226 THE MATHEMATICAL PRINCIPLES [BoOK 1 cases to revolve in conic sections, and composing sphaerical bodies whose centripetal forces observe the same law of increase or decrease in the recess from the centre as the forces of the particles themselves do ; which is verv remarkable. It would be tedious to run over the other cases, whose con clusions are less elegant and important, so particularly as I have done these. I choose rather to comprehend and determine them all by one gen eral method as follows. LEMMA XXIX. ff about the centre S there be described any circle as AEB, and about the centre P there be. also described two circles EF, ef, cutting the Jirst in E and e, and the line PS in F and f ; and there be let fall to PS the perpendiculars ED, ed ; I say, that if the distance of the arcs EF; ef be supposed to be infinitely diminished, the last ratio of the evanscent linr Dd to the evanescent line Ff is the same as that of the line PE to the live PS. For if the line Pe cut the arc EF in q ; and the right line Ee, which coincides with the evanescent arc Ee, be produced, and meet the right line PS in T ; and there be let fall from S to PE the perpendicular SG ; then, because of the like triangles DTE, '!>, DES, it will be as Dd to Ee so ))T to TE, or DE to ES : and because the triangles, Ee?, ESG (by Lem. VIII, and Cor. 3, Lem. VII) are similar, it will be as Ee to eq or F/soES to SG ; and, ex ceqno, as Dd to Ff so DE to SG ; that is (because of the similar triangles PDE; PGS), so is PE to PS. Q.E.D. PROPOSITION LXXIX. THEOREM XXXIX. Suppose a superficies as EFfe to have its breadth infinitely diminished, and to be just vanishing ; and that the same superficies by its revolution round the axis PS describes a spherical concavo-convex solid, to the several equnJ particle* of which there tend equal centripetal forces ; I soy, that the force with which thit solid attracts a corpuscle situate in P is in a ratio compounded of the ratio of the solid DE2 X Ff and the ratio of the force with which the given particle in the place Ff would attract the same corpuscle. For if we consider, first, the force of the spherical superficies FE which