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XLVIII. Intelligence and Miscellaneous Articles. 



NOTE ON SPIRAL NEBULA. BY T. S. ALDIS, M.A. 



HPHE following are some points connected with nebulae which I 

 -*- have not seen noticed. 



The spiral structure can, of course, only be seen when we view a 

 nebula nearly perpendicularly to its plane of rotation. 



The nebulae we see are selected. All those composed of denser 

 substances have long since compacted into stars. Those which are 

 left are of [small density; consequently motion in them will be 

 slow. It may be noticed in passing, that to know the density of a 

 nebula of known shape by the period of an outlying mass, the paral- 

 lax of the nebula is not needed. At twice the distance, on twice the 

 scale, with the same density the period is not altered. 



Now in our own system we have reason to believe * that the pla- 

 nets thrown off the denser nebula (viz. the interior planets) were 

 thrown off in comparatively compact portions ; the planets thrown 

 off when the nebula was rare (viz. the exterior planets) were thrown 

 off in crescent-shaped masses or ansae, extending round some consi- 

 derable portion of the interior mass. It is in such crescent shapes 

 that we find the outer portions of the nebulae now seen. The cres- 

 cent shape, then, of the detached body is probably a further criterion 

 of their low density, confirming our expectation that the motion will 

 be slow and therefore not easily detected. 



The spiral formation itself is easily explained. The portion left 

 behind by the contracting nebula is of a crescent shape, the concavity 

 embracing the central mass. As this cools, the interval between 

 the two increases, whilst the outer slowly pursues its now free orbit. 

 If it underwent no further change it would rotate in its periodic 

 time, keeping its concavity constantly towards the central nebula. 

 But it, as well as the nebula, still contracts, and in so doing acquires 

 increased speed of rotation, and thus the foremost cusp will sweep 

 round slowly into the central body, the hinder cusp will sweep out 

 from it. In this way a spiral arrangement of the different detached 

 portions will arise. Of course the foremost cusp, after swinging 

 round a considerable angle, will fall foul of the central mass and be 

 reabsorbed in it, and the hinder cusp, too, will merge in outer masses; 

 so that masses which have rotated considerably will not appear as 

 elongated masses lying across the coils of the spiral, being shorn of 

 their length in the process, and thus the nebula will take its com- 

 plexion from the coils which are in the earlier stages of development. 



Manchester Free Grammar School, 

 October 11, 1870. 



* See the paper on the Nebular Hypothesis in the Phil. Mag. Oct. 1869. 



