Wooden double cone (height 32 cm, diameter 17.5 cm) on inclined wooden tracks (not the originals) with variable slope.
This device demonstrates a well-known mechanical paradox: when the double cone is placed on the diverging tracks, which are slightly higher at the wider part, it is in unstable equilibrium and rolls in the direction of the part where the tracks are farther apart and higher, thus giving the impression that it is rolling uphill. In reality, this occurs because its barycentre is lower in the wider part of the tracks; in fact, starting from this situation and raising the tracks slightly, we can create the condition in which the barycentre of the double cone is at the same height in all positions and is in stable equilibrium, as if the tracks were parallel and horizontal.
Daguin (1878), T. I, p. 96
Turner (1987), p. 201