Dynamics of Nature - Ball Bearings

This invention by Leonardo is an ancestor of the modern ball bearing, used to reduce friction. Wooden spheres are alternated with spindles with concave sides. The spheres are free to move in all directions and the spindles rotate on their own axis. Because the balls do not touch each other, wear on them is reduced.
This device was used by Leonardo in a design for a large revolving stage for a theatrical extravaganza at the Court of Milan, but the stage was never built.