Mr. A. S. Davis on a Theory of Nebula and Comets. 409 



it, and a spiral nebula would be formed. The dark shading, as 

 before, shows the visible part. 



This represents the formation of a spiral nebula : — 



Lassell gives several drawings of S -shaped nebulae having 

 nuclei in the middle. 



The above diagram is intended to represent the mode of for- 

 mation of a nebula of this kind. 



Two masses approaching each other move in parabolas, having 

 their centre of gravity as a common focus. On approaching 

 each other they are drawn out into an elongated form ; and if a 

 collision occurs, the common boundary will be shaped as in the 



figure. 



Arthur Sladen Davis. 



Roundhav Vicarage, 

 March 18, 1870. 



