By letter

U

Underdrift
Millstones driven from below; the customary arrangement in a watermill.
Underdriven
see UNDERDRIFT.
Underhoose
The lower compartment of a Norse mill, containing the TIRL.
Under Runner
The lower revolving MILLSTONE of a horizontal pair.
Undershot Wheel
A form of WATERWHEEL, driven by only IMPULSE of water passing beneath & striking the lowest FLOATS. Of use where the fall of water is insufficient to turn an OVERSHOT or BREAST WHEEL.
Under Stone
see BEDSTONE.
Union Dress
Union dress.
Union Dressing
see STRAIGHT DRESSING.
Union Mill
see ANTI-MILL.
Universal (Centre) Iron
see GIMBLES.
Universal Joint
A coupling between the ends of two SHAFTS which allows for the shafts to be aligned in different directions. A "Hook Coupling". Comprising yokes and cross-type trunnions
Unventilated Buckets
Waterwheel buckets in which no air-escapes are provided to assist with rapid filling. The lower edges of the buckets fit closely to the SOLE plate. See also VENTILATED BUCKET & BUCKET.
Uplongs
(1) The intermediate longitudinal battens of a SAIL FRAME(1). (2) see SHUTTER BAR.
Upper Side Girts
Side beams supporting the tailbeam under the wind-shaft in a post mill.
Upper Stone
see RUNNER STONE.
Upright Shaft
The main vertical driving shaft of a wind or water mill upon which the WALLOWER, the GREAT SPUR WHEEL and the CROWN WHEEL are mounted. Also called a MAIN SHAFT.