278 Mr. J. S. Stuart Glennie on the 
dent that «! will act as an 
accelerating force of rota- 
tion in the direction £; 
and that this rotation will 
be retarded, and e’ partly 
neutralized by the resist~ 
ance in the direction #9’. 
10. In the actual case 
of theplanets, their masses 
and velocities of rotation 
are such that the solar medium can be of course conceived, not 
as causing, but only as tending slightly to accelerate their 
rotations. And a problem is by this theorem suggested of ex- 
treme interest, but also, in the present state of hydrodynamics, 
of extreme difficulty, as to how far this accelerating force of rota- 
tion is neutralized. The earth’s rotation has been hitherto 
considered and proved to be invariable, only in respect of the 
action of the sun and moon; and it is to be remembered that 
doubts have been thrown on its actual invariability even during 
the short period of 4000 years; that one-tenth of a second in 
10,000 years would be a large astronomical quantity; and that 
their actual times are all that, at best, we know of the rotations 
of the other planets. I shall not at present offer any further 
remarks on this problem, considered either as a purely hydro- 
dynamical one, or with the data afforded by the planetary system, 
except to note that nothing seems as yet to have been done towards 
determining the relative effect of a resisting medium on (what 
may at any moment be called) the back of a revolving and rota- 
ting body. And it should seem that little further* can be done 
towards the solution of this problem without experimental data on 
this point especially. The determination of the secular inequality, 
the result of the variously directed and most improbably equili- 
brating forces of the medium, becomes still more complicated 
when such a triple motion as that of a satellite is considered. 
Such, then, is the theorem I would venture to offer as, if not 
giving as yet the demonstrable explanation of the effect of the 
resisting medium on the bodies of the solar systemy, at least sug- 
gesting new and very interesting experimental and analytical 
problems m hydrodynamics. 
11. (III.) The condition of Translation is a difference of 
polar pressures on a point; the condition of Rotation is equal 
* Stokes, ‘On Fluid Friction.’ 
+ I may refer to, though I cannot here discuss, the remarks on this sub- 
ject of Sir John Herschel, ‘ Outlines of Astronomy,’ 5th edit. p. 389 note ; 
and of Prof. Challis in his paper “ On the Resistance of the Luminiferous 
Medium,” Phil. Mag. May 1859. ; 
