EVOLUTION OF THE SOLAR SYSTEM. 
531 
friction may be assigned as a reason to explain how it happened that the terrestrial 
planet had contracted to nearly its present dimensions before the genesis of a satellite, 
but that this was not the case with the exterior planets. 
The numbers given in Table III., § 8, show that the efficiency of solar tidal friction 
is very much greater in its action on the nearer planets than on the further ones. 
But the total amount of rotation of the various planetary masses destroyed from the 
beginning cannot be at all nearly proportional to the numbers given in that table, for 
the more remote planets must be much older than the nearer ones, and the time 
occupied by the contraction of the solar nebula from the dimensions of the orbit of 
Saturn down to those of the orbit of Mercury must be very long. Hence the time 
during which solar tidal friction has been operating on the external planets must be 
very much longer than the period of its efficiency for the interior ones, and a series of 
numbers proportional to the total amount of rotation destroyed in the several planets 
would present a far less rapid decrease, as we recede from the sun, than do the 
numbers given in Table III. Nevertheless the disproportion between these numbers 
is so great that it must be admitted that the effect produced by solar tidal friction on 
Jupiter and Saturn has not been nearly so great as on the interior planets. 
In § 8 it has been shown to be probable that, as a planetary mass contracts, the rate 
of tidal retardation of rotation, and of destruction of rotational momentum increases, 
rises to a maximum, and then diminishes. This at least is so, when the acceleration 
of rotation due to contraction exceeds the retardation due to tidal friction; and this 
must in general have been the case. Thus we may suppose that the rate at which 
solar tidal friction has retarded the planetary rotations in past ages was greater than the 
present rate of retardation, and indeed there seems no reason why many times the 
present rotational momenta of the planets should not have been destroyed by solar 
tidal friction. But it remains very improbable that so large an amount of momentum 
should have been destroyed as to materially affect the orbits of the planets round 
the sun. 
I will now proceed to examine how the differences of distance from the sun would 
be likely to affect the histories of the several planetary masses. 
According to the nebular hypothesis a planetary nebula contracts, and rotates 
quicker as it contracts. The rapidity of the revolution causes its form to become 
unstable, or, perhaps a portion gradually detaches itself; it is immaterial which of 
these two really takes place. In either case the separation of that part of the mass, 
which before the change had the greatest angular momentum, permits the central 
portion to resume a planetary shape. The contraction and increase of rotation proceed 
continually until another portion is detached, and so on. There thus recur at intervals 
a series of epochs of instability or of abnormal change. 
Now tidal friction must diminish the rate of increase of rotation due to contraction, 
and therefore if tidal friction and contraction are at work together, the epochs of 
instability must recur more rarely than if contraction acted alone. 
MDCCCLXXXI. 3 Z 
