134 THE MATHEMATICAL PRINCIPLES [BOOK 1. PQ, PR are drawn. In all other cases the locus of the point P will be one of the three figures which pass commonly by the name of the conic sections. But in room of the trapezium A BCD, we may substitute a quadrilateral figure whose two opposite sides cross one another like diago nals. And one or two of the four points A, B, C, D may be supposed to be removed to an infinite distance, by which means the sides of the figure which converge to those points, will become parallel ; and in this case the conic section will pass through the other points, and will go the same way as the parallels in, infinitum. LEMMA XIX. To find a point P from which if four right lines PQ, PR, PS, PT an drawn to as many other right lines AB, CD, AC, BD, given by posi tion, each to each, at given angles, the rectangle PQ X PR, under any two of the lines' drawn, shall be to the rectangle PS X PT, under the other tivo. in a given ratio. Suppose the lines AB, CD, to which the two right lines PQ, PR, containing one of the rect angles, are drawn to meet two other lines, given by position, in the points A, B, C, D. From one of those, as A, draw any right line AH, in which you would find the point P. Let this cut the opposite lines BD, CD, in H and I ; and, because all the angles of the figure are given, the ratio of PQ to PA, and PA to PS, and therefore of PQ to PS, will be also given. Subducting this ratio from the given ratio oi PQ X PR to PS X PT, the ratio of PR to PT will be given ; and ad ding the given ratios of PI to PR, and PT to PH, the ratio of PI to PH. and therefore the point P will be given. Q.E.I. COR. 1. Hence also a tangent may be drawn to any point D of the locus of all the points P. For the chord PD, where the points P and D meet, that is, where AH is drawn through the point D, becomes a tangent. In which case the ultimate ratio of the evanescent lines IP and PH will be found as above. Therefore draw CF parallel to AD, meeting BD in F, and cut it in E in the same ultimate ratio, then DE will be the tan gent ; because CF and the evanescent IH are parallel, and similarly cut in E and P. COR. 2. Hence also the locus of all the points P may be determined. Through any of the points A, B, C, D, as A, draw AE touching the locus, and through any other point B parallel to the tangent, draw BF meeting the locus in F ; and find the point F by this Lemma. Bisect BF in G, and, drawing the indefinite line AG, this will be the position of the dia meter to which BG and FG are ordinates. Let this AG meet the locus