested Kant's attention, and he sketched its birth-history 

 on lines anticipatory of those adopted by Laplace for the 

 solar system iu its entirety. Laplace himself regarded the 

 formation as the one surviving relic of the annular stage 

 of planet-building — as a witness from the dim past to a 

 condition of things elsewhere transitory. Tet the witness 

 has turned king's evidence, and betrayed the whole situa- 

 tion. The innermost Saturnian ring has a period far too 

 short to lie compatible witli the requirements of theory. 

 For its meteoric constituents, which are known to revolve 

 as independent units, complete their circuits iu between 

 five and six hours, while the planet needs just 10| for its 

 axial rotation. Moreover, tidal friction is here far less 

 available than on Mars ; yet no other retarding agency has 

 Ijeen invented. The deadlock appears final and hopeless. 



An objection quite as formidable, and even more funda- 

 mental, was raised by Kirkwood in 1869. The nebulous 

 material of the uncondensed sun must have been, at the 

 outset, of the last degree of tenuity. Atmospheric air is, 

 by comparison, a dense and massive substance. Yet no 

 reasonable person could ascribe to aerial matter the least 

 power of resisting strain. We know perfectly that a 

 rotating globe of air, and, a fortiori, a globe of matter 

 thousands of times less compact than air, would uninter- 

 mittently disintegrate at the surface with the progress 

 of acceleration. The disturbance and restoration of 



* ^ovXton, Asfrojihi/ncal Journal, Vol. XI., p. 110. 

 + Nolan, Nature, Tol. XXXIV., p. 287. 

 ; Bull. Astr., t. II., p. 223. 



equilibrium would be virtually simultaneous. There could 

 lie no accumulation of internal stress, and consequentlv 

 no distinctly separated epochs of instability. At the first 

 solicitation, at the first instant that centrifugal velocity 

 gained the upper hand over gravity, nebulous wisps would 

 have become detached ; and their detachment would have 

 gone on without pause. Space would have been strewn 

 with the di'hris of the condensing nebula ; and there should 

 have resulted a vast cloud of cosmic dust, not a majestic 

 array of i-evolving spheres. 



Moreover, even if the nebulous material had possessed 

 the fabulous cohesion indispensable for its division into 

 voluminous rings with wide intervening empty gaps, their 

 ultimate agglomeration into planetary globes would never 

 have lieen eii'eclually accomplished. Kirkwood long ago 

 questioned the feasibility of the process ; Mr. Moulton has 

 gone far towards demonstrating that it must have had an 

 abortive outcome. 



Another grave objection to Laplace's scheme is founded 

 ou the marked deviations visible in the solar system from 

 conformity to a fundamental plane of motion. Apart 

 from unexplained modifying influences, all the planets 

 should circulate along the level of the sun's equator, and 

 rotate on axes perpendicular to it. How far this is from 

 being realised in nature we have only to look around us to 

 perceive. We owe the changes of our seasons to the tilted 

 fashion of the earth's spinning. Yet it is by no means easy to 

 understand how the pole of its equator comes to be situated 

 in the Tail of Ursa Minor, while the pole of the ecliptic is 

 involved in the folds of Draco. They should have 

 coincided if the simple prescription of the Nebular Hypo- 

 thesis had been followed in the making and modelling of 

 the planets. Nor are the terrestrial arrangements 

 exceptional. The Saturnian equator and the Saturnian 

 rings have a still higher inclination ; while, in the systems 

 of Uranus and Neptune — since we may thus interpret their 

 retrogade revolutions — the angle exceeds the limit of a 

 cjuadrant. These and other similar discrepancies prove 

 the solar mechanism to have originated by a more complex 

 method than that imagined by Laplace ; and an hypothesis 

 which invokes the aid of a multitude of auxiliary devices 

 for its extrication from accumulating embarrassments falls 

 thereby under the suspicion of not being worth the trouble 

 of extricating. It forfeits, at any rate, all claim to com- 

 mendation for directness and simplicity. 



The Cosmogony turned out at Paris has thus proved 

 ^Tilnerable on a number of point s ; but all the blows aimed 

 at it have not told with such deadly effect as those just 

 referred to. Some have fallen harmlessly, or glanced 

 aside. One hostile argument in particular, which for a 

 time seemed irresistible, has been completely overthrown 

 by the logic of facts, and deserves mention only as a 

 historical curiosity. Towards the middle of the nineteenth 

 century, the progress of sidereal astronomy seemed to take 

 the direction of showing all nebulae indiscriminately to l>e 

 of stellar composition. With Lord Rosse's great reflectors, 

 a good many such objects were genuinely, and some 

 Ijesides were decejitively, resolved into stars, the illusory 

 effects being contirraed \>\ Bond's observations with the 

 deservedly celebrated fifteen-inch refractor then recently 

 built by Merz for Harvard College. Hence the rash 

 inference was drawn that resolution was wholly a c^uestion 

 of optical power, and that no real distinction existed 

 between the stellar and the nebular realms. Herschel's 

 •■ shining fluid " assumed a mythical air ; " island- 

 universes " came into popular vogue ; and all but a few 

 careful thinkers held nebulae and clusters to be differentiated 

 merely by degrees of remoteness. But if space contained 

 onlv full-grown stars, and no stars iu the making — 

 no star-spawn, no star-protojilasm — then the imagined 



