Mick Harper wrote: Bit of a long shot but at least and at last we have a hypothesis for why the Megalithics built flat-top causewayed islands (jelly-moulds) eg Jethou (pic, please). If these glow in some fashion then they would make ideal navigational beacons. Presumably the tidal element is necessary to re-charge them.
Sorry to burst your jelly-mould bubble, but if you want rechargeable battery islands, you have to pay extra. Or use a different method. Piezoelectric hills would work best in dry conditions, not beside the seaside beside the sea. It gets wet. What you need there is a voltaic pile
. Salt water would be an effective electrolyte, given the right metals. But the only ones I can find in known history were small novelty items, not industrial-grade lighthouse material. The best I can find (so far) is the Baghdad Battery
But our learned friends say:
While some researchers refer to the object as a battery, the origin and purpose of the object remains unclear. In March 2012, Professor Elizabeth Stone, of Stony Brook University, an expert on Iraqi archaeology, returning from the first archaeological expedition in Iraq after 20 years, stated that she does not know a single archaeologist who believed that these were batteries.https://en.wikipedia.org/wiki/Baghdad_Battery
More TME research required?