Publisher's proof of the folding engraved chart that would be included in volume VI of Freycinet's 'Voyage autour du Monde' (1824-1844), 'Magnetisme terrestre' (1842), as 'Tracé de l'Equateur magnétique pour les époques moyennes de 1773 et 1821, 1826', by Jacques Marie Hacq, after Freycinet, and (probably) Brue.

The chart shows the track of Freycinet's ship, the 'Uranie', as well as three lines of variation of the magnetic equator in 1773, 1800 (after Humboldt's observations), and in 1821 (after the observations of Freycinet's voyage). Upper left is the outline of the inset that would appear on the published map comparing the older lines with the newer ones.

The map itself appears only in outline, and is probably based on the charts of Andre Herbert Brue, who had accompanied Nicolas Baudin's expedition to the Pacific (1800-1803), as had Freycinet, and whose 'Atlas universal de géographie physique, politique, ancienne et moderne', was published from 1822.

In 1817, Louis Freycinet took command of the corvette 'L'Uranie' charged, in part, "with the investigation of the shape of the earth, terrestrial magnetism, meteorology, and natural science. Further adding to the comprehensive nature of the venture, the expedition report was to be presented under many varied headings, viz. geography, history, observations of the people, government, commerce, primary produce, industries and art. With 596 sub-classes listed under these headings according to Leslie Marchant in his seminal work, France Australe, it made the Uranie voyage 'one of the most significant anthropological expeditions conducted by the French'" (McCarthy, WA Museum's "Journeys of Enlightenment" exhibition 2008).

In order to help determine the sphericity of the earth, Freycinet and his crew measured the magnetism of the earth at three key different locations: Mauritius, Guam (between the Philippines and the Marshall Islands), and Maui in the Hawaiian Islands. He then calculated the degree of flattening for each hemisphere, and at the equator, and concluded that the earth was basically round.