The discovery of the site of two tidal water mills adjacent to the monastery at Nendrum near the shore of an island in Strangford Lough on the east coast of Northern Ireland prompted a remarkable excavation, which commenced in April 1999 and ended in 2001. In less skilful hands a site that was flooded twice daily by the tides might not have been so painstakingly excavated or recorded and the published report might have been couched in technical language, to be consigned to the library shelf save for occasional reference by specialists. Instead, the excavators have produced an exemplary and highly readable yet detailed narrative description of a competent excavation in an extremely attractive format, supported by useful contextual background and specialist reports. The background material includes a review of the life of the Irish saint Mo-Choí and a highly readable account of the history of the monastery of Nendrum from the sixth to the twelfth century. The context of the mills is further explored through a reassessment of the archaeology of Nendrum, with its triple concentric enclosure containing the standing remains of the church and a round tower.

It has been estimated that in medieval England there was one mill for every forty-six peasant households, and 5,624 mills are listed in the Domesday Book. But many such sites have been subject to disruptive reconstructions, and, relative to its overall scale, the potential archaeological resource has hardly been touched. In marked contrast, at least eighty horizontal-wheeled water mills have been recorded in Ireland, thirty-two of which have been tree-ring dated to earlier than the twelfth century, and there is now a considerable literature on the subject; the Nendrum report is the latest distinguished addition to that corpus.

The date of construction ascribed to the earliest of the two mills is based upon the dendrochronology of timbers felled in ad 619–21, some of which were intended for other structures; the second mill, which replaced the first, was dated to around ad 789. Each mill was driven by a horizontal wheel with a vertical shaft, and each was powered by sea water that was impounded at high tide in a pond behind a dam on the foreshore. It is plausibly argued in the report that some of the timber found in association with the first phase had been stockpiled for only a short time, and if this interpretation is accepted the mill is the oldest with a horizontal wheel yet found in Ireland; the same structure is also claimed as the oldest tide mill yet found anywhere.

The introduction to the Nendrum report includes a clear explanation of the operation of horizontal-wheeled mills, leading the reader into the terms employed throughout the description of the evidence and obviating the need for a glossary. Additional site context is provided by summaries of the origins and distribution of horizontal-wheeled mills and of tide mills. Horizontal-wheeled water mills did not require a set of gears to transfer power from a horizontal shaft. Instead the wheel, comprising a number of paddles mounted in a hub, carried a shaft, which extended vertically into a wheelhouse on the upper floor. A spindle set into the top of the shaft passed through a ‘bedstone’ on the millhouse floor and was fixed into sockets in the upper or ‘runner’ stone so that it could be rotated at the same speed as the water-wheel in the wheelhouse. The water-wheel was mounted on to a simple mechanism known as the tentering system, comprising a horizontal plank or ‘bridge-tree’, which could be raised or lowered from above by means of a lever or ‘lightening-tree’ in order to adjust the space between the bedstone and the runner stone.

On conservation grounds the first mill assemblage could only be partially excavated, and in any case the stone-built wheelhouse was incomplete. Interpreted partly on the basis of ex situ evidence, the operation of the first mill had involved directing water via an intake channel with a stone-lined base into a hollowed-out oak flume, or penstock, which delivered a jet of water on to the water-wheel. A more complete picture was gained of the clay-lined first millpond, which was contained behind a dam with a palisade, internal revetments of timber and wattle and an inner stone revetment. A major component of the first mill complex was a timber platform, possibly a landing-place, which retained numerous worked timbers, some of them with mortices and tenons.

After a working life of up to 167 years, much of the old dam was dismantled, the intake channel and the first wheelhouse and its tail-race were filled in, and a second mill was built to an improved design; it is the most complete example of its kind yet to be discovered and excavated in Ireland. The much smaller second millpond supplied a stone-lined intake channel which was 2.6m wide at its entrance but only 0.8m wide at the mouth of the penstock. For the first time at any Irish mill the evidence of a penstock sluice gate was found, including a stone with a socket for a pivot. Opening this gate would have released the impounded sea water into the elaborately built stone penstock – a narrowing flume which directed a powerful jet of water precisely on to the wheel. In the wheelhouse of the second mill three complete oak paddles were found, but unfortunately the only part of the tentering system recovered was the beam on which the bridge-tree had rested.

The hub of a wheel, complete with one paddle, was associated with the first mill, whereas three paddles, but no hub, were found in the second. The latter probably originally had twenty-four paddles and a diameter of 0.98m. The millstones from the second mill were made of Mourne granite. In marked contrast with the wheelhouse, the millhouse had probably been plank-built – a conclusion derived from limited evidence but supported by a summary of other ‘accomplished’ seventh-century buildings. The specialist reports include an examination of the extractable energy and milling capacity of the second mill by Max Brown. Compiled by Caroline Earwood, the description and analysis of the worked wood from both mills will have general applications for the study of early medieval carpentry; the finds in this section include an oar made of ash, and two turned bowls made of alder. Eileen Murphy assessed the small assemblage of animal bones.

It is in the nature of archaeology that the excavator must make do with what remains on each site, and that when sufficient similar sites have been excavated information can be drawn from each of them and comparisons made. The absence of a bridge-tree from Nendrum is regrettable, as no comparison could be made with that found in the ninth-century Tamworth mill (Staffordshire), where it would appear that the shaft revolved on a steel bearing. On the other hand, at Nendrum the lower or bed-stone retained a wooden ‘bush’ around the central hole – part of the means by which the centrifugal force of the spindle at the top of the shaft would have been contained. This is the only feature of its kind yet found in Ireland, and it is unlikely that the Tamworth bed-stones contained such wooden bushes.

A particularly cogent presidential Anniversary Address to this Society was that delivered by R M Robbins in 1988, much of it devoted to the duty of scholars to communicate to the widest possible audience. ‘Every writer should believe before he starts that he has something to say, that it is worth saying, and that he can say it better than anyone else; and when he looks up from his work he should see in his mind’s eye that last reader; and his concern about that last reader must constantly be: “Will he turn the page?” ’ Every reader of Harnessing the Tides will wish to turn the page, for it is a transparently written and handsomely presented volume – a pleasure to hold and a joy to read. Furthermore, the publishers are to be congratulated for the exceptional quality and price.