54 G. Hinrichs on Planetology. 
by the fact that the sum of the four great exterior planets is 4 
times the mass of the earth, while the sum of the masses of 
VI. The figure of the planets (being a 
an oblate spheroid of 
VIL. Sinall ellipticity, because 
VIII. The velocity of rotation is but small (compare § 1, 2, 3; 4 
and § 8, 7). : 
This last proposition is based upon the fact that the mom 
of rotation is but the difference between the moment of re 
tion of the exterior and interior part of the planetary ring. 
Still, the exact amount of this velocity, as well as the period 
rotation of the sun, has not yet been deduced from the neb 
hypothesis; we have often attempted it, but as yet have ! 
been able to solve this difficult problem. ~ 
Prof. Kirkwood has found” the empirical law of the velo 
f rotation, a law analogous to the third law of Kepler, Weh 
repeatedly arrived at expressions similar to (but not iden 
with) Kirkwood’s law. 
IX The Plane; and 
** As above, this Journal, 1850, ix, 395. 
