(tony 
the Calculation thereof : Whk^Center of Gravity) 
he does not onely demonftratc to be (which others 
have hitherto Poftulated, but not any, that I know 
of, demonftrated i) but doth from General Princi- 
ples (hew^ How by Calculation to aflign the fame, in 
infinite forts of Lines, Surfaces, Solids^ aswellfuch 
as are bounded, or take their rife f rom Crooked Lmes, 
as thofe that are bounded onely with Streight Lines 
and Plains- 
And from the General Principl^^, here laid down, 
he doth, in his Third Part C which i^ to follow} de- 
rive the Doftrine of t\tQ VeSlis or l.ea^^r ^ the trecb^ 
lea or PHlley j the C^hlea or Scretp y the Axis' in Peri- 
trochio^ or feveral fores of Wheel- work ^ and other 
fuch Mechanical Engins derived from thefe. As like- 
wife the Doftrine of Ferchjj^on (on wlvch depends 
that of the Cuneus or Wedge , with many other Spe- 
culations of a like kinde :) And that of Reftlition or 
Rebounding ; which ( as appears by a (hott Specimen 
formerly Printed in M/i^^. 43. of thefe Trafits^ he 
derives from a Rcpercufliori , either of fome other 
Body in Motion, which it meets with, or from the 
Elaftick Force or Spring in one or both of the Meet- 
ing Bodies, which, being com preiled by thcColhfi- 
on, doth endeavor ro reftore it felf by caftingoff 
thefe Bodies, one or both ways, ( ^onfonant tohis 
Principles here laid down in the Tenth, eleventh, and 
twelfth Propolitions of his Firji Cbafter , where the 
Impedimentum or C^/l^f/e is made -fufficient to retard 
or flop a Motion j but a ^is contraria or contrary 
Force, 
