BiCKERTON. — On the General Problem of Stellar Collision. 185 



motion of translatiou being originally in the plane of the paper, that ail 

 motion but heat motion and original rotation will still tend to occupy that 

 plane, and that it is consequently the plane of the rotation of the mass. 

 According to the modern theory of heat, all those parts, whose motion of 

 mass is destroyed, will have an equivalent heat energy given them. 



Here I approach one of the striking peculiarities of this theory. For 

 the temperature developed will not be in any way dependent on the propor- 

 tion struck off. It will depend upon the molar velocity destroyed, and this, 

 again, chiefly upon the mass of the attracting bodies, and the nearness of 

 their centres of gravity. Thus the merest graze will develope nearly as 

 high a temperatm-e in the coalesced part as though one half of each were 

 struck off. But in these two cases the gravitating powers of the coalesced 

 mass are altogether different, and hence, in some cases, where a small ratio 

 is struck off, the molecular velocity may carry every particle away into free 

 space, and, in other cases, where the collision is more complete, the great 

 attractive powers of the body may hold the gaseous mass in a definite posi- 

 tion. I need not refer to the fact that the former case is typical of tempo- 

 rary stars, and the latter of nebiili© of definite form. On reference to fig. 

 4, it will be seen that the wounded retreating stars are considerably heated 

 in and about the plane of section. On escaping the influence of the other 

 bodies they would recover their sphericity, exhibiting a very highly-heated 

 part on one side. I hope, in a future paper, to bring before the Institute a 

 number of facts and conclusions which, I believe, will actually demonstrate 

 that in these bodies we recognize the variable stars which stud our galaxy. 

 It is j)owerful evidence in favour of this theory, that many variable stars 

 are in pairs. In figure 5, the arrows show the general tendency of the 

 motion in the several parts of the spindle-shaped mass of heated matter. 

 I need scarcely mention that the mass in this form is a figure of a large 

 number of the characteristic nebula, such as the nebulse of Andromeda. 

 In figure 6, the rotation of the centre of the mass has begun to give itself a 

 spiral form, and an almost exact figure of Herschel's drawing of the nebulas 

 of the Lion is produced. The other figures trace the later possible j)heno- 

 mena which the various motions may produce. They represent systems of 

 bodies, sph'al, annular, and planetary nebulae. I again state, that these 

 figures were geometrically drawn on a well-considered estimation of the 

 probable residual motion and attractions left in the mass. When some 

 discussion has elicited all the difficulties which beset the question, I hope 

 to offer you an approximate geometrical demonstration of the problem. I 

 believe, however, that it is only in the impact of rare bodies, such as 

 nebulse, that nebulae showing a spiral reaching near the centre could be 

 produced. 17 



