http://demonstrations.wolfram.com/Parabolograph/[br][br][i]"Prior to the advent of computers, draftsmen had to use dedicated mechanisms to plot curves. One such instrument was the parabolograph, simulated in this Demonstration and on display in the London Science Museum."[br][/i][br]The equation of the parabola is [math]y=x\left(1-\frac{x}{a}\right)cot\left(\alpha\right)[/math] where a is the x coordinate of the red Drag me point.