130 THE MATHEMATICAL PRINCIPLES [BOOK I. CASE 3. If all the three are equal, the point Z will be placed in the centre of a circle that passes through the points A, B, C. Q.E.I. This problematic Lemma is likewise solved in Apollonius's Book oi Tactions restored by Vieta. PROPOSITION XXL PROBLEM XIII. About a given focus to describe a trajectory that shall pass through given points and touch right Hues given by position. Let the focus S, the point P, and the tangent TR be given, and suppose that the other focus H is to be found. On the tangent let fall the perpendicular ST, which produce to Y, so that TY may be equal to ST, and YH will be equal to the principal axis. Join SP, HP, and SP will be the difference between HP and the principal axis. After this manner, if more tangents TR are given, or more points P. we shall always determine as many lines YH, or PH, drawn from the said points Y or P, to the focus H, which either shall be equal to the axes, or differ from the axes by given lengths SP ; and therefore which shall either be equal among themselves, or shall have given differences ; from whence (by the preceding Lemma). that other focus H is given. But having the foci and the length of the axis (which is either YH, or, if the trajectory be an ellipsis, PH -f SP ; or PH — SP, if it be an hyperbola), the trajectory is given. Q.E.I. SCHOLIUM. When the trajectory is an hyperbola, I do not comprehend its conjugate hyperbola under the name of tins trajectory. For a body going on with a continued motion can never pass out of one hyperbola into its conjugate hyperbola. The case when three points are given is more readily solved thus. Let B, C, I), be the given points. Join BC, CD, and produce them to E, F, so as EB may be to EC as SB to SC ; and FC to FD as SC to SD. On EF drawn and pro duced let fall the perpendiculars SG, BH, and in GS produced indefinitely E take GA to AS, and Ga to aS, as HB is to BS ; then A will be the vertex, and Aa the principal axis of the tra jectory ; which, according as GA is greater than, equal to, or less than