Mr. A. S. Davis on a Theory of Nebula, and Comets. 403 



of this kind might be produced by a 



small mass of comparatively dense gas 



rushing obliquely into a large mass of 



rarer gas*. Such a mass would tend 



to the centre of gravity of the larger 



mass; and since those parts which 



are nearest the centre of gravity would 



be most accelerated, it would become Lassell, pi. 1. fio-. 3f. 



drawn out into an elongated spiral 



form. 



After a time the whole of the 

 smaller mass of gas will have col- 

 lected about the centre, and a glo- 

 bular or planetary nebula will be T ^J^Pl „ on 

 formed. Lassell, pi. 7. flg. 30. 



In the annexed diagram, copied from a drawing by Lassell, 

 we appear to have a globular nebula «.,. 



in the process of formation. That \-; - . 



the smaller mass of gas has entered "%w'£.\«.--. a 



obliquely is shown by the fact that %;v! v:\ 



the ending, a b, of the tubular portion / %^y ; :>. 



is oblique to the axis of the tube ; for / %;>.•;*:■ 



the line ab will indicate the boundary <&agfb. '%^0'. 



of the larger mass. That the gas is ( , >' - ' -'• •> ' '">' 

 collecting about the centre of the ',., 



larger mass is indicated by the fact >^_- ; " .';;// 

 that the line o c joining the centre of i#.-"l;'a ; ; ' . " ",• /? 

 the globe with the end of the tube is ^fev- : i;; ' " 4-.\ v!; ; 

 perpendicular to a b — as it should be, ».; ' > "., tl ' ,'>'*' 



supposing the larger mass of a sphe- '"-sm&Ssbw*"- 



rical form. Lassell, pi. 2. fig. 9. 



Planetary nebula? consisting of more than one envelope might 

 also be formed in the same way as those having 

 only one envelope, the envelopes being formed ^*z_ 



in succession at different periods of time. j^m sSUfk, 



Annular nebula? might be formed in the fol- - H| 



lowing manner. If a small mass of gas were 

 attracted to a larger mass so as just to graze 

 its surface on passing it, its path (previously Lassell, pi. I. fig. 4. 

 parabolic) would become elliptic, and its motion 

 would be changed into a periodical revolution round the larger 



* When I speak of one gas as being rarer than another, I mean that, 

 when subject to equal pressures, the one gas is specifically lighter than the 

 other. 



t See Memoirs of the Royal Astronomical Society, vol. xxxvi. 



2D 2 



