This device has two grooves, one in the shape of an arc of a circle, the other of a cycloid, which is the curve described by a point on the circumference of a disc as it rolls over a plane.
The instrument serves to show that by simultaneously releasing two spheres at the top of the two guides, the one on the cycloid arc descends faster than the other, thus demonstrating that the cycloid is brachistochronous, i.e. the quickest path, compared to the arc of a circle or a straight line, between two points.
Device on display.