Harnessing the Tides: the Early Medieval Tide Mills at Nendrum Monastery, Strangford Lough ‐ by Thomas McErlean and Norman Crothers

Ship Museum, Voyages to North America, devoted a chapter, again by S. Thirslund, to the enigmatic object (see review in IJNA 23:1). Since its discovery and a generally accepted interpretation, there continues to be praise for the ancient Scandinavian inventors of such a navigational tool—or was it, in fact, a development for northern latitudes following an early expedition by Norsemen to the Mediterranean? Gnomonis and the measuring of sun-shadows were known to the Romans, as illustrated in fig. 37. Is it possible that navigation then from endto-end of the Mediterranean was based on parallels of latitude and on similar distances, as in later voyaging between Scandinavia and Greenland, and so was dependent on some form of sun-compass with its gnomon and shadow lines? Have its interpreters missed evidence of this? Sir Robin Knox Johnson in his Foreword to Viking Navigation wonders about the Scandinavian gnomon’s origin without coming to any conclusion. This slim volume describes the discovery of the unrecognised piece of a sun-compass and then of its sequestration in an archaeological store. Time was lost because it needed astronomers and professional seamen to interpret its technical points, and then other scholars to appreciate its significance for the early history of Scandinavian settlements in the North Atlantic. Copies were made and trial voyages proved its remarkable success when used to maintain navigable sailing courses far out to sea, if there was sunshine or a bright star. The remainder of the book discusses the practical needs and design of the sun-compass so that modern versions and other experimentation may be undertaken. Discussion of the method of calibration is linked with possible ancient uses and the results of modern sailing trials. Throughout the 42 pages there are clear illustrations of the original device and of the modern versions sometimes used to maintain courses successfully over long distances. For further technical discussion readers are referred to Considerations of the Viking Sun Compass (2004) at http//www. sjolander.com/viking/museum/compass/rebobsun.htm. What separates the modern sailor from the ancient Norse seaman in his experiments with the gnomon is the use of a chart for plotting the information gained through the instrument. The ancient assessment of the track as ‘a course made good’ is confirmed after sailing perhaps for days, checking bearings with the gnomon and maintaining a steady direction. But a steady direction relative to what? —to wind and wave direction? to cloud movement? to patches of drifting weed? to soundings including arming the lead? to observing birds and their flight? all or any of which should eventually reveal a recognisable coastline. However, that mix of imprecision, observation and outstanding rote-learning was sufficient to maintain contact between Scandinavia, Iceland, Greenland, Labrador and the land to its south for 400 years. One shudders at the thought of the probable failure rate but must admire the evident success of the system. Ultimately Scandinavia lost contact with its western relatives due to the onset of a lesser Ice Age which settled over Europe and lasted a few hundred years. This was more than long enough for tales of settlement in lands to the west (including those of British-Celtic origin? ) to be doubted when combined with ignorance of how such courses could have been sailed—and so left it to Christopher Columbus to ‘take the line honours’ centuries later. Many years ago this reviewer read that during the Second World War the British Army in North Africa had a long-range, roaming motorised desert group which steered by the shadows of gnomons mounted on the bonnets of their vehicles. This avoided the complication which arose from trying to steer straight courses over unmarked desert terrain using fixed, adjusted magnetic compasses surrounded by moveable weaponry. Søren Thirslund’s latest book on the sun-compass is to be recommended to all who would understand early Scandinavian navigation. The subject is lucidly explained and contains as much as is at present known about ancient sun-compasses. But was it being carried on the Stallion of the Sea during her circuit of Britain which ended triumphantly back in Roskilde last August (2008)?