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Creative Commons AttributionBinnacle - Credit: Andyrob, Flickr
A binnacle is simply a robust, built-in case to hold and protect navigational instruments on deck.   Over time, engineers have learnt to avoid the use of iron nails in their construction, as the ferromagnetic material distorts the compass.   Later developments on iron ships include Lord Kelvin’s invention of a binnacle incorporating two magnets to counter the effects of the ship’s plates.