48 



GEOLOGY. 



pendence of parts. It is, therefore, conceived that the innumerable 

 solid or liquid particles which the continuous spectrum implies, 

 revolve about the common center of gravity as though they were 

 planetoidal bodies. If this were certainly known to be the case, 

 these might be well called planetesimal nebulae. 



It is clear from the tenuity of these nebulae, as seen from the side 

 of the spiral, that they are thin and disk-like, and this is directly shown 

 to be so in the case illustrated in Fig. 18, which is seen obliquely. The 

 slender ellipse shown in Fig. 19 probably represents a spiral seen edge- 

 wise, but this is not certain. In their disk-like shapes, these nebula? 



Fig. 18. Fig. 19. 



Fig. 18. — A spiral nebula in Pegasi (HI 53), seen obliquely, showing the disk-like 



form. (Photo, from Lick Observatory.) 

 Fig. 19. — An elliptical nebula (HV 41), interpreted uncertainly as a possible spiral 



seen from a point in its own plane. (Photo, from Lick Observatory.) 



conform to the mode of distribution of matter in the solar system. 

 Within the area of their disks also, the distribution is irregular as 

 it is in the solar system — a fact too much overlooked by reason of our 

 predilection for symmetry, under the influence of the symmetrical 

 Laplacian conception. 



If these nebula? are composed of minute aggregates of matter re- 

 volving as planetesimals about the nebular centers, the momentum of 

 the outer parts is necessarily high in proportion to that of the inner 

 portions, and in this essential particular they meet the most stubborn 

 of the difficulties that confront the Laplacian hypothesis. 



All of the spiral nebula? of our illustrations have dimensions that 



