194 WARREN UPHAM, M.A., F.G.S.A., ON THE NEBULAR AND 



" In attempting to follow the probable evolution of such a spiral 

 nebula, three elements stand out conspicuously; (1) the central 

 mass, obviously to become the sun ; (2) the knots on the arms that 

 ■are assumed to be the nuclei of the future planets and perhaps 

 .satellites ; and (3) the diffuse nebulous matter to be added to the 

 Jiuclei as material of growth. In the particular case of the solar 

 nebula it is assumed (I) that the central mass was relatively very 

 great ; (2) that the knots were very irregular in size and placed 

 -at irregular distances from the center ; and (3) that the nebulous 

 portion was very small relative to the central mass and probably 

 large relative to the knots. 



" . . . Since all the planetesimals and planetary nuclei were 

 revolving in the same direction about the solar mass, the collisions 

 were all overtakes, and could have been violent only to the extent 

 •of their differences of orbital velocity, modified by their mutual 

 attractions. These velocities are of a much lower order than the 

 average velocities of meteoritic collisions. Many of the overtakes 

 would obviously be due to differences of velocity barely sufficient to 

 faring about an overtake. When the relative mildness of impact is 

 -considered in connection with the intervals between impacts at a 

 given spot, the conviction can scarcely be avoided that the surface 

 temperature would not necessarily have been high. It seems probable 

 that it would have been moderate throughout most of the period of 

 aggregation, and certainly so in the declining stages of infall. 



"By graphical inspection of all probable cases, it may be seen 

 that the possibilities of overtake favourable to forward rotation 

 -exceed those favourable to retrograde rotation. This holds true on 

 the assumption of an equable distribution of planetesimals, which 

 may fairly be assumed as an average fact, but not necessarily as 

 •always the fact ; and hence the conclusion is not rigorous, and a 

 backward rotation is not impossible. From the nature of the 

 ■case, a varying rotation for the several planets is more probable 

 than a nearly uniform one. 



"It is also obvious that the impacts on the right and left sides of 

 .a growing nucleus, as well as those on the outer and inner sides, 

 might be unequal, and hence obliquity of rotation of varying kinds and 

 ■degrees might arise. As the solar system presents these variations, 

 the method of accretion here postulated seems to lend itself happily 

 to the requirements of the case. 



"... A planetary nucleus gathers planetesimals' that have 

 orbits both smaller and larger than itself, and hence in effect it 

 sweeps a space both outside and inside its own zone. The breadth 

 of this space is dependent on the eccentricity of its own orbit and 

 on the eccentricities of the orbits of the planetesimals it gathers in 

 on either hand. 



"... For the large planets that have dominated their 



