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This reminds me of something I read (and a Google search couldn't supply a reference) about a brilliant scheme some Nordic country (I think Sweden) devised to mine its harbour and not jeopardize its own ships. A room overlooking the harbour was turned into a camera obscura and as the ship set the mines, its location was noted on the wall. One of their own ships could leave the harbour, guided by a set of visual signals from this room, but any approaching enemy ship would be in jeopardy. This was a two or three hundred years ago.
