Periodic Wave Generated by a Pendulum Sandglass
This Demonstration shows a wave generated by a pendulum sandglass. The wave is well approximated as a sinusoid, but not exactly since this is a spherical pendulum. The pendulum consists of a hollow tube with sand running down from a funnel at the top. It is assumed that the pendulum's mass and period are constant and that frictional losses can be neglected.
 Wikipedia. "Pendulum." (Dec 29, 2011) en.wikipedia.org/wiki/Pendulum.
 Wikipedia. "Sandglass." (Nov 17, 2010) en.wikipedia.org/wiki/Sandglass.