The penstock for the gristmill, discharging on to an backshot wheel ion the far side of the picture. The setup here is interesting in that the penstock is fed by a ground level pipe (that is, as an inverted siphon under pressure. The water supply comes from a nearby millpond with the pipe run underground, perhaps this solved cold weather freezing problems (the mill is in Raphine, VA 10 miles south of Staunton in the Shenandoah Valley). For more information see wadesmill.com.