ORIGIN OF THE SOLAR SYSTEM JEANS 143 



succession of figures, if looked at along all possible lines of sight, 

 will exhibit precisely the series of shapes which are found to be 

 exhibited by the regular nebula? under discussion. There are, then, 

 good grounds for conjecturing that these nebula? are rotating masses 

 of gas; but we can test this conjecture further before finally ac- 

 cepting it. 



As a mass of gas radiates its energy away it must shrinlc. If it 

 is in rotation, its angular momentum will remain constant, and the 

 shrunken mass can only carry its original dose of angular momentum 

 by rotating more rapidly than before. This conception, which 

 formed the corner-stone of the cosmogonies of Kant and Laplace, is 

 still of fundamental importance to the cosmogonist of to-day. Thus 

 every nebula, as it grows older, will rotate ever more and more 

 rapidly and, barring accidents, will in due course reach the configura- 

 tion shown in Plate 1, Figure 1. This configuration marks a verita- 

 ble landmark in the evolutionary path of a nebula. Until this con- 

 figuration is reached the effect of shrinkage can be adjusted, and 

 is adjusted, by a mere change of shape; the mass carries the same 

 angular momentum as before, in spite of its reduced size, by the 

 simple expedient of rotating more rapidly, and restores equilibrium 

 by bulging out its equator. But mathematical analysis shows that 

 this is no longer possible when once this landmark has been passed. 

 Further shrinkage now involves an actual break-up of the nebula, 

 the excess of the angular momentum beyond that which can be 

 carried by the shrunken mass being thrown off into space by the 

 ejection of matter from the equator of the nebula. 



We have so far spoken of the nebular equator as being of circular 

 shape, as it undoubtedly would be if the nebula were alone by itself 

 in space. But an actual nebula must have neighbors, and these 

 neighbors will raise tides on its surface, just as the sun and moon 

 raise tides on the surface of the rotating earth. 'V\^iatever the neigh- 

 bors are, there will always be two points of high tide antipodally 

 opposite to one another, and two points of low tide intermediate be- 

 tween the two points of high tide. Thus the equator, instead of 

 being strictly circular, will be slightly elliptical. 



If the equator of the nebula had been a perfect circle, and if the 

 nebula had been in all respects symmetrical about its axis of rota- 

 tion, the ejection of matter would have started from all points of 

 the equator simultaneously. Indeed, there could be no conceivable 

 reason why it should start at one point rather than at any other 

 point. But in nature we do not expect to find perfect balances of 

 this kind; if the main factors are of exactly equal weight some 

 quite minor factor invariably intervenes to turn the balance in one 

 direction or another. In the present problem there could be no 



