Peaucellier's Linkage

Invented in 1864 by Charles - Nicolas Peaucellier, a French Army Officer, this is probably the most ingenious, interesting and utterly useless piece of geometrical drawing apparatus ever devised. Points A and B were both fixed and all of the points were joined by rods. The yellow point could be moved along a circular path. A lead was inserted at the red point and the whole apparatus was placed on a sheet of paper so that the path of the red point could be traced.

This is what the apparatus did.