Broadly speaking these are ancient computers that tell you when the tide is in. You might think you could do that with a pair of open-toes sandals, but being on a sailing ship, or a smugglers longboat, approaching the coast on a stormy night precise calculations are essential. I challenge you to stop and imagine just how you would work it out - then give credit to minds that hadn't the benefit of a technological upbringing yet perhaps had all the more clarity and vision for that. I give you - Our Ancestors. Cheers