Wrought iron, not magnetized by the loadstone attracts iron. A is cork holding an iron wire reaching a little out of the water; C refers to the end of the second wire, which pulls B. [ZB Magnet 2] William Gilbert, De Magnete (London: Petrus Short, 1600), Zentralbibliothek Zürich, Alte Drucke und Rara, XVI.28, p. 29.
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[ZB Magnet 2] William Gilbert, De Magnete (London: Petrus Short, 1600), Zentralbibliothek Zürich, Alte Drucke und Rara, XVI.28, p. 158.
Deviation* is due to Earth’s Elevation. Gilbert explains this thesis with his “terrella” (a spheric magnet). He adds mass to the terrella and observes that a needle is “attracted out of the true direction”. [ZB Magnet 2] William Gilbert, De Magnete (London: Petrus Short, 1600), Zentralbibliothek Zürich, Alte Drucke und Rara, XVI.28, p. 157.
The rising and setting of the sun or the moon or stars is observed with a rule, which has two sights (figure above). This rule is connected with a compass rose and needle (figure below). William Gilbert, De Magnete (London: Petrus Short, 1600), Zentralbibliothek Zürich, Alte Drucke und Rara, XVI.28, p. 173.
Variation compass, instrument for measuring magnetic deviation*; the instrument relates observations of the sun to observations of the magnetic needle. The instrument shows the variation by the shadow of the sun. The shadow is observed before and after noon and correlated with the direction of the needle. The observations are marked with chalk or ink on the plate (circles). Afterwards they are compared to each other to study the deviation. William Gilbert, De Magnete (London: Petrus Short, 1600), Zentralbibliothek Zürich, Alte Drucke und Rara, XVI.28, p. 171.
* Deviation: The direction of a compass needle is deviating from the north-south-direction at different places of the globe. Sebastain Cabot first discovered and reported this fact, except for Columbus who didn’t report it officially. Deviation is also named “variation”.
