Sf.ptkmber, 1910. 



KNOWLEDGE. 



353 



tidal evolution, accordinc' to w hich 



Moon once 



formed part of the earth's substance. It is supposed 

 that once upon a time the latter was rotating niuch 

 more quickly than at present, and that a portion 

 separated from the rest and became the moon. The 

 time of the earth's rotation was estimated to be 

 then about two and a half hours, so that the 

 "centrifugal force" caused its rupture. The separ- 

 ated portion ma}- ha\e gradualh' receded to 

 its present position hv tidal action. Difficulties 

 as to the possibilities of the Moon's holding 

 together under tidal strain when so much closer 

 to the earth than at present, and the cause of the 

 supposed ver\- rapid rotation of the earth in earl\- 

 ages, have been raised by various writers, so that \\e 

 may at least regard the alternative capture theor\- and 

 action of the resisting medium 

 as a possible explanation, though 

 it seems difficult to imagine that 

 all the relations pointed out by 

 Sir George Darwin are pureK' 

 accidental. The Laplacian 

 hypothesis, with the modifi- 

 cations suggested by Faye and 

 others and supplemented by 

 the theory of tidal evolution, 

 has been ne\ertheless show n 

 to be untenable, and the 

 evolution of the planets bv 

 separation of rings of matter 

 from the central condensation 

 as it contracted must be defin- 

 itely abandoned. Briefly stated 

 the theorv is as follows. The 

 matter now forming Sun, 

 Earth and other planets was at 

 one time in the form of a hot 

 gas, and was of approximately 

 spherical form. It rotated 

 slowlv on its axis, the rota- 

 tion increasing in swiftness as it 

 and contracted. In time rings of 



The Spiral Nebula M, 51 Caniini Venaticorum 

 (after Seel. 



IllusUat 



matter were 



left behind. Each of these rings gradually 

 collected into a single globe, and thus the planets 

 came into existence. A planet thus formed 

 continuing to revolve itself, in turn abandoned 

 rings in contracting, and thus arose the satellites. 

 This process is illustrated hv a well-known 

 experiment, devised by Plateau. He formed a 

 mixture of alcohol and water of the same density as 

 oil (water being heavier, alcohol lighter), and 

 poured into this mixture a quantity of oil. The 

 oil sank about halfway and floated in the 

 middle of the mixture as a round ball. I)\- 

 means of a disc attached to a wire. Plateau made 

 it rotate. The rotation caused the ball to spread 

 out into the form of a spheroid ; then, as the speed 

 increased, a ring was thrown off which revolved 

 round it. The ring broke up after a time and 

 collected into a smaller globe, which rotated and 

 also revolved round the larger ; then another ring 

 was thrown off as the speed again increased, and so 



on, liabinet, however, in 1861 showed that the 

 original nebula imagined by Laplace could not have 

 rotated with sufficient speed to detach any of the 

 planetary masses, and his argument has been extended 

 bv Professer See to show that, similarh', the satellites 

 could not have separated from their primaries. 

 There has therefore been proposed an alternative 

 fission theory, that secondar)- condensation nuclei 

 might be formed bv ^mvitational instabilit\- within 

 the gaseous nebula, as also the capture theory 

 referred to above, together w ith the " spiral " nebula 

 form, A spiral nebula is supposed to be formed 

 from a Sun by the near passage of another star, the 

 consequent ejection of matter forming two spiral 

 arms bearing nuclei at irregular intervals. The par- 

 ticles of these arms move round the central nucleus in 

 elliptic orbits, the planets being 

 formed by the gradual growth 

 of these nuclei, bv the addi- 

 tion of small ""planetesimals." 

 Professor See considers that 

 our solar svstem was formed 

 from a spiral nebula and that 

 the planets have not been 

 detached from the central mass 

 li\- rotation, but "captured," 

 or added on, from the outer 

 parts of the nebula, their orbits 

 lia\'e been "rounded off" by 

 the resisting action of the 

 medium in which they move, 

 and the particles moving in 

 orbits confined to regions of 

 the nature of those indicated 

 in the researches of Darwin 

 alluded to in the earlier part of 

 the paper. Moving against 

 resistance they will all drop 

 nearer to one or other centre of 

 attraction : some will become 

 planet, some of another, whilst 

 collide with and be absorbed 

 or fall upon the Sun, Thus 



the Spiral Theory. 



colder 



satellites of one 

 \-et others will 

 by these latter 



the greater masses will increase, and m some 

 such manner the "secular acceleration" of the 

 Sun and the unexplained acceleration of Mercury, 

 ma\" be explained. In our svstem we have a 

 large central mass, the Sun, with attendant planets, 

 but in some cases we may have a more nearly 

 equal division of the matter, as in the double 

 stars. Figure 2 shows the equipotential surfaces 

 about a double star with e<]ual components (or 

 rather their traces on the plane of the paper). In 

 such cases the planets which are developed, if they 

 can continue revolving in stable orbits, will have to 

 be near either one or other of the large masses, or 

 else at great distances from both, if their motion is 

 to be of a permanent character. However, the 

 existence of such bodies, though probable, is not 

 likely to be revealed by any increase in telescopic 

 power of our present instruments. Great numbers 

 of sjiiral nebulae are now known to exist, scattered 



