roller-ball clock

[inner_languor]

Помогите пожалуйста, как правильно перевести roller-ball clock?

The roller-ball clock was beautiful, glittering as it moved. The boys put their faces close to the glass case. It was an inclined metal plane with a shallow groove, along which a silver ball rolled, zigzagging from one end to the other. When the ball reached the bottom it struck a catch reversing the tilt and rolled back the way it had come. Each journey took a known amount of time and the hands moved on accordingly.

Спасибо.