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Illustration of an experiment by William Gilbert which appeared in his book De Magnete, published in London 1600. This picture is from the 2nd edition published in 1628. The experiment was designed to show the dip in the Earth's magnetic field, which varies with latitude, and can therefore be used to determine position. The picture shows a glass goblet with a magnetised iron wire passing through a cork, which is partially submerged in water. The magnet points north south, but also tilts according to latitude.
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