SEC. IV.] OF NATURAL PHILOSOPHY. 125 the body with that motion, which the impulse alone would generate, you will have the motion with which the body will go off from a given place of impulse in the direction of a right line given in position. COR. 4. And if that body is continually disturbed by the action of some foreign force, we may nearly know its course, by collecting the changes which that force introduces in some points, and estimating the continual changes it will undergo in the intermediate places, from the analogy that appears in the progress of the series. SCHOLIUM. If a body P, by means of a centripetal force tending to any given point R, move in the perimeter of any given conic sec tion whose centre is C ; and the law of the centripetal force is required : draw CG parallel to the radius RP, and meet ing the tangent PG of the orbit in G ; and the force required (by Cor. 1, and CG3 Schol. Prop. X., and Cor. 3, Prop. VII.) will be as SECTION IV. Of the finding of elliptic, parabolic, and hyperbolic orbits, from ttu. focus given. LEMMA XV. If from the two foci S, II, of any ellipsis or hyberbola, we draw to any third point V the right lines SV, H V, whereof one HV is equal to the principal axis of the figure, thai is, to the axis in which the foci are situated, the other, SV, is bisected in T by t/ie perpendicular TR let fall upon it ; that perpendicular TR will somewhere touch the conic section : and, vice versa, if it does touch it, HV will be equal to the principal axis of the figure. For, let the perpendicular TR cut the right line HV, produced, if need be, in R ; and join SR. Be cause TS, TV are equal, therefore the right lines SR, VR, as well as the angles TRS, TRV, will be also equal. Whence the point R will be in the conic section, and the perpen dicular TR will touch the same ; and the contrary. Q.E.D.