## Peaucellier's linkage

Here is a demonstration of Peaucellier's linkage, a machine which
converts linear into circular motion.

Initially, you are allowed to move the red dot in any way you'd like.
However, if you press "Constrain," you will see a new dot which allows
you to position a line to which the motion of the red dot will be
constrained. When you have the desired line, press "Line Set." You
may now move the original red dot, but it will be constrained to lie
on the line you choose. You will see that the cyan dot traces out an
arc of a circle.