NavList:

Just two historical comments: As the isogonic lines west of the Cape of Good Hope in the 16^th – 18^th centuries were indeed mostly oriented in a north-south direction Portuguese navigators used the value of variation to assess their distance from the cape (not the longitude) on the East India run. For this reason there are quite a few Portuguese Azimuth compasses in different museums (for instance the National Maritime Museum, Greenwich, Whipple Museum, Cambridge, Whaling Museum, New Bedford, or the Museé de la Marine, Paris). On the nautical practice see: Malhão Pereira, José Manuel, The Magnetic Variation and the Variation Compass in Portuguese Navigation Techniques, XVI to XVIII Centuries, in: idem, Estudos de História da Náutica e das Navegações de Alto-Mar. Vol. IV, Lisboa 2022, 129 - 160.

Using variation as a means to find longitude was discussed as early as 1514 by João de Lisboa and was also mentioned by Alonso de Santa Cruz in his ‘Libro de las Longitudes’. Michel Coignet in his ‘Instruction Nouvelle’ (1581) supported the idea whereas Robert Norman in ‘The newe attractive’ refuted it in the same year. Still the method was propagated for more than another century; the Board of Longitude in the 1760s still had to deal with such proposals and usually replied that “no notice will be taken” of such schemes.