Some Early Contributions to the History of Geomagnetism—II AND III

Abstract

(II) Francisco Falero—During the great age of discovery which followed the voyages of Columbus, Spain and Portugal took an important part in maritime enterprises and the exploration of lands beyond the sea. These ventures were responsible for the improvement of instruments and methods of navigation and the production of charts and maps. At that time it was widely believed that the determination of longitude at sea could be obtained from magnetic data, particularly from those of the declination, a belief which persisted throughout the seventeenth century and encouraged the making of many magnetic observations which, although useless as far as longitude‐determinations were concerned, at least furnished data of great value in advancing knowledge of geomagnetism.The first person who announced practical methods of determining the magnetic declination in printed form was Francisco Falero or Faleiro, a Portuguese in the service of the Spanish Navy, to whom we are indebted for the first real manual of navigation. This work entitled “Tratado del esphera y del arte del marear; con el regimie˜to de la altura; cõ algũas reglas nueuame˜te escritas muy necessarias,” is extremely rare—so rare in fact that its existence has sometimes been doubted. The National Library in Madrid, however, possessed a copy and Hellmann was enabled to reproduce its title‐page which we in turn present herewith in Plate 2. (A free translation of the title is as follows: 'Treatise on the sphere and the art of navigation with manual of altitudes with some very necessary written rules. With imperial privilege. A. D. 1535.”) The work was printed in gothic type and consists of 52 unnumbered folio pages. In the eighth chapter of the second part under the title "Del nordestear de las agujas,” a translation of which we present (page 80), magnetic declination is discussed in detail for the first time in print and three methods are given for its determination, namely (1) by the azimuth‐determination of the magnetic needle at true noon when the shadow of the pin falls to the north, (2) by observation of the shadow‐azimuths at corresponding altitudes of the Sun before and after noon, (3) by observation of the azimuth at sunrise and sunset.1 These methods are perhaps intended for an instrument (brújula de variación) devised by Filipe Guillen although no mention is made of it.