156 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



high tide. Each particle of this jet moves under the combined forces 

 of the primary and of the visiting star, and the problem of determin- 

 ing its orbit is a special case of the problem of three bodies, which 

 unfortunately is not soluble. But the general result is that the jet 

 undergoes various contortions while moving all the time in the plane 

 which contains the orbit of the visiting star. 



If such a jet had been thrown off the sun simply by an increase of 

 rotation consequent on shrinkage, its gravitational attraction would, 

 as we have seen, be inadequate to resist the expansive effect of its own 

 gas presstire, and it would have been rapidly dissipated away into 

 space. In the present situation conditions are very different, the 

 essential difference being that, while shrinkage from loss of radiation 

 is a very slow process, tidal disruption may be a very rapid process. 

 The rate of a star's rotation will alter but slightly in a thousand 

 years, whereas 10 years may suffice for a tide-raising body to come, 

 do its work, and go away again. The filament of gas set free by 

 increase of rotation would be of extreme tenuity ; a filament set free 

 by a tidal cataclysm might easily be of sufficient substance for its 

 own gravitation to hold it together as a compact whole. 



If gravitation is potent enough to do this, it will also be potent 

 enough to break up the filament into condensations, just as the fila- 

 ments of spiral nebulae are broken up into condensations. But here 

 again an essential difference must be taken into account. The shrink- 

 age of a spiral nebula is a slow secular process. Year after year and 

 century after century the filament will be ejected without change of 

 character — the process may be compared to the paying out of a coil 

 of rope. But the tidal disruption of a star is a rapid, even cataclys- 

 mic event; within a few years the emission of the filament starts, 

 reaches a maximum, declines, and ends. There is no steady paying 

 out here ; the process ought rather to be compared to the discharge of 

 a torpedo, or other body which is thickest in the middle and tapers 

 off at the two ends. When a filament of this shape breaks up into 

 condensations it will form no long chain of similar masses, but a 

 small number of unequal masses. It is natural to conjecture a priori 

 that large masses are likely to form out of the central portions where 

 matter is most plentiful, and smaller masses at the ends where matter 

 is scarce. Such a question can not, of course, be finally settled by a 

 priori conjectures, but in the present case an exact discussion of the 

 problem indicates that the a priori view is the right one, and suggests 

 that the comparative abundance of matter in the central part of the 

 filament may provide an explanation of the appearance of the more 

 massive planets, Jupiter and Saturn, near the center of the sequence 

 of planets. 



Obviously, if a tidal cataclysm can explain the existence of the 

 planets, it can also, in general terms at least, explain the existence of 



