428 THE SUPPOSED SELF-LUMINOSITY 



The Earth and Venus, which are very nearly the same size, are both 

 larger than Mercury, but Mars, outside the Earth, is much smaller 

 than the Earth, its diameter being only 4,100 miles : the Earth's 

 equatorial diameter being 7925 miles. Jupiter is the largest body 

 in the system, except the Sun ; Saturn, next in size, is next in distance ; 

 but Uranus and Neptune are much smaller than either Jupiter or 

 Saturn, their diameter in round numbers being severally about 

 35,000 miles. 



We might also naturally expect to see the number of moons be- 

 longing to the outside planets increased in proportion to their dis- 

 tances from the Sun — but this is not the case, for although Jupiter 

 has four moons and Saturn eight, and several rings, yet Uranus, 

 which is outside Saturn, is now known to have only four satellites, 

 and Neptune, the most remote body in the system from the Sun, (as 

 far as yet known) being 2,700 millions of miles more distant from 

 the Sun than our Earth, has, like our globe, but one moon. Thus we 

 see that although the denser bodies are nearest the Sun generally, 

 yet they are not uniformly so ; that whilst the larger bodies of the 

 system are those most remote from the Sun, yet increase of size is 

 not uniformly proportioned to increase of distance, and that although 

 the planets most distant from the Sun as a general rule are those 

 which have the greater number of moons, still the number of moons 

 belonging to any planet is not necessarily contingent on its distance 

 from the Sun. Variety in physical condition is therefore a character- 

 istic of the planetary bodies, as it is indeed of every other work of 

 the Creator, and in all probability not any two of the planets are in 

 any way similarly constituted. 



But if this self-luminosity of Neptune be confirmed by further 

 observation, it will certainly be an unique feature in the Solar System. 

 It has always occurred to me that one great difficulty, of which I 

 have never met any explanation, attends what is called the Nebular 

 Theory. This is — if all the bodies of the system had a common 

 origin, being formed from a rotating ISebula throwing off rings and 

 planets, &c, why in such a case should the luminous atmosphere be 

 confined to one of these bodies only, viz., the central one ? and why 

 should the others have very different atmospheres or envelopes ? 



But if Neptune's atmosphere is self-luminous, it will at any rate 

 shew that it is not incompatible with the conditions of the Solar Sys- 

 tem, for one body besides the central one to be provided with a 

 Photosphere. I must observe here that I have not seen any con- 

 firmation of the self-luminosity of Neptune in astronomical works to 

 which I have access ; but the recentness of the discovery of this 



