Precejfion of the Equinoxes. 521 
fuppofe the body to move along the chord ac, and fay, 
that the angle cad meafures the deflection of the path 
in the time of the body’s moving through the arc or 
chord ac, we fir all miftake by one half of the true quan- 
tity; for draw the tangent at c, and fince A*/ is equal to 
d c from the property of the circle, the angle cc?d of de- 
viation is equal to twice the angle ca d. The practice of 
newton in a fimilar cafe, where he is inveftigating the 
horary motion of the lunar nodes in a circular orbit, is 
entirely confiftent with this. See the Principia, lib. III. 
prop. 30. 
§ 15. M. d’alembert has lately charged simpson’s 
account of the preceflion of the equinoxes with fome 
miftakes of this nature in his fecond lemma; but, in 
juftice to Simpson, I mult fay, that, whatever other de- 
feats there may be in his paper, I am convinced, after the 
molt diligent attention, that thofe alluded to are without 
foundation. 
§ 16. Sir Isaac newton firfl: obferved, that an ho- 
mogeneous globe could not poflibly retain many diftindt 
motions, without compounding them all into one, and 
revolving with a Ample and uniform motion about an 
invariable axis. When two forces imprefs upon a globe 
two diftindt circular motions he briefly concludes in- 
(b) See Principia, lib, I. prop. ixvi. coroll. 22. . 
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