This video of the Da Vinci 'Piano-Viola' shows the foot pedal in action
briefly at the beginning of the video:

  http://www.chonday.com/Videos/pianoviola1 

And something in a similar vein: the Glass Harmonika, which was
invented by Benjamin Franklin:

  http://www.chonday.com/Videos/glassharmonika1 

D.F. Rowe
Bronx, New York