750 Dr. H. Wilde on the Origin of 



Applying the foregoing principles of direct and retrograde 

 motion of cometary bodies to the explosion of a whole planet 

 between Mars and Jupiter, the fragments projected opposite 

 to the orbital motion would be retarded, and by the action 

 of solar gravity revolve in a smaller orbit than that of the 

 planet before the explosion. On the other hand, the motion 

 of the fragments coincident with the orbital direction would 

 be increased, and by the diminished action of the sun's 

 attraction, revolve in a larger orbit in accordance with the 

 observations. In neither of these cases, however, would the 

 orbits of the fragmentary bodies a^ain intersect each other 

 at the point of the planet's orbit where the explosion 

 occurred. 



All the observations which I have made on the evolution 

 of the Jovian satellites and cometary ejecta, are applicable 

 alike to the Saturnian and other systems of planetary satel- 

 lites. The evidence of orderly progression in the periodic 

 times of the inner satellites of Saturn differs in one respect 

 from that indicated by the satellites of Jupiter in similar 

 positions, as the times of revolution of the first and third 

 satellites are in the ratio of 1 and 2, and the times of the 

 second and fourth are also in the same ratio, as was first 

 pointed out by Sir John Herschel*. 



Notwithstanding that the actual surface of Jupiter is 

 covered with dense vapours of great depth, just as the ter- 

 restrial globe at one period of its history was enveloped with 

 an atmosphere of aqueous vapour which has since condensed 

 to form the oceans, several facts, in addition to those ad- 

 vanced, indicate that the Jovian planet has a solid crust of 

 considerable thickness. 



The remarkably bright round spots which suddenly appear 

 on the planet at irregular intervals, and have been described 

 by Lassell, and also by Dawes, as having some resemblance 

 to lunar craters t, indicate considerable volcanic activity 

 below the atmospheric envelope. The eruptive matter from 

 the Jovian craters also produces the appearance of belts on 

 his outer surface, as well as those seen on Saturn and Uranus. 

 That these belts and bands are caused by volcanic dust 

 ejected to great heights from the interior parts of planetary 

 bodies is highly probable from observations made on the 

 great eruption of Krakatoa in 1883 J. 



The ejecta from this volcano reached a height of more 



* ' Outlines of Astronomy,' p. 368, 1864. 



t Monthly Notices Roy. Ast. Soc. vol. x. 1850 ; ibid. vol. xviii. 1857. 

 'I " The Eruption of Krakatoa and Subsequent Phenomena," Report 

 of the Krakatoa Committee of the Royal Society, 1888. 



