What rotating machine did Leonardo da Vinci design to produce a continuous bowed sound?

Leonardo da Vinci designed the viola organista as a rotating machine intended to produce continuous bowed sound.

Unlike an ordinary keyboard instrument, the proposal used moving bowing elements to make strings vibrate. A keyboard controlled which strings were engaged, while a crank or other mechanism kept the bowing surfaces in motion. The result was meant to combine the expressive sustain of a string instrument with the organization of a keyboard.

Leonardo drew related versions during the Renaissance, but the surviving sketches do not establish a single definitive design. Because he left no authenticated completed viola organista, later makers have reconstructed the instrument from his notebooks and experimented with different mechanical solutions.

The instrument’s name can cause confusion: it does not mean a conventional viola fitted with organ pipes. It refers to a keyboard-based instrument whose sound-producing principle was intended to resemble that of bowed violas.