The World on Mercator's Projection. This rare and unusual piece is a reduction of a larger map, also engraved by Harris, which illustrates Dr. Edmund Halley's magnetic variations.

In 1699, Halley was put in temporary command of a vessel and sailed to the South Atlantic to study the magnetic variations of the globe. The result of this landmark voyage was to produce the first ever map of the world to show magnetic delineation. This map combines both these delineations and Halley's calculations for the position and direction of the trade winds and monsoons, a study he had made while on the island of St. Helena in 1676. The symbols he used for these are still in use today.

The chart's main geographical concentration is on coastal detail but it also shows the Island of California and there is a small inset of the North Pole on the lower left, which also advertises the sale of Halley's large map by Richard Mount and Thomas Page in Tower Hill, London. [WLD4343]