September, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



525 



form of a relatively thin disc," and instead of beintf 

 liomogeneous, as in Laplace's Hypothesis, "it may have 

 had almost any degree of heterogeneity." In this 

 theory fluid pressure, which was of fundamental im- 

 portance in Laplace's Hypothesis, is of minor con- 

 sideration, " and no general shrinkage with loss of 

 heat " plays any part in the evolution of planets from 

 the parent mass. 



^Ioulton shows that on this theory the resulting 

 planets will all probably revolve round the nucleus in 

 the same direction as the original rotation, and that the 



also agrees with the known facts of the solar system. 

 The orbits of the so-called "terrestrial planets," 

 Mercury, \'enus, the Earth, and Mars, are, on the 

 average, more eccentric than those of the large planets, 

 Jupiter, Saturn, Uranus, and Neptune; and those of the 

 small minor planets between Mars and Jupiter are still 

 inori' so. 



With reference to the rotations of the planets on 

 their axes, Moulton shows that these would probably 

 be direct, that is, in the same direction as the orbital 

 re\olutions, and that the inclinations of the axes of 



The Spiral Nebula, 51 Messier. 



(/•-/■,»« :i PJiologiath hy II-. /:. U'ihoti. r.RS) 



pl;incs of Ihcir (irl)ils " will ncarh , ihnugh not exactly, 

 coincide " ; also that the (jibils ol the larger planets 

 will show smaller deviations from the g-eneral plane 

 than those of the smaller planets, like Mercury and the 

 asteroids. This we know to be the case in the solar 

 system. He shows that the present rotation of the sun 

 is due to the original rotation of the m;iss from which it 

 was formed, combined with the disturbance caused by 

 the body which approached it, and that the more rapid 

 rotation of the sun's equator is due to the same cause. 

 He also shows that the larger the planet "the more 

 nearly circular in general " ils orbit would be; and this 



rotation to the planes of the orbits might be different 

 for different planets. Any well-marked deviation from 

 this rule might be expected in the outer planets of the 

 system. This we see in the case of LVanus and Nep- 

 tune, which have always proved stumbling blocks in 

 Laplace's Hypothesis. \Ve should also expect, he 

 thinks, " to find the larger planets rotating more rapidly 

 than the smaller," and this we know to be the case. 



With reference to the satellites, the direction of their 

 motion round the primary might, on the new hypothesis, 

 be either direct or retrograde, according to circum- 

 stances. Retrograde motion might bo expected in 



