Nicholas Simons wrote:

> The Watt's linkage consists of three pin-jointed links that create
> a straight line motion without any recourse to a straight edge.

Just to be nit-picky: It only creates an _almost_ straight line.
However, the small "S swing" can be made as small as one wants.

Regards
Harald M. Mueller

 [ I am studying Watt's linkage right now as I am fitting a
 [ replacement pneumatic action stack into a different piano.
 [ I want to minimize the scrubbing motion of the vertical pushrod
 [ where it contacts the tail of the whippen, which means that the
 [ contact surface at the whippen must lie on the line through the
 [ whippen fulcrum (pin joint).  Naturally, the whippen construction
 [ of the old piano action doesn't come close to the ideal geometry.
 [ :-(  -- Robbie