cerning the Seas Ebbing and Flowing : Had I not been already 
too tedious , I fhould now proceed Co give a further reafon, 
why I do introduce this confideration ot the Common Center of 
Gravity in reference to Aftr Gnomical Accounts. For indeed, that 
which may poffibly feem at firft to be an Objection 
with me one reafon/or it. 
It may be thought perkaps, that if the Earth fhould thus de- 
fcribe an Epicycle about theCommon Center of Gravity, it would 
(by this its change of plaeej) difturbe the Ccehflial motions - and 
make the apparent places of the Planets , efpecially fome of 
them, different from what they would otherwife be. For 
though fo fmall a removal of the Earth , as the Epicycle would 
caufe (efpecially if its Semidiameter fhould not be above of 
the Earths Semidiameter) would fcarce be fenfible (if at all)to 
the remoter Planets ^ yet as to.the nearer it might. 
Now though what Galilee o anfwers to a like Obje&ion in his 
Hypothecs ; (that its poffible there may be fome fmall difference, 
which Aflronomers have not yet been fo accurate, as to obferve) 
might here perhaps ferve the turn Yet my anfweris much 
otherwife ; to wit, that fuch difference hath been obfervcd.and 
hath very much puzzeled Aflronomers to give an account of. 
About which you will find Mr, Eiorrocky ( in fome of his Let- 
ters, whereof I did formerly, upon the Command of the Royal 
Society , make an ExtraB ) was very much perplexed ; and was 
fain, for want of other relief , to have recourfe to fomewhat 
like Ky piers amicable Fibres , which did according to the feve- 
ral pofitions of the Moon , accelerate or retard the Moon’s 
motion - which amicable Fibres he had no affection to at all ( as 
there appears ) if he could any other waies give account of 
thofe little inequalities ; and would much rather (I doubt not) 
have embraced this Notion of the Common Center of Gravity, 
to falve the Phenomenon , had it come to his mind, qr been fug- 
gefted to him. And you find, that other Aflronomers have been 
feen to bring in (fome upon one fuppofition, fome upon ano- 
ther) feme kind of Menflrual Mquation , to folve the inequalities 
of the Moon’s motion, according to her YyzWzW Revolution, 
or different Afpedfs ( ofNew-moon,Full Moon, See.) befide 
what concerns her own Periodical motion. 
For 
