38 D. Trowbridge on the Nebular Hypothesis. 
the difference between the mean distances of the planets, in | 
terms of the mean distance between the earth and sun; and also 
the breadth of the primitive rings, according to Kirkwood’s 
Analogy.” We have at present no method probably more ac 
curate than Kirkwood’s to determine the breadth of the primi — 
tive rings. oe 
We thus see that, generally speaking, there is an increase of — 
distance between the orbits of the planets, and also in the 
breadth of the rings abandoned by the sun; and yet it is not 
absolutely the case in every instance; nor is the increase accord: — 
ing to any simple law, as we had occasion to peint out from : 
what was assumed to be the probable condition of the primitive 
solar spheroid. , 
55. If we compare the masses and the densities of the plan- 
ets, we shall see here, also, a general increase in the masses, and 
a decrease in the densities, as we get farther from the sun, as can 
be seen in the following table: 
Taste VI. 
Planet. Magnitude. Mass, | Density. 
spheroid, threw off a satellite-ring, taking off the rarer part and 
leaving the denser, so that the ultimate mean density of the 
earth became a little greater than that of Venus. There seems 
to be some peculiarity about Saturn in respect to density that 
© This Journal, [2], xiv, 213. 
* According to Kirkwood’s Analogy, this Journal, [2], xiv, 213. 
