This apparatus, invented in 1699 by the French Carmelite monk Jean Truchet, is used to provide an experimental demonstration of the Galilean law of falling bodies.

It consists of a hexagonal wooden base on which are attached six curved brass rods. Joined at the apex, they form a paraboloid, in other words, a surface generated by the rotation of a parabola around its own axis. A pair of metal wires is wound in a spiral forming a track that rises from the base to the top of the apparatus. At the apex of the paraboloid there is a cup, connected to the spiral track through a hole. A brass plumb line ensures that the apparatus is exactly vertical, an essential prerequisite for the success of the experiment.

A ball is dropped on the track. When the ball has completed the first coil, a second ball is dropped. The lengths of the coils increase in proportion to the sequence of odd numbers starting with one. We can verify that the two balls take the same time to move through each coil. The base of the apparatus conceals a clockwork mechanism fitted with a spoon that reactivates the experiment by releasing the ball again.