538 MR. G. H. DARWIN ON THE PRECESSION OE A VISCOUS SPHEROID, ETC. 
arisen from a planet which ever rotated about an axis at all nearly perpendicular to 
the ecliptic. 
The system of planets revolving round the sun present so strong a resemblance to the 
systems of satellites revolving round the planets, that we are almost compelled to believe 
that their modes of development have been somewhat alike. But in applying the 
present theory to explain the orbits of the planets, we are met by the great difficulty 
that the tidal reaction due to solar tides in the planet is exceedingly slow in its 
influence ; and not much help is got by supposing the tides in the sun to react on 
the planet. Thus enormous periods of time would have to be postulated for the 
evolution. 
If, however, this theory should be found to explain the greater part of the configura¬ 
tions of the satellites round the planets, it would hardly be logical to refuse it some 
amount of applicability to the planets. We should then have to suppose that before 
the birth of the satellites the planets occupied very much larger volumes, and possessed 
much more moment of momentum than they do now. If they did so, we should not 
expect to trace back the positions of the axes of the planets to the state when they 
were perpendicular to the ecliptic, as ought to be the case if the action of the satellites, 
and of the sun after their birth, is alone concerned. 
Whatever may be thought of the theory of the viscosity of the earth, and of the 
large speculations to which it has given rise, the fact remains that nearly all the effects 
which have been attributed to the action of bodily tides would also follow, though 
probably at a somewhat less rapid rate, from the influence of oceanic tides on a rigid 
nucleus. The effect of oceanic tidal friction on the oblicpiity of the ecliptic has already 
been considered by Mr. Stone, in the only paper on the subject which I have yet 
seen.* His argument is based on what I conceive to be an incorrect assumption as to 
the nature of the tidal frictional couple, and he neglects tidal reaction ; he finds that 
the effects would be cpiite insignificant. This result would, I think, be modified by a 
more satisfactory assumption. 
* Ast. Soc. Monthly Notices. March 8, 1867. 
