Scale: circa 1:35,000,000. Facsimile. Reproduction of Halley's world map with isogonic lines, on one sheet. Mercator's projection. George Biddell Airy, then Astronomer Royal, felt that it would be valuable to reproduce this chart, which was requently referred to in works on magnetism, but very difficult to find. A reduced version of this chart was included as an appendix to the Greenwich Observations for 1869. This chart has Airy's signature on it.