Laplace's tiieoky of planetaky evolution. 265 



each of the revolving rings thus periodically detached, 

 eventually became ruptured at its weakest point, and con- 

 tracting on itself, gradually aggregated into a rotating 

 mass ; that this, like the parent mass, increased in rapidity 

 of rotation as it decreased in size, and, where the centrifu- 

 gal force was sufficient, similarly threw off rings, which fi- 

 nally collapsed into rotating spheroids; and that thus out of 

 these primary and secondary rings there arose planets and 

 their satellites, while from the central mass there resulted the 

 sun. Moreovei^ it is tolerably well known that this d pri- 

 ori reasoning harmonizes with the results of experiment. 

 Dr. Plateau has shown that when a mass of fluid is, as far 

 may be, protected from the action of external forces, it 

 will, if made to rotate with adequate velocity, form detach- 

 ed rings ; and that these rings will break up into spheroids 

 which turn on their axes in the same direction with the 

 central mass. Thus, given the original nebula, which, ac- 

 quiring a vortical motion in the way we have explained, 

 has at length concentrated into a vast sj^heroid of aeriform 

 matter moving round its axis — given this, and mechanical 

 principles explain the rest. The genesis of a solar system 

 displaying movements like those observed, maybe predicted ; 

 and the reasoning on which the prediction is based is coun- 

 tenanced by experiment.* 



• 

 * It is true that, as expressed by him, these propositions of Laplace 

 are not all beyond dispute. An astronomer of the highest authority, who 

 has favoured me with some criticisms on this essay, alleges that instead of 

 a nebulous ring rupturing at one point, and collapsing into a single mass, 

 " all probability would be in favour of its breaking up into many masses." 

 This alternative result certainly seems to be more likely. But granting 

 that a nebulous ring would break up into many masses, it may still be con- 

 tended that, since the chances are infinity to one against these being of 

 equal sizes and equidistant, they could not remain evenly distributed round 

 tl\eii orbit : this annular chain of gaseous masses would break up into 

 groups of masses ; these groups would eventually aggregate into larger 

 groups ; and the final result wo' ild be the formation of a single mass. I 

 12 



