When in a more or less vast region the value of the magnetic declination and inclination of a sufficiently large number of points is known, it is possible to produce a magnetic map of that region by tracing on a geographic map the lines passing through the points at which the declination is the same (isoclinal lines) and those passing through the points at which the intensity of the earth's magnetic field is the same (isodynamic lines).
The map conserved in the museum is a map of the magnetic equator; this is to say, of the isocline passing through the points on the earth's surface at which the inclination is null.
In drawing up this map, a large-scale scientific expedition was organized under the command of L. I. Duperrey; he personally did the mapping, using the results of the campaigns in 1822 - 23 - 24 and 25 and distinguished his determinations from those of Sabine and those of Blosseville.
Murani (1906), p. 402
Roiti (1914), p. 152
Perucca (1949), p. 419