THE EVOLUTION OF STELLAR SYSTEMS 87 



planets Jupiter and Saturn should rotate most rapidly, as is 

 known to be the case. 



It has long been known that the effect of a resisting medium 

 on the paths of bodies moving in it, in a manner analogous to 

 the planets moving round the sun is (i) to reduce their distances 

 from the central body ; (2) to diminish the eccentricity of their 

 orbits, i.e. to make these more nearly circular. 



The proof of this is given in works on analytical dynamics 

 (Cheyne's, Planetary Theory, and other books) and was of course 

 well known to Laplace, who says, "At the same time the planet 

 approaches the sun, by the effect of the resisting medium the 

 orbit also becomes rounder." The well-known comet of Encke 

 is thought to be gradually drawing nearer to the sun by such 

 an action. 



Thus the present shape of the planetary paths is accounted 

 for, the action of the resisting medium having changed their 

 orbits. Around each planet circulates a vortex of cosmical 

 dust, and the descent of this material upon the surfaces of sun 

 and planets is considered to give rise to the accelerations of 

 their equatorial regions — i.e. the fact that the parts of the sun, 

 Jupiter and Saturn near their respective equators have a 

 shorter period of rotation than those farther north or south. 

 However, there is no perceptible difference of rotation in different 

 regions of our own earth or of Mars, so far as known and the 

 amount of matter required to produce such an effect (at present) 

 seems greater than can reasonably be supposed to fall upon 

 the surfaces of the planetary bodies. 



A similar difficulty ;occurs in the meteoric theory of the 

 sun's heat, attributing the latter to the impact produced by the 

 fall of countless meteorites upon its surface. No doubt such 

 bodies do fall upon the surface of the sun in considerable 

 quantities, but the amount required to maintain the sun's output 

 of heat is so enormous that there should be an enormously 

 greater quantity in regions near the sun, so that our own 

 earth ought to receive nearly half as much heat as she gets from 

 the sun by impact with meteors. This is certainly not the 

 case. 



The descent of matter upon the sun increasing its mass may 

 also account for the small secular acceleration of the earth 

 indicated by the observations of eclipses, and the outstanding 

 motion of the perihelion of Mercury, which Leverrier attributed 



