534 
MR. G. H. DARWIN ON THE 
In the review referred to I examined the eccentricities and inclinations of the orbits 
of the several other satellites, and found them to present indications favourable to the 
theory. In the present paper I have given reasons for supposing that the tidal fric¬ 
tion arising from the action of the other satellites on their planets cannot have had so 
much effect as in the case of the earth. That those indications were not more marked, 
and yet seemed to exist, agrees well with this last conclusion. 
The various obliquities of the planets’ equators to their orbits were also considered, 
and I was led to conclude that the axes of the planets from Jupiter inwards were 
primitively much more nearly perpendicular to their orbits than at present. But the 
case of Saturn and still more that of Uranus (as inferred from its satellites) seem to 
indicate that there was a primitive obliquity at the time of the genesis of the planets, 
arising from causes other than those here considered. 
The satellites of the larger planets revolve with short periodic times; this admits of 
a simple explanation, for the smallness of the masses of these satellites would have 
prevented tidal friction from being a very efficient cause of change in the dimensions 
of their orbits, and the largeness of the planets’ masses would have caused them to 
proceed slowly in their evolution. 
If the planets be formed from chains of meteorites or of nebulous matter the rotation 
of the planets has arisen from the excess of orbital momentum of the exterior over that 
of the interior matter. As we have no means of knowing how broad the chain may 
have been in any case, nor how much it may have closed in on the sun in course of 
concentration, we have no means of computing the primitive angular momentum of a 
planet. A rigorous method of comparison of the primitive rotations of the several 
planets is thus wanting. 
If however the planets were formed under similar conditions, then, according to 
the present theory, we should expect to find the exterior planets now rotating more 
rapidly than the interior ones. It has been shown above (see Table IV., note to § 8) 
that, on making allowance for the different degrees of concentration of the planets, 
this is the case. 
That the interior satellite of Mars revolves with a period of less than a third of its 
planet’s rotation is perhaps the most remarkable fact in the solar system. The theory 
of tidal friction explains this perfectly,and we find that this will be the ultimate 
* It is proper to remark that the rapid revolution of this satellite might perhaps be referred to another 
cause, although the explanation appears very inadequate. 
It has been pointed out above that the formation of a satellite out of a chain or ring of matter must be 
accompanied by a diminution of periodic time and of distance. Thus a satellite might after formation 
have a shorter periodic time than its planet. 
If this, however, were the explanation, we should expect to find other instances elsewhere, but the case 
of the Martian satellite stands quite alone. 
[I believe that 1 now (July, 1881) see some reason to suppose that the earliest form of a satellite may 
not be annular. The investigation necessary to test this idea seems likely to prove a difficult one.] 
