470 THE POPULAR SCIENCE MONTHLY. 



Saturn's rings, be a prey to great commotions, sustaining a constant 

 loss of energy and contracting the circles which its parts described 

 around the sun. In endeavoring to account for the direct motion in 

 secondary systems, Laplace contends that, in consequence of friction, 

 the supposed primitive solar rings would have a greater velocity in 

 their outer than in their inner zones. Now, if friction were so potent 

 as to counteract to such an extent the normal effects of gravitation, it 

 must be an eternal bar against the origin of worlds by nebulous dis- 

 memberment ; and if a ring of attenuated matter were placed under 

 the circumstances suggested by the eminent astronomer, it would be 

 ultimately doomed, not to form a planet, but to coalesce with the im- 

 mense spheroid of fiery vapor which it is supposed to have environed. 



For further progress in the task of tracing the course of celestial 

 events and thus obtaining materials for an astronomical history of 

 worlds, it is necessary to consider, not only the theory of motion and 

 stability in comparatively small orbits, but also the effects Avhich a 

 resisting medium of space may produce during long periods of time. 

 Though the evidence which cometary motion gives of this rare fluid is 

 far from being satisfactory, more reliable information on the subject 

 may be gleaned from other sources, as I pointed out in an article in the 

 " Philosophical Magazine " for June, 1861. In the September number of 

 "The Popular Science Monthly " I alluded to the form of Mars as bear- 

 ing marks of a former rapid rotation which appears to have been con- 

 siderably reduced by the friction of a space-pervading fluid. But by 

 far the most acceptable evidence on this question has been lately derived 

 from the peculiarities of the nearest moon of Mars, as the small size of 

 its orbit and its brief period of revolution have been ascribed to a resist- 

 ing medium, even by advocates of the nebular hypothesis. The bearing 

 of the new discoveries on certain astronomical doctrines has been already 

 pointed out, in the July number of this journal, by my long-esteemed 

 friend whose useful life has been since lost to the cause of science. 

 The efiiects of the space-pervading fluid will appear more decided if we 

 regard the diminutive satellites as former asteroids which became so far 

 a prey to the Martian attraction as to be reduced to their present sub- 

 ordinate condition. Such views respecting their origin were first sug- 

 gested by Prof. Kirkwood, and were subsequently advocated by Prof. 

 Alexander in a paper read before the National Academy of Science. 



The possibility that some straggling fragments from the ruins of one 

 world might become the satellites of another, depends on a principle 

 which has been long applied in tracing the origin of the cometary mem- 

 bers of the solar family. It has been maintained by Laplace that 

 comets were at first strangers in our system, and that many of them, 

 coming from remote interstellar regions and entering the sphere of the 

 sun's attraction, obtained a permanent domicile in his extensive domain ; 

 their orbits being changed from hyperboles to ellipses chiefly by some 

 planetary disturbances. There would, however, be less probability that 



