The two triangles CET, CAN have then the angle c common, and the sides about that angle reciprocally proportional; those triangles are therefore equal, namely, the ▲ CET = A CAN. Corol. 1. From each of the equal tri. CET, CAN, take the common space CAPE, and there remains the external ▲ PAT = A PNE. Corol. 2. Also from the equal triangles CET, CAN, take the common triangle CED, and there remains the ▲ TED ≈ trapez. ANED. THEOREM XIX (19). The same being supposed as in the last Proposition; then any Lines кQ, QG, drawn parallel to the two Tangents, shall also cut off equal Spaces. That is, AKQG=trapez.ANHG, and AKqg=trapez. ANhg. 19 T K Ag D H G C : GQ2. : GQ2; DE. Then For, draw the ordinate DE: KQG :: DE2 TED th. by div. the trap. 'ANED: trap. ANHG :: CA2- CD2 : CA2 — CG2, ANED = A TED; : KQG. Corol. 3. And from the equals ghнG, gqq¤, B Corol. 4. Again from the equals KQG, TEHG, TELKA LQH. Corol Corol. 2. Hence also, CA CD2 + cď2,· and ca2 DE2 + de2. Corol. 3. Further, because CA2: ca3:: AD. DB or cď2: DE2, therefore CA: ca :: cd: DE. likewise CA: ca :: CD: de. THEOREM XVIi (15). If from any Point in the Curve there be drawn an Ordinate, and a Perpendicular to the Curve, or to the Tangent at that Point: Then, the Dist. on the Trans, between the Centre and Ordinate, CD: For, by theor. 2, CA2: ca2 :: AD.DB : DE, CD. DTAD. DB; = CA2: ca2 :: TD.DC: TD. DP, or AC2 ça2: DC : DP. Q. E. D. THEOREM XVIII (18). If there be Two Tangents drawn, the One to the Extremity of the Transverse, and the other to the Extremity of any other Diameter, each meeting the other's Diameter produced; the two Tangential Triangles so formed, will be equal. The two triangles CET, CAN have then the angle c common, and the sides abou: that angle reciprocally proportional; those triangles are therefore equal, namely, the ▲ CET = A CAN. Corol. 1. From each of the equal tri. CET, CAN, take the common space CAPE, and there remains the external ▲ PATA PNE. Corol. 2. Also from the equal triangles CET, CAN, take the common triangle CED, and there remains the ▲ TED ≈ trapez. ANED. THEOREM XIX (19). The same being supposed as in the last Proposition; then any Lines кQ, QG, drawn parallel to the two Tangents, shall also cut off equal Spaces. That is, For, draw the ordinate DE. Then The three sim. triangles CAN, CDE, CGH, are to each other as CA, CD, CG; th. by div. the trap. 'ANED: trap. ANHG :: CA2-CD2 : CA2 — CG2. theref. by equ. trap. ANED: trap. But, by sim. As, tri. TED: tri. theref. by equality, ANED : But, by cor. 2, theor. 18, the trap. ANED = A ted; and therefore the trap, ANHG ▲ KQG. In like manner the trap. ANhg = A Kqg. Q. E. D. Corol. 1. The three spaces ANHG, TEHG, KQG are all equal. Corol. 2. From the equals ANHG, KQG, take the equals Anhg, K48, and there remains ghHGgqqG. Corol. 3. And from the equals ghнG, gqQG, take the common space gqLHG, and there remains the ▲ LQH = ▲ Lgh. Corol. 4. Again from the equals KQG, TEHG, take the common space KLHG, and there remains TELKA LQH. Corol. Corol. 5. And when, by the lines KQ, GH, moving with a parallel motion, KQ comes into the position IR, where CR is the conjugate to CA; then the triangle N E KQG becomes the triangle IRC, and the space ANHG becomes the triangle ANC ; and therefore the A IRC A ANCA TEC. 1 Corol. 6. Also when the lines кQ and нQ, by moving with a parallel motion, come into the position ce, me, the triangle LQн becomes the triangle ceм, and the space TELK becomes the triangle TEC; and theref. the A ceMA TECA ANCA IRC. THEOREM XX (20). Any Diameter bisects all its Double Ordinates, or the Lines drawn Parallel to the Tangent at its Vertex, or to its Conjugate Diameter. For, draw aн, qh perpendicular to the transverse. Then by cor. 3 theor. 19, the A LQH = ▲ Lgh ; but these triangles are also equiangular; consequently their like sides are equal, or LQ=Lq. Corol. Any diameter divides the ellipse into two equal parts. For, the ordinates on each side being equal to each other, and equal in number; all the ordinates, or the area, on one side of the diameter, is equal to all the ordinates, or the area, on the other side of it. THEOREM ·THEOREM XXI (21). As the Square of any Diameter : That is, CE ce2 :: EL'. LG or CE2 - CL2 : LQ2. : For, draw the tangent TE, and produce the ordinate QL to the transverse at K. Also draw IQH, eм perpendicular to the transverse, and meeting EG in н and M. Then, similar triangles H being as the squares of their like sides, it is, M ▲ CET: trap. TELK :: ce2: CE2 — CL2. Again, by sim. tri. A cem : ▲ LQH :: ce2: LQ2. But, by cor. 5 theor. 19, the A cem = ▲ CET, and, by cor. 4 theor. 19, the ▲ LQH = trap. TELk; theref. by equality, CE ce :: CE2 CL2: LQ2, CE2 Ce2 :: EL`.LG: LQ2. Q. E.D. or : Corol. 1. The squares of the ordinates to any diameter, are to one another as the rectangles of their respective abscisses, or as the difference of the squares of the semidiameter and of the distance between the ordinate and centre. For they are all in the same ratio of CE2 to ce2. Corol. 2. The above being the same property as that belonging to the two axes, all the other properties before laid down, for the axes, may be understood of any two conjugate diameters whatever, using only the oblique ordinates of these diameters, instead of the perpendicular ordinates of the axes; namely, all the properties in theorems 6, 7, 8, 14, 15, 16, 18 and 19. If any THEOREM XXII (22). Two Lines, that any where intersect each other, meet The Rectangle of the Segments of the one : That |