Kirkwood.] iUo [Nov. 21, 



lite's origin, was about 2.5.* With this valueof p, and the value of x 

 already obtained for each planet, the ratio of the mean density of the solar 

 mass to that of the planets at the respective epochs of their separation 

 would have been as follows : 



For Neptune 1.31 : 1 



" Uranus 1.31:1 



" Saturn 1.39:1 



" Jupiter *. 1.39:1 



" Mars 1.25:1 



" Earth 1.29 : 1 



" Venus 1.27 : 1 



" Mercury 1.22 : 1 



From these numbers we infer that central condensation had com- 

 menced in the solar nebula before the origin of Neptune, and that the 

 ratio of the mean density to the density of the equatorial parts near the 

 surface was approximately the same at the successive epochs of planetary 

 formation. 



Were the Planets formed from nebulous Rings? 



If the original solar mass, like most nebulae, was irregular in form, the 

 Srst matter detached would not probably be a ring, but a nebulous planet. 

 As condensation advanced, the centrifugal force would increase until 

 approximately equal to the central attraction. The disturbing influence 

 of the planet already formed wovild produce, when in perihelion, an in- 

 creasing tidal-wave, resulting in the separation of a second planet. The 

 origin of other planets is accounted for in like manner. If, in the ancient 

 history of the system, nebulous matter, left at first exterior to the orbit ot 

 a new planet, should subsequently fall upon the central body, the effect 

 would be not only a shortening of the period, but probably also a lessen- 

 ing of the orbit's eccentricity. 



III. T7ie Peculiar Belations of the Martian, System. — Professor Pick- 

 ering estimates the diameter of Phobos at seven miles.f Adopting 

 this value, and supposing the ratio between the densities of Phobos 

 and Mars equal to that between the moon and the earth, we shall 

 lind the limit of the satellite's equilibrium to be 6.5 miles from its centre, 

 or three miles from its surface. Were the density reduced to that of 

 Saturn, the limit would be almost exactly at the surface ; or, with a den- 

 sity equal to that of Mars when the radius of the latter was that of the 

 satellite's orbit, the limit would be at a considerable distance within the 

 surface. Since, therefore, the satellite could never have existed at its 



• It was shown by Laplace ttiat a rotating homogeneous fluid cannot retain 

 its spheroidal form when p is greater than 2.7197. Mec. Cel. Ill, Hi, (S20 [1605'], 

 Bowditch's Trans. The ratio would be less in the case of central condensation. 



t Annals of the Observatory of Harvard College, Vol. xi. Professor Seth C. 

 Chandler makes the diameter still less. See Sci. Obs. for Sept., 1877. 



