Ml TKORITIC HYPOTHESES 



303 



tin- diftVrent directions of motion and the various velocities of 

 meteorites are such as to forbid the belief that the solar system, with 

 its symmetrical discoidal form and its harmonious motions, could 

 have been formed in this way. 



A more logical meteoritic hypothesis is based on the conception 

 that meteorites may be aggregated into swarms and constitute 

 nebulae. This hypothesis is, 

 therefore, nebulo-meteoritic. 

 Sir George Darwin came to the 

 conclusion that such a swarm 

 of meteorites would act very 

 much like a gas, and that the 

 laws of gases could be applied 

 in determining its mechanics. 

 If the meteorites of such a 

 nebula move in various direc- 

 tions, this hypothesis, as ap- 

 plied to the origin of the earth, 

 is practically identical with the 

 gaseous hypothesis; and as 

 applied to the solar system, it 

 is subject to the criticisms al- 

 ready urged against that hy- 

 pothesis. The term meteoritic 

 hypothesis is used commonly 

 in the above sense. It was ap- 



Fig. 288. A spiral nebula in Canes 

 \Vn;ititi, 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 sym- 

 metrical in the inner parts, but departs 

 from symmetry in the outer parN. A 

 notable feature is the comet-like streamers 

 of some of the Jcnots and denser portions. 

 If these are true streamers, curved by 

 motion, they imply an active rotation, 

 and strengthen the inference drawn from 

 the coiled condition. (Photo, by Ritchey, 

 Yerkes Observatory.) 



plied by its authors (Lockyer 

 and Darwin) chiefly to the 

 earlier and more scattered con- 

 ditions of the nebulae, and has 

 not been applied specifically to 

 the formation of a planet. If, 

 on the other hand, the meteorites were so assembled as to have con- 

 centric orbits and form a disk-like system, the system, to all intents 

 and purposes, falls into class 3. 



3. The planetesimal hypothesis. When the shortcomings of 

 the Laplacian hypothesis were seen to be so serious that there was 

 no apparent way of escape from them, an alternative better in accord 

 with the facts was sought. 



It has been shown by photography that there are a multitude of 



