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Dunster Watermill (also known as Castle Mill) is a restored 18th century watermill situated in the grounds of Dunster Castle in Dunster, Somerset, England.

The present mill, which was built around 1780, is built on the site of a mill mentioned in the Domesday Book and was restored to working order in 1979. It has been designated by English Heritage as a grade II listed building.
Source: Wikipedia

In a mill context, tentering is changing the distance (gap) between the runner stone and the bed stone. Decreasing the distance results in a finer grain. As the stones go faster there is a natural tendency for the runner to rise and the tentering mechanism can be adjusted to counter this. In many windmills this is done by a governor that automatically compensates for the tendency of the runner to rise. In watermills it is done by hand with mechanisms of varying elegance. In this case, the second order lever at the bottom raises or lowers the footstep bearing that the spindle carrying the stone nut is supported by. The adjustment is by a vertical lead screw on the other side of the frame and in easy reach of the miller.