Some Early Contributions to the History of Geomagnetism—VIII

Abstract

We present as the eighth contribution to the early history of geomagnetism extracts from the English version of the “Portuum investigan‐dorum ratio.”1 This work was written by Simon Stevinus (1548–1620), an eminent Dutch mathematician, at the instance of Count Moritz of Nassau, Prince of Orange, Admiral of the Dutch Fleet, who was convinced of the value of a knowledge of the magnetic declination for navigation. It contains the oldest list of values of the magnetic declination (43 stations), emphasizes the great importance of a knowledge of the declination to mariners, and contains an introduction to the determination thereof with the aid of the azimuth‐compass.The booklet appeared simultaneously in Dutch, Latin, English, and French. P. Plancius, a Calvinist preacher and geographer, is mentioned as the authority for the declination‐values. It appears that he had collected these data for a geographic chart which he is thought to have prepared in 1592 but of which no copy is known to exist.