50 GEOLOGY. 



bring within the reach of successful study some of the smaller spiral 

 nebulae that represent more nearly, in mass and proportions, the solar 



Fig. 21. — A spiral nebula in Ursa Major, Messier 18, showing a large dominant central 

 mass, with symmetrical arms, well coiled. Form and proportions not unlike 

 those assigned to the parent^ solar nebula. Interpreted as somewhat advanced 

 in evolution. (Photo, from Lick Observatory.) 



system, and that some of these will be found sufficiently near to per- 

 mit an early determination of their motions. 



