Voyages Making Geomagnetic Measurements
The magnetic compass (q.v.) has been used in Western Europe since the early or middle 12th century (see History of Geomagnetism). By 1187 a form with pivoted needle was described by Alexander Neckam, a monk at St. Albans, in his De naturis rerum. At this time, however, and for the next two and a half centuries, it was believed that the compass pointed toward true north. As long as this was the case, there was no reason to make or record any form of magnetic measurement.
Even when (probably sometime about the middle of the 15th century) it was realized that the magnetic needle did not, in general, point true north the situation with regard to making measurements did not change. The discrepancy in pointing was regarded more as a fault of the needle itself or of the method of magnetizing it rather than as being caused by some external agency.
And yet, some glimmering of the truth seems to have begun to dawn. Instrument makers in different parts of Europe began to compensate...
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