¥ 
t 6 7 .] 
to which they meant to apply Sir Ifaac Newton’s rule, 
is widely different. The earth and moon revolve 
about their common centre of gravity : their distances 
from which being inverfely as their mattes, and the 
forces, by which either is attradted by the other, a$ 
alfo the forces of the fun to difturb their motions, 
being in the fame ratio ; it follows, that the earth in 
her motion round the common centre of gravity will 
fuffer dittiurbances every way ttmilar to thofe of the 
moon. And the whole motion of the apfii of the 
moon’s orbit, refulting from the two difturbing forces, 
will be near the double of what either of them could 
produce feparately, round a fix’d centre *. 
9. To 
* “ Shtatenus terra et luna ctrcum commune gravitatis centrum 
i( revolvuntur , perturbabitur etiam motus terra: circa centrum illud 
6t a • viribus confimil'tbus ; fed fummas tarn virium quam motuum re- 
“ ferre licet ad lunam Princip. p. 429. 
And/). 141. Jpfis lunce ejl duplo velocior circiter : but this has 
been ftrangely miftak^n, as it the author having revifed and printed 
his 9th fe&ion a third time, and above forty years after it was in- 
vented, fhould, after all, own, that it fignrfied nothing to his pur- 
pofe. VVould this be the nil moliturdnepte, fo juftly applied to 
Newton ? 
See likewife, p. 423 ; where having deduced the motion of the 
apfuls of Jupiter’s fatellites from that of our moon’s, he adds, 
“ Diminui tamen debet motus augis fic inventus in rati one 5 ad 9, 
<e vel 1 ad 2, circiter , ob caufatn , quam Inc exponere non vacat 
The reafon is not, that the orbits of Jupiter’s moons are lefs ex- 
centric than that of ours, as fome have imagined ; for, “ augends 
“ vel diminuendo excentricitatem et inclinationem orbis , non mutatur 
xi motus augis fenfibihter, nifi ubi ccedemfunt nimis magnae f p. 180. 
Is it not rather, becaufe the adlion of the feveral fatellites upon their 
primary and upon one another, in all the poffible variety of direc- 
tions, reduces the cafe of any particular fatellite to that of a fingle 
tfeody revolving round a fix’d centre, viz. that of Jupiter’s fyftem? 
I 2 
