nnnAL motion of nebulous matter. 253 



but also towards neighbouring flocculi. Hence the whole 

 assemblage of flocculi will break up into subordinate 

 groups : each group concentrating towards its local centre 

 of gravity, and in so doing acquiring a vortical movement, 

 like that subsequently acquired by the whole nebula. 

 Now, according to circumstances, and chiefly according to 

 the size of the original nebulous mass, this process of local 

 aggregation will produce various results. If the w^hole 

 nebula is but small, the local groups of flocculi may be 

 drawn into the common centre of gravity before their con- 

 stituent masses have coalesced with each other. In a 

 larger nebula, these local aggregations may have concen- 

 trated into rotating spheroids of vapour, while yet they 

 have made but little approach towards the general focus of 

 the system. In a still larger nebula, where the local aggre- 

 gations are both greater and more remote from the com- 

 mon centre of gravity, they may have condensed into 

 masses of molten matter before the general distribution of 

 them has greatly altered. In short, as the conditions in 

 each case determine, the discrete masses produced may 

 vary indefinitely in number, in size, in density, in motion, 

 in distribution. 



And now let us return to the visible characters 

 of the nebulae, as observed through modern telescopesc 

 Take first the description of those nebulse which, by the 

 hypothesis, must be in an early stage of evolution. 



" Among the irregular nebulm^'' says Sir John Herschel, " may 

 oe comprehended all which, to a want of complete^ and in most 

 instances^ even of partial resohahility by the power of the 20-feet 

 reflector, unite such a deviation from the circular or elliptic form, 

 or such a want of symmetry (with that form) as preclude their 

 being placed in Class 1, or that of regular nebulaa. This second 

 class comprises many of the most remarkable and interesting ob- 

 jects in the heavens, as icell as the most extensive in respect of the 

 area they occupy. '^^ 



