I was interested to see this in the fantastic new Maritime Museum in Amsterdam. Those big canisters on each side of the ship compass are iron slugs designed to compensate for the "hard iron" magnetic distortions of the iron ships they were mounted on. Now we do it in software, but the principle is the same.
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Even the compasses you buy for modern Aluminum and fiberglass boats have calibration adjustments which move a piece of iron or a magnet.
That said, even calibrated as well as possible, they are still generally off a bit in anything other than the direction they were calibrated in or possibly it's opposite.
For the compass shown above, it was common to produce a deviation table for every degree on the compass.
Really used to help when you were navigating between Continents, probably wouldn't have helped with the Costa Concordia though, you'd actually need to look at it for it to do you any good.
thanx chris. actually couple of us did not know about the exact principal of compass calibration. now it is much more clear.