FUNDAMENTAL PROBLEMS OF GEOLOGY. 213 



ously known. From the superior number of spiral nebulae it is a 

 safe inference that their peculiar forms represent some prevalent 

 process in celestial dynamics. This is in itself a reason why re- 

 search should turn to them, by preference, for the origin of the 

 present solar system.* 



Nothing is yet positively known of the motions of the parts of these 

 spirals, for time enough has not yet elapsed since they were first 

 sharply photographed to permit the requisite comparisons. Infer- 

 ences from their remarkable forms are the only present resource. 

 To me these peculiar forms seem to imply that the spirals sprang 

 from a combined outzvard and rotatory ^novement. The outward move- 

 ment may no longer exist, as it may have been already checked by 

 the gravity of the central mass, and the rotatory motion be the dom- 

 inant one at present, but their forms seem still to bear the impress 

 of an outward movement. If the outward movement has ceased, or 

 when it ceases, the rotatory movement must tend to wrap the spiral 

 up more and more closely and symmetrically, because the revolutions 

 of the inner parts must be more rapid than those of the outer parts. 

 By this it is not meant that the matter of the nebulae is necessarily 

 drawn nearer the center of the system, but merely that the arms are 

 stretched and more closely coiled. The forms that seem to be the 

 more mature appear to betray this, for their inner parts are coiled 

 more closely and symmetrically than their outer parts. In the 



* The profoundly lamented death of Professor Keeler, just as he was beginning 

 to reap the rich fruits of his skill and patience in nebular investigations, gives 

 historical value to his latest statement of results, published about two months 

 before his death. 



" I. Many thousands of unrecorded nebulae exist in the sky. A conservative 

 estimate places the number within reach of the Crossley reflector at about 

 120,000. The number of nebulse in our catalogues is but a small fraction of this. 



' ' 2. These nebulae exhibit all gradations of apparent size from the great nebula 

 in Andrcnneda down to an object which is hardly distinguishable from a faint 

 star disk. 



" 3. Most of these nebulae have a spiral structure. * * * 



" While I must leave to others an estimate of the importance of these conclu- 

 sions, it seems to me that they have a very direct bearing on many, if not all, 

 questions concerning the cosmogony. If, for example, the spiral is the form 

 normally assumed b}^ a contracting nebulous mass, the idea at once suggests 

 itself that the solar sj'stem has been evolved from a spiral nebula, while the pho- 

 tographs show that the spiral nebula is not, as a rule, characterized by the sim- 

 plicity attributed to the contracting mass in the nebular hypothesis. This is a 

 question which has already been taken up by Professor Chamberlin and Mr. 

 Moulton of the University of Chicago. ' ' Astrophys. Jour. , June, 1900, pp. 347-348. 



