Page:Philosophical Transactions - Volume 004.djvu/202

 the Calculation thereof: Which (Center of Gravity) he does not onely demonstrate to be (which others have hitherto Postulated, but not any, that I know of, demonstrated;) but doth from General Principles shew, How by Calculation to assign the same in infinite sorts of Lines, Surfaces, Solids, as well such as are bounded, or take their rise from Crooked Lines, as those that are bounded onely with Streight Lines and Plains.

And from the General Principles, here laid down, he, doth, in his Third Part (which is to follow) derive the Doctrine of the Vectis or Leaver; the Trochlea or Pulley; the Cochlea or Screw; the Axis in Peritrochio, or several sorts of Wheel-work; and other such Mechanical Engins derived from these. As likewise the Doctrine of Percussion (on which depends that of the Cuneus or Wedge; with many other Speculations of a like kinde:) And that of Resilition or Rebounding, which (as appears by a short Specimen formerly Printed in Numb. . of these Tracts) he derives from a Repercussion, either of some other Body in Motion, which it meets with, or from the Elastick Force or Spring in one or both of the Meeting Bodies, which, being compressed by the Collision, doth endeavor to restore it self by calling of these Bodies, one or both ways. Consonant to his Principles here laid down in the Tenth, eleventh, and twelfth Propositions of his First Chapter; where the Impedimentum or Obstacle is made sufficient to retard or stop a Motion; but a vis contraria or contrary