A link-gear mechanism to trace a parabola. It uses a slider C moving along a line, representing the directrix of the parabola. A rhombus linkage with side d connects point C with the focus A of the parabola. Slider D moves along the vertical line CD connected by a turning pair to the diagonal of the rhombus. The diagonal line of the rhombus is simultaneously the tangent of the parabola at D.
Taken from Atobolevsky vol. II, p. 156