Johnson's World, Showing the Lines of Equal Magnetic Declination.
1858 (dated) 6 x 10.5 in (15.24 x 26.67 cm)
This is a beautiful example of Johnson's 1858 map or chart of the world's magnetic curves of equal variation. The chart depicts the magnetic curves over the entire world including the isomagnetic lines over every continent. It marks the North Magnetic Pole at the Boothia Felix (now the Boothia Peninsula) based on the first expedition by James Clark Ross in 1831. Due to the magnetic changes in the earth's core, the position of the magnetic poles change over time. In 2005, it was estimated that the North Magnetic Pole was positioned to the west of ellesmere Island in Canada. Johnson based this map on th work of Arnold Henry Guyot, an influential Swiss-American geologist.
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