Edmond Halley and the magnetic field of the earth

Edmond Halley was interested in magnetism from his schooldays to the end of his life, and of all his notable contributions to natural knowledge, his survey of the magnetic field over the Atlantic Ocean surely stands out both for its achievement at the time and for its lasting value. His identification of the westerly drift and his speculations about its origin, his description of aurorae and his connection of them with the geomagnetic field, all demonstrate his physical insight, the grounds for which have only quite recently become apparent. How profound were Halley's studies of geomagnetism has become much more evident in the 57 years that have passed since Sidney Chapman's Halley lecture on the same topic; the advances in geophysics that have taken place in the intervening years are my justification for returning to the subject.