Designing Four-Bar Linkages
The four-bar linkage can be regarded as a basic mechanism. Although simple, the equations describing its dynamic are far from simple and form an important topic in mechanical engineering. This Demonstration shows the steps to construct the Fermat linkage, which is a particular four-bar linkage used to draw a lemniscate of Bernoulli. You can also change the position of a marker and the length of one of the arms.
 Wikipedia. "Lemniscate of Bernoulli." (Jul 31, 2012) en.wikipedia.org/wiki/Lemniscate_of_Bernoulli.