264 THE MATHEMATICAL PRINCIPLES [BjOK II. thod, which extends itself, without any troublesome calculation, not ojdy to the drawing of tangents to any curve lines, whether geometrical or mechanical, or any how respecting right lines or other cnrves, but also to the resolving other abstrnser kinds of problems about the crookedness, areas, lengths, centres of gravity of curves, &c. ; nor is it (as Hudd^ri's method de Maximis & Minimia) limited to equations which are free from surd quantities. This method I have interwoven with that other oj working in equations, by reducing them to infinite serie?. So far that letter. And these last words relate to a treatise I composed on that sub ject in the year 1671. The foundation of that general method is contain ed in the preceding Lemma. PROPOSITION VIII. THEOREM VI. If a body in an uniform medium, being uniformly acted upon by the force of gravity, ascends or descends in a right line ; and the whole space described be distinguished into equal parts, and in the beginning of each of the parts (by adding or subducting the resisting force of the medium to or from the force of gravity, when the body ascends or de scends] yon collect the absolute forces ; I say, that those absolute forces ire in a geometrical progression. For let the force of gravity be expounded by the given line AC ; the force of resistance by the indefi nite line AK ; the absolute force in the descent of the body by the difference KC : the velocity of the I tody F/ by a line AP, which shall be a mean proportional be tween AK and AC, and therefore in a subduplicate ratio of the resistance; the increment of the resistance made in a given particle of time by the lineola KL, and the contemporaneous increment of the velocity by the lineola PQ ; and with the centre C, and rectangular asymptotes CA, CH, describe any hyperbola BNS meeting the erected perpendiculars AB, KN, LO in B, N and O. Because AK is as AP2 , the moment KL of the one will be as the moment 2APQ of the other, that is, as AP X KC ; for the in crement PQ of the velocity is (by Law II) proportional to the generating force KC. Let the ratio of KL be compounded with the ratio KN, and the rectangle KL X KN will become as AP X KC X KN ; that is (because the rectangle KC X KN is given), as AP. But the ultimate ratio of the hyperbolic area KNOL to the rectangle KL X KN becomes, when the points K and L coincide, the ratio of equality. Therefore that hyperbolic evanescent area is as AP. Therefore the whole hyperbolic area ABOL is composed of particles KNOL which are always proportional to the velocity AP; and therefore is itself proportional to the space described with that velocity. Let ,that area be now divided into equal parts