14-ie] Theories of Cosmogony 17 



points at which the tides are highest. But the tide-raising body does not 

 stand still ; it is always somewhat ahead of the diameter through the two 

 highest tides, and so exerts not only a tide-generating force, but also a couple 

 which tends to set up rotation in the primary body. The two jets of nebulous 

 matter are therefore being ejected from a slowly rotating body, and instead 

 of forming straight lines, form spiral curves. 



The authors of the planetesimal theory claim that these conceptions 

 explain the origin of the spiral nebula formation, which they regard as a half- 

 way stage in the process of planetary formation, just as Kant and Laplace 

 regarded the rings of Saturn. The authors further believe that the ejection 

 of matter will take place by " pulsations " hence the nuclei observed in the 

 arms of the typical spiral nebula and that the condensations of these nuclei 

 ultimately form planets by agglomeration. If all this can be shewn to happen 

 according to the authors' programme, then clearly the planetary structure and 

 the spiral nebula structure are explained at one sweep. But whether all this 

 happens or not can only be decided by exhaustive mathematical investigation. 



Perhaps the most obvious criticism that can be brought against this 

 and all other tidal theories is that they require the close approach of large 

 astronomical bodies, and that such close approaches are very rare events. 

 Calculations which will be given later seem to shew that this consideration 

 must lead to the abandonment of all tidal theories, including the planetesimal, 

 as explanations of normal cosmogonic processes. It must not of course be 

 asserted that no system has ever been broken up by tidal forces this would 

 be contrary to all statistical laws but it will be found that only a small 

 proportion of the stars in the universe are likely to have been broken up in 

 this way. 



III. OTHER THEORIES 



16. In addition to the theories just mentioned, there are a great number 

 of others in the field which claim to explain the origin of the solar system. 

 Many of these start from a nebulous mass or swarm of meteorites in chaos, 

 and regard the spiral nebula formation as an intermediate stage towards the 

 development of a solar system. Thus in addition to Moulton and Chamberlin, 

 See* and Arrheniusf both contemplate the possibility of spiral nebulae 

 forming out of the collision or near approach of two stars, the condensations 

 in the arms of the spiral being supposed ultimately to form planets circling 

 around a central nucleus. Sutherland J has suggested that Bode's well- 

 known law of planetary distances is readily explained in terms of a spiral 

 origin ; for Bode's law, usually expressed in the form 



r = 0-4 + 0-15 x 2 n (n=l, 2,3, ...), 



* Researches of the Evolution of Stellar Systems, Vol. i. 



t Worlds in the Making. (London, 1908.) J Astrophys. Journ. 34, p. 251. 



J. C. 2 



