Construction Of A Parabola
An illustration showing how to construct a parabola using a pencil and a string. "Having given the two axes, vertex, and focus of the parabola. Take a square cde, and fix to it a string at c; fix the other end of the string at the focus f. The length of the sting should be such that when the square is in the position of the axis Af, the string should reach to the vertex A. Move the square along BB, and the pencil P will describe the parabola."