Take a vessel filled with water and set afloat in it a cork into which you have stuck an iron needle. Then pass the stone over the surface of the water, until the needle has acquired the same properties as the stone. And at this point the needle—though the stone would also have done it if it had had the capacity to move around a pivot—will turn and point north, and if you move it with the vessel, it will always turn in the direction of the north wind. Obviously, if you bear north in mind and also mark on the edge of the vessel the positions of east, south, and west, you will always know which
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