THE ORIGIN OF THE EARTH. 



71 



rings, revolving as units, happened to collect about some point other 

 than the center of the cross-section, the foregoing conclusions would 

 not hold, but if the matter were drawn together by gravity simply, as 

 usually supposed under the Laplacian hypothesis, it is not evident why 

 it should not collect about the middle part. 



Now as matter of fact, the six inner planets and their satellites 

 rotate forward. The satellites of Uranus revolve backward in a plane 



Fig. 27. Fig. 28. 



Fig. 27. — RR represents a ring of gas moving as a unit and hence the outer portion 

 the faster. If converted into a spheroid, E, centrally located, the rotation is 

 forward, as shown by the arrow. 



Fig. 28. — PP represents a belt of planetesimals revolving concentrically about the 

 center, S. If these collect about the central point of the belt into a spheroid, E, 

 by the enlargement of the inner orbits or the reduction of the outer ones, the con- 

 centric arrangement remaining, the rotation will be retrograde, as shown by the 

 arrow. 



inclined 82.2° to the ecliptic; those of Neptune also revolve backward, 

 in a plane inclined 34.5° to the ecliptic. The rotations of these planets 

 themselves have not been determined. These exceptional inclina- 

 tions and revolutions have been interpreted as very oblique or partially 

 overturned revolutions. Accepting the foregoing premises, the prev- 

 alence of direct rotation has been regarded as strongly confirmatory 

 of an origin from gaseous rings rotating as units, and as strongly 

 adverse to accretion from bodies revolving independently. The force 

 of this line of reasoning has apparently been felt to be so strong as to 

 be essentially fatal to the latter conception. It, therefore, requires 

 critical consideration. 



The reasoning is good for the special case cited, that of a symmetrical 



