The III. and lafl: Book is entituled de Syfiemate Mundi^ 
wherein the Denionftrations of the two former Books are 
applyed to the Explication of the principal Phemmena of 
Nature ; Here the verity of the Hyfcthefts of Kjpler is de- 
monftrated ; and a full Refolution given to all the diiBcul- 
ties that occur in the Aflronomkd Science ; they being no- 
thing el(e but the neceifary confequencesof the Sun^ Earth, 
Moon^ and Planets, having all of them a gravitation or ten- 
dency towards their Centers f)roportionate to the Quan- 
tity of Matter in each of them, and whole Foxe abates in 
duplicate proportion of the Diftance reciprocally. Here 
likewife are indifputably folved the Appearances of the 
Tides, or Flux and Reflux of the Sea ; and the Spheroidical 
Figure of the Earth and Jupiter determined, (from which 
the preceflion of the Equinoxes, or rotation of the Earths 
Axis is made out, ) together with the retroceflTion of the 
Moons Nodes, the Quantity and inequalities of whofe Mo- 
tion are here exadly ftated a pricre : Laftly the Theory of 
the Motion of Comets is attempted with fuch fuccels, 
that in an Example of the great Comec which appeared 
in i68f, the Motion thereof is computed as exaftly as we 
can pretend to give the places of the primary Planets; and 
a general Method is here laid down to ftate and determine 
thoTraje^ort^ of ComQts, by an eafy (jeometrical Con- 
fl:ruQ:ion;upon fuppofition that thofeCurves are Para wlick, 
or fb near it that the Parahola may ierve without fenfible 
Error ; tho^ it be more probable, faith our Author, that 
thefe Orbs are £////?to^/, and that after long periods Co- 
mets may return again. But fiich Ellipfes are by Reafon 
oftheimmenfe diftance of the f(?tr/, and fmaiinels of the 
Latus Return, in the Parts near the Sun where Comets 
appear, not eafily diftinguifhed from the Curve of theP^- 
rahoU : as is proved by the Example produced. 
The whole Book is interfperfed with Lernnns of Gene- 
ra! u{h in Geometrj, and feveral new Methods applyed, 
which 
