Kopie materiálu Huygens pendulum
The evolute of a cycloid is a shifted cycloid. Tying a string of length 4R in the cusp of the upper cycloid (R is the radius of the circle generating the cycloid), the mass at the endpoint of the string describes the lower cycloidal arc. Since the cycloid is a tautochrone curve, we've just built a isochrone pendulum.
A way to hypnotize yourself!