The fourth and lowest adit connected with the deepening of the West Twll (NPRN 408820), the driving of Level 4 adit was engineered by Charles Spooner of the Festiniog Railway, commenced in February 1857 and completed in 1859. It is almost 500 metres long, and was 2.9 metres wide and 2.29 metres high. It was excavated from two intermediate shafts 18 and 24 metres deep, which were drained during their sinking by what is thought to have been a pair of water balance engines housed in a building midway between the shafts, and transmitting power via reciprocating rods whose paths can be traced by lines of hummocks to the tops of the shafts. Lines of hummocks to the west of the building would have supported wooden launders bringing water to the balance engines, which worked by filling a tank at one end of a seesaw until it was heavy enough to descend, lifting the pumps in the shaft. At the end of the stroke an automatic valve emptied the tank, and the weight of the pumps flipped the seesaw, raising the tank to its starting position for the next stroke.
W J Crompton, RCAHMW, 22 April 2009.