THEIR ARRANGEMENTS AND FORMATION. 31 



previous condition of matter. Without any knowledge of these 

 speculations, M. Laplace started one of his own, founded on 

 the known features of the constitution of the solar system, as 

 the uniformity of plane and of motions. When he afterwards 

 became acquainted with Herschel's idea, he regarded it as in. 

 harmony with his own. He showed that, if a luminous matter 

 such as had been described by Herschel, existed, and if nuclei 

 were established in it, these might become centres of aggrega- 

 tion for the neighbouring diffused matter ; on such centres a 

 rotatory motion would be established, wherever, as was the 

 most likely case, there was any obliquity in the lines of direc- 

 tion in which the opposing currents met each other : this 

 motion would increase as the agglomeration proceeded : at 

 certain intervals, the centrifugal force acting in the exterior of 

 the rotating mass would overcome the agglomerating force, and 

 a series of rings would thus be left apart, each possessing the 

 motion proper to itself at the crisis of separation. These, 

 again, could only continue in their annular form, if uniform in 

 constitution. There being many chances against this, they 

 would probably break up, and be agglomerated into either one 

 or several masses, which would then become representatives of 

 the primary mass, and perhaps give rise to a similar progeny 

 of inferior masses. The result would be Planets and Satel- 

 lites ; each having an orbitual speed corresponding with that 

 of the zone constituting it ; each obtaining a rotatory motion 

 from the excess of speed observed by the outer, over the inner 

 portion of the zone. All this Laplace showed to be possible 

 under the physical laws of the universe. In the case of our 

 solar system, the results tally in the nicest way with the hy- 

 pothesis ; for example, the rapidity of the revolutionary 

 motions of the planets is in proportion to their nearness to the 

 sun ; so also, do the primaries rotate more rapidly than the 

 satellites revolve in their orbits. There are even singularities 

 tending to support the hypothesis j the small planets between 

 Mars and Jupiter may be regarded as an example of a zone 

 which agglomerated in distinct parts, while the rings of Saturn, 

 now believed to be three in number, are instances of zones still 

 in their original state. 



A law presiding over the solar system, which may be said 

 to constitute a fourth of the series of which the three first are 

 associated with the name of Kepler, has lately been discovered 

 (1850) by Mr. Kirkwood of Pennsylvania, and it is important 

 to observe how entirely it accords with the Laplacian hypo- 



