164 [Oct. 6, 
Kirkwood. ] 
Il. 
Tue SATURNIAN SYSTEM. 
Pe: Vile 4 
tp esp + (2) 
y "4 h m s 
beh ce Mee oceee Gaegly ae | 
pve lp 10 bo! 13 
VES 
Tur URANIAN SYSTEM. 
pel. ne pan 
api’ pl +6 
iy aes 5 
apt =p — 1 
It is infinitely improbable that all these coincidences should be purely 
accidental. Their physical cause is a legitimate object of research, and 
the writer is vain enough to believe that he has suggested the true one.* 
Before proceeding with our discussion, however, it may be proper to in- 
dicate such modifications of the nebular hypothesis as seem to be de- 
manded by recent discoveries. 
The view generally received in regard to the formation of the solar sys- 
tem has been that equatorial rings were abandoned only in the vicinity 
of the present planetary orbits. As the writer has elsewhere observed, 
however, ‘‘it seems highly probable that, after first reaching the point 
at which gravity was counterbalanced by the centrifugal force arising 
from the rotation of the contracting spheroid, a continuous succession 
of narrow rings would be thrown off in close proximity to each other, 
and revolving in different periods according to Kepler’s third law.” But 
in this matter we are not left to mere speculation. The zone of minor 
planets has evidently not been produced by a single annulus, all the parts 
of which had, at first, nearly equal velocities. On the contrary, it must 
have resulted from an almost continuous abandonment of narrow rings, 
from the exterior limit at the mean distance 3.50, down to the interior, 
at 2.20. The rings of Saturn, moreover, afford a similar index to the pro- 
cess of planetary formation. 
Let us assume, then, the existence of a central mass 8, with a ring R, 
and an exterior planet P. The particles of the ring having different dis- 
tances from the centre of motion will move with different velocities. Let 
*Met, Astr., Ch. XIII., and Monthly Notices of the It. A. S., vol. XXIX. 
they 
