122 SEE— THE PAST HISTORY OF THE EARTH. [April 23, 



system was formed from a nebula, and to-day we know that this 

 nebula was of the spiral type, due to the automatic coiling up under 

 mutual gravitation of two or more streams of cosmical dust. Wher- 

 ever such streams meet, or pass near one another, there is developed 

 a cosmical vortex, with rotation about a center, and a definite mo- 

 ment of momentum about an axis. This is due to the fact that the 

 Impact is never central, but always unsymmetrical, and thus gives 

 rise to a rotation. 



The two or more streams which meet continue to wind up, under 

 the effects of mutual gravitation, and thus we have the different 

 observed types of spiral nebulae. The nebula continues to rotate 

 and the coils are drawn closer and closer together, and the whole 

 mass slowly settles towards its center. The planets, which are 

 formed by the agglomeration of cosmical dust in the convolutions 

 of the nebula, revolve constantly in the surrounding nebular medium. 

 As the planetary bodies grow by the gathering in of the cosmical 

 dust in which they revolve their orbits are reduced in size and 

 rounded up under the secular action of the resisting medium. 



It is shown by this line of inquiry, and especially by the round- 

 ness of Neptune's orbit, that our system extends much beyond Nep- 

 tune ; and that the orbits now observed to have a round form were 

 originally much larger and also much more eccentric than they are 

 now seen to be. It is impossible to determine definitely how much 

 the orbits have been reduced in size, but owing to the almost total 

 obliteration of the eccentricity, it seems certain that they were origi- 

 nally two or three times larger than they are now. 



Moreover, it is proved that in a resisting medium of given den- 

 sity the secular effect is proportionally greater on a small planet 

 than on a large one. This is owing to the fact that the mass, and 

 therefore the moment of momentum, is proportional to the cube of 

 the planet's radius, but the surface, and therefore the resistance of 

 the medium, proportional to the square of the radius; so that the 

 changes in the orbit of a small body are greater than in that of a 

 large body in the inverse ratio of the radius, for masses of the same 

 mean density. 



Accordingly it follows that small planets, such as the asteroids 

 or inner planets were at a former epoch, when revolving in a. 



