Reverse overshot water-wheel

Frequently used in mines and probably elsewhere (such as agricultural drainage), the reverse overshot water wheel was a Roman innovation to help remove water from the lowest levels of underground workings. It is described by Vitruvius in his work De Architectura published circa 25 BC. The remains of such systems found in Roman mines by later mining operations show that they were used in sequences so as to lift water a considerable height.

The Roman author Vitruvius gives explicit instructions on the construction of dewatering devices, and describes three variants of the "tympanum" in Chapter X of De Architectura. It is a large wheel fitted with boxes, which in the first design, encompass the whole diameter of the wheel. Holes are bored in the boxes to allow water into them, so that as a box dips into the water, it enters and is raised as the wheel turns. When it reaches to the top of the turn, the water runs out into a channel. He then describes a second variant where the boxes are only fitted to the ends of the wheel, so that although the volume of water carried is much smaller, it is carried to a greater height. The final variant is an endless chains of buckets, and much greater lifts can be achieved, although greater effort is needed.

Pliny the Elder is probably referring to such devices in a discussion of silver/lead mines in his Naturalis Historia. Spain produced the most silver in his time, many of the silver mines having been started by Hannibal. One of the largest had galleries running for between one and two miles into the mountain, "water-men" (in Latin "aquatini") draining the mine, and they

That they stood suggests that they operated the wheels by standing on the top to turn the cleats, and continuous working would produce a steady stream of water.

Fragments of such machines have been found in mines which were re-opened in the Victorian era in Spain, especially at Rio Tinto, where one example used no less than 16 such wheels working in pairs, each pair of wheels lifting water about 3.5 m (12 feet), so giving a total lift of 30 m (96 feet). The system was carefully engineered, and was worked by individuals treading slats at the side of each wheel. It is not an isolated example, because Oliver Davies mentions examples from the Tharsis copper mine and Logroño in Spain, as well as from Dacia. The gold deposits in Dacia, now modern Romania were especially rich, and worked intensively after the successful Roman invasion under Trajan. According to Oliver Davies, one such sequence discovered at Ruda in Hunedoara County in modern Romania was 75 metres in depth, or over 200 feet. If worked like the Rio Tinto example, it would have needed at least 32 wheels.

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