Some ideas for finding longitude at sea did not rely on time. One of them was to use patterns in the Earth’s magnetic field, but these needed to be mapped for practical use. There had been numerous proposals for using magnetism since the 16th century and scholars continued to investigate its potential for navigation.

Terrellas were magnetic spheres used to investigate the Earth’s magnetic field. This one is contained in a brass case, which is engraved with lines of latitude and longitude.

Terrella

In 1698–1700 the English astronomer, Edmond Halley, charted magnetic variation in the Atlantic Ocean.

Halley chart

These are the first world charts to plot magnetic variation. They were based on data collected by Halley during his command of the Paramore, on two voyages in 1698 and 1699. Halley noted that the charts might be useful for finding longitude in places where the lines of similar variation ran almost north to south.

By the early 1800s longitude-finding by timekeeper and lunar distances was proving successful and commercially viable. However, magnetic variation continued to arouse interest.  One solution came from Ralph Walker who advocated the use of a compass of his own invention, in which a mariner would use the sundial attachment on the compass to align the instrument to the true north-south plane. Comparing this reading with the direction in which the compass needle was pointing gave the magnetic variation. This could, in theory, be used to discover the longitude, by finding where supposed ‘magnetic meridians’ intersected with the observed latitude. Walker believed that his method was simpler than lunar distances, cheaper than chronometers, and deserving of a substantial reward from the Commisioners of Longitude. And although Nevil Maskelyne didn't think it was an effective longitude method, the Commissioners gave Walker a £200 reward for an interesting new compass design.

With the development of the battery, the late eighteenth-century saw a re-emrgence of interest in the relationship between electricity and magnetism, leading to an increase in correspondence on the subject being sent to the Board of Longitude. The volume of correspondence was great enough to result in Astronomer Royal George Airy having to dedicate two volumes of papers from the Board of Longitude to correspondence on magnetic variation when he sorted the papers.

Although using magnetism to find longitude is straightforward in principle, the Earth’s magnetic field changes over time. This makes it impossible to produce maps of magnetic variation that remain accurate for long periods.