In his book Lo specchio ustorio overo trattato delle Settioni coniche: et alcuni loro mirabili effetti, the Italian mathematician Bonaventura Cavalieri described in 1638 an instrument designed to draw parabolas. The line segment CD, that slides in the ray AD, has a fixed length. The line segment CF is perpendicular to CD and the angle AFD is always a right angle.

You can interact with it below. Click in the button Start to begin the animation that shows the drawing of a parabola using the instrument invented by Cavalieri. You can use the button Stop to stop the animation at any point to verify the equalities in both boxes, green and blue.