748 Dr. H. Wilde on the Origin of 



less elongated, but the vast tabulated periodic times of comets 

 supposed to move in parabolic and hyperbolic curves are 

 necessarily ultra-speculative. 



As the attraction of solar gravitation extends far beyond 

 the orbit of Neptune, the motion of a body on the line of an 

 open curve would ultimately be arrested, and a comet would 

 necessarily return over the same track, approximately, with 

 a retrograde motion as an unknown member of the solar 

 system. Halley's comet, however, is considered to move in 

 an elliptical orbit, and has, therefore, the longest periodic 

 time of which astronomers have certain knowledge. 



As the principle of conservation holds good alike for 

 celestial and terrestrial bodies, the moving force of comets 

 will not exceed the attraction of gravitation beyond the limits 

 of the solar system, and will be much less through the con- 

 version of molar into molecular motion by friction of the 

 discrete particles of cometary matter among themselves 

 during the act of ejection, as also from the resistance of the 

 medium through which they move in their orbits, and 

 especially near the sun. 



The principle of conservation, as will be obvious, will hold 

 equally for the comets ejected from the planets of other 

 stellar systems. Hence the absurdity of bringing cometary 

 bodies into the solar system which contains within itself the 

 power of evolving its own comets. Moreover, it will be 

 further evident that this immigration motion might be ex- 

 tended to include the earth and other planets as bodies from 

 other stellar systems, captured by the sun in their wanderings 

 from outer space. 



Jupiter, with his system of satellites, is generally regarded 

 as a miniature solar system formed by the successive con- 

 densations of a nebular substance surrounding the planet. 

 The laws of attraction, moving force, and Kepler's laws have 

 the same relations among his satellites as in the planetary 

 system. The binary progression of the periodic times of the 

 three adjoining major satellites, Io, Europa, and Ganymede 

 (which are very nearly in the ratio of J, 2, 4), indicates an 

 orderly process of evolution similar to that of the binary 

 progression of the planetary distances. 



The erratic movements and irregular orbits of the three 

 outer Jovian satellites recently discovered have, however, 

 presented a new problem for solution in connexion with the 

 nebular theory of the evolution of satellites, as it was found 

 that the orbital motion of the outermost one was in a 

 retrograde direction. 



An attempt has been made to explain the anomaly by 



