46 GEOLOGY. 



ceases, the rotatory movement must tend to wrap the spiral up more 

 and more closely l and symmetrically, because the revolutions of the 

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

 forms that seem to be the more mature appear to betray this, for the 

 inner parts are coiled more c'osely and symmetrically than the outer 



Fig. 14. Fig. 15. 



Fig. 14. — A brilliant spiral nebula in Canes Venatici, Messier 51. The exposure was 

 long and has given relative exaggeration to the fainter parts. The nucleus is 

 apparently dense and relatively massive; the coiling is pronounced and rather 

 symmetrical in the inner parts, but departs from symmetry in the outer parts. 

 A second nucleus is attached to the extremity of one arm. If this be interpreted 

 as the representation of the disturbing star, it should perhaps be regarded as made 

 of colder, heavier material, much less subject to expulsive elasticity, and hence less 

 dispersed, and only slightly affected by rotating influences. A notable feature is 

 the comet-like streamers of some of the knots and denser portions. If these are 

 true streamers, curved by motion, they imply an active rotation and strengthen 

 the similar inference drawn from the coiled condition. The system is perhaps to 

 be interpreted as young, but as having advanced rapidly in its rotatory evolution 

 because of its massive nucleus. (Photo, by Ritchey, Yerkes Observatory.) 



Fig. 15. — A spiral nebula (Messier 61) in which the central mass is sharply denned, 

 the immediately surrounding portion diffuse and hazy, while the outtying portion 

 retains its spiral distribution. The middle of the outer portion appears to con- 

 tain most of the outlying matter, the distribution somewhat resembling that in 

 the solar system. The dimensions here are presumptively much greater than those 

 of the solar nebula, but this is not certainly known. (Photo, from Lick Observa- 

 tory.) 



parts. This is notable in Figs. 13 and 14. In the remarkable nebula 

 in Canes Venatici (Fig. 14) there are curved streamers, like the tails 

 of comets, stretching outward from some of the knots in the arms. 

 If these are indeed streamers driven outward from the knots and 



1 This does not mean that the matter is drawn toward the center, but merely that 

 the arms are stretched and more closely coiled. 



