Section VIII.
Of the invention of orbits wherein bodies will revolve, being acted upon by any ſort of centripetal forces.
Proposition XI. Theorem XIII.
If a body, acted upon by any centripetal force, if any how moved, and another body aſcends or deſcends in a right line; and their velocities be equal in any one caſe of equal altitude, their velocities will be alſo equal at all equal altitudes.
Let a body deſcend from A (Pl. 17. Fig. 3.) through D and E, to the centre C, and let another body move from V in the curve line VIKk. From the centre C, with any diſtances, deſcribe the concentric circles DI, EK, meeting the right line AC in D and E, and the curve VIK in I and K.
