On Derwen Hall Farmstead (nprn 408946) and some 140m north of the Hall (nprn 27107), is a reservoir (shown as marked out on the first edition Ordnance Survey 25in map of 1875 and depicted as water-filled on the 1899 map), constructed on Nant Pen-y-parc, which measures approximately 40m east to west by 30m. It fed a water turbine via an open leat which ran eastwards for 180m long to a tank, thence in a pipe about 40m long to the turbine itself (all features appear on the 1913 map), 250m from Derwen Hall agricultural outbuildings (nprn 408947). Power was transmitted from the turbine to the outbuildings by an endless wire rope running on pulleys mounted on wooden poles.
B.A.Malaws, RCAHMW, 22 April 2009.