142 CLASSICS OF MODERN SCIENCE 

 particles, several concentrical rings of vapours circulating about the 

 Sun. But mutual friction of the molecules of each ring ought to 

 accelerate some and retard others, until they all had acquired the 

 same angular motion. Consequently the real velocities of the mole- 

 cules which are farther from the Sun, ought to be greatest. The 

 following cause ought likewise to contribute to this difference of 

 velocities : The most distant particles of the Sun, and which, by 

 the effects of cooling and condensation, have collected so as to con- 

 stitute the superior part of the ring, have always described areas 

 proportional to the times, because the central force by which they are 

 actuated has been constantly directed to this star ; but this constancy 

 of areas requires an increase of velocity, according as they approach 

 more to each other. It appears that the same cause ought to di- 

 minish the velocity of the particles, which, situated near the ring, con- 

 stitute its inferior part. 



If all the particles of a ring of vapours continued to condense with- 

 out separating, they would at length constitute a solid or a liquid 

 ring. But the regularity which this formation requires in all the 

 parts of the ring, and in their cooling, ought to make this phenomenon 

 very rare. Thus the solar system presents but one example of it ; 

 that of the rings of Saturn. Almost always each ring of vapours 

 ought to be divided into several masses, which, being moved with 

 velocities which differ little from each other, should continue to 

 revolve at the same distance about the Sun. These masses should 

 assume a spheroidical form, with a rotatory motion in the direction 

 of that of their revolution, because their inferior particles have a less 

 real velocity than the superior ; they have therefore constituted so 

 many planets in a state of vapour. But if one of them was suffi- 

 ciently powerful, to unite successively by its attraction, all the others 

 about its centre, the ring of vapours would be changed into one sole 

 spheroidical mass, circulating about the Sun, with a motion of 

 rotation in the same direction with that of revolution. This last case 

 has been the most common ; however, the solar system presents to us 

 the first case, in the four small planets which revolve between Mars 

 and Jupiter, at least unless we suppose with Olbers, that they origi- 

 nally formed one planet only, which was divided by an explosion into 

 several parts, and actuated by different velocities. Now if we trace 

 the changes which a further cooling ought to produce in the planets 



