1888.] 



of the various Species of Heavenly Bodies. 



9 



as a very extended spheroid, it is obvious that meteorites approaching 

 it in directions parallel to its minor axis will have fewer chances of 

 collisions than those which approach it, from whatever azimuth, in 

 what we may term the equatorial plane. These evidently, at all 

 events if they enter the system in any quantity, will do for the 

 equatorial plane exactly what their fellows were supposed to do 

 for the section in fig. 1, and we shall have on the general back- 

 ground of the symmetrically rotating nebula, "which may almost be 

 invisible in consequence of its constituent meteorites all travelling 

 the same way and with nearly equal velocities, curves indicating the 

 regions along which the entrance of the new swarm is interfering 

 with the movements of the old one ; if they enter in excess from any 

 direction, we shall have broken rings or spirals. 



This was suggested in my last paper. Various segments of rings 

 will indicate the regions where most collisions are possible, and the 

 absence of luminosity in the centre by no means demonstrates the 

 absence of meteorites there. 



Researches by Lord Rosse and others have given us forms of 

 nebulae which may be termed sigmoid and Saturnine, and these 

 suggest that they and the elliptical nebulas themselves are really pro- 

 duced by the rotation of what was at first a globular rotating swarm 

 of meteorites, and that in these later revelations we pick up those 

 forms which are produced by the continued flattening of the sphere 

 into a spheroid under the meteoric conditions stated. It is worthy of 

 remark that all the forms taken on by the so-called elliptic nebulas 

 described by the two Herschels, and by the spiral, sigmoid, and 

 Saturnine forms which have been added to them by the labours of 

 Lord Rosse and others, are recalled in the most striking manner by 

 the ball of oil in Plateau's experiment, when rotations of different 

 velocities are imparted to it. 



The Saturnine form may, indeed, in some cases represent either the 

 first or last stages in this period of the evolutionary process. I say 

 may represent, in consequence of the extreme difficulty in making the 

 observations so that in the early stages a spherical nebula, beginning 

 to change into a spheroid, may have its real spheroidal figure cloaked 

 by varions conditions of illumination. 



The true Saturnine form must, as in the case of Saturn itself, 

 represent one of the latest forms in the meteor-swarm, because, if it 

 be not continually fed from without, collisions must sooner or later 

 bring all the members of the swarm to the centre of figure. 



Cometic Nebulce. 



I do not know r that any explanation has, so far, been suggested as 

 to the origin of these curious forms, which were first figured by 

 Sir William Herschel, and of which a number have recently been 



