IN PHYSICAL ASTRONOMY. 
279 
On the Precession of the Equinoxes, supposing the Earth to revolve in a resisting 
medium. 
In my last paper on Physical Astronomy, I gave expressions for the varia- 
tions of the six constants which enter into the solution of this problem, upon 
the hypothesis that the body revolves in a medium devoid of resistance. 
If we suppose a plane to revolve in a resisting medium, about an axis per- 
pendicular to itself, the resistance of the medium can produce no effect, and 
the phenomena will only be modified in a slight degree by the friction of the 
plane surface against the medium. If, however, the inclination of the plane 
on the axis of rotation differs from 90°, the effect of the resistance of the 
medium becomes sensible, tending to retard the motion of the plane ; the 
effect being greatest when the axis of rotation is parallel to the plane. 
This principle is used in some machines, as in self-playing organs, to regu- 
late the motion by means of a vane, of which the inclination to its axis of rota- 
tion can be varied at pleasure. 
In the case of a sphere, whatever be the direction of the axis of rotation, this 
effect of the resistance is insensible ; and also in the case of a solid of revolu- 
tion when the axis of rotation coincides with the axis of the figure, but not 
otherwise. If the difference of the latitude of the axis of rotation from 90° 
(supposing the equator from which the latitudes are reckoned to coincide with 
the equator of the figure) be at any time small, the mathematical investigation 
appears to show, that the effect of the resistance of the medium is to diminish 
continually this difference. In the case of the earth, this quantity is now 
insensible ; hut as the probability is small that this was the case in the 
first instance, may this circumstance arise from the resistance of a medium 
of small density acting for a great length of time ? and can the change of 
climate on the surface of the earth, a change of which the probability is in- 
dicated by many geological phenomena, be explained in the same manner ? 
It may be remarked, however, that the effect of a resisting medium in dimi- 
nishing the eccentricities of the orbits of the planets is of the same order, and 
that these, although for the most part small, are far from having reached zero. 
The tendency of a resisting medium is also to diminish the major axes of the 
orbits of the planets ; these effects, if they exist, will probably be most sensible 
2 o 
MDCCCXXXI. 
