This experimental apparatus is used to demonstrate an important property of the cycloid known as tautochronism: a body made to oscillate along the arc of a cycloid will always take the same time to travel the distance, whatever the width of the arc. The phenomenon was discovered by Christiaan Huyghens. In 1659, he designed the perfectly tautochronous cycloidal pendulum, that is, a pendulum with perfect harmonic motion.

The apparatus is described in the leading eighteenth-century treatises on experimental physics. It consists of a wooden frame bearing two identical parallel cycloidal tracks, both ending in a straight track. There are three adjustable screws in the base of the frame for ensuring that the device is perfectly vertical—an essential prerequisite for the accuracy of the experiment. A plumb line, now missing, served to check the horizontal level.

Two balls dropped simultaneously from different heights reached the end of the cycloidal path at the same time. This was an experimental demonstration of the tautochronism of the cycloid.