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but when there is but one point, as L, or, which is 
the fame, one moon, which revolves in the circum- 
ference of the ring • this motion fubfifts, and difturbs 
the motion of the moon in the plane of its orbit. 
In like manner the force of the point L parallel to 
y C y or according to c L, is deftroyed, in a ring, by the 
force of another equal point a' placed on the other 
fide with regard to and at the fame diftance as the 
point L. But this force fubfifts in the cafe where 
there is but one fingle point L, or one moon, and this 
motion diforders, befides, the motion of the moon in 
the plane of its orbit. 
Laftly, If the ring, which circulates about the 
centre C, be elliptical, the motion of every point of 
the ring in the orbit is proportional to its diftance 
from the centre ; becaufe, on account of the conti- 
nuity of the ring, all the points complete their revo- 
lution in the fame time, and have equal angular mo- 
tions; but when it is but one fingle moon that re- 
volves, in an elliptic orbit, it defcribes areas propor- 
tional to the times, and has an angular motion fo 
much the greater, as it approaches nearer the centre ; 
which is the contrary of the motion of the points of 
the elliptic ring, the more diftant whereof Irom the 
centre, have the moft motion. And it arifes from 
hence, that the motion of the pole of the lunar orbit 
is much the fame as that of the ring, taking it during 
the entire revolution ; but that there is a fmall varia- 
tion, or inequality, in the inftantaneous motion of the 
pole of a moon ; whereas this inftantaneous motion 
is conftant through every inftant of the revolution of 
the ring. 
I (hall 
