Jonkers, A. R. T.  Earth’s Magnetism in the Age of Sail xvii + 300 pp., figs., index. Baltimore: The Johns Hopkins University Press, 2003. $45 (cloth)

This work is an interdisciplinary venture bridging the dissimilar fields of geomagnetism and history.  The author, trained in the earth sciences, has two main goals.  In Part I, Earth’s Magnetism, he focuses on the changing theoretical speculations regarding terrestrial magnetism from the sixteenth to the eighteenth centuries for Europe’s maritime powers (i.e., England, France, the Dutch Republic).  In Part II, In the Age of Sail, he examines differing navigational practices for the same period and concludes with a geomagnetic model using data culled from thousands of ships’ logbooks, sailing directions, and other historical sources.  In the introduction, Jonkers explains: “Tens of thousands of scattered notes, which by themselves carry little explanatory value beyond their immediate context, conceal both a planet-wide geophysical picture and a distinctive imprint of maritime expertise.  On the most fundamental level, this book will try to establish to what extent these hidden layers of information can be exploited by both disciplines concerned” [5].

Even though the Jonkers reiterates the groundbreaking nature of his research, the book’s two sections are less than satisfactorily integrated.  The two parts read largely as separate works.  Though both contain information of interest to historians of science, they also reflect an older historiography common among scientists seeking the “firsts” or “origins” of their modern disciplines.  The repeated use, for example, of phrases like “geomagnetic field” or “earth’s field” throughout anachronistically imposes a concept developed in the nineteenth century onto earlier centuries.  

In addition, the use of words such as “unfortunately,” “regrettably,” or “sadly” assumes a modern rather than an historical perspective.  While discussing the lack of mathematical and physical concepts “still in their infancy,” Jonkers notes that seventeenth- and eighteenth-century natural philosophers were “unable to mathematically describe a magnetic field in terms of a vector field,” and that their misconceptions about the geomagnetic poles lie “at the root of their failure to adequately describe, explain, and predict measurements” [32].  Later he opines: “Even when secular [geomagnetic] variation was eventually recognized, the missing temporal dimension prevented them from reappraisal for time-dependent analysis (both in the past and at present), which is most unfortunate” [61].  The book is ripe with this type of language.      

As a result, Jonkers’ goal of a symbiotic interaction between the history of science and the study of geomagnetism appears as tension between the construction of a modern model and the development of historical context.  Undoubtedly, the quantitative analysis and modeling of geomagnetic secular variation will interest geophysicists, but the statistical analysis of magnetic measurements will be of lesser worth to historians of science.  Upon reading these latter measurements “have been statistically quantified for the first time” [228] and that this is “the world’s biggest compilation of historical magnetic observations,” [231] one wonders about the broader significance to the historian of science.  Of greatest value is the discussion in Part I of the shifting relationship between geomagnetic theory and observation.  Jonkers argues quite persuasively that the primary reason for greater theoretical complexity among natural philosophers was the growing quantity and quality of data collected by navigators.  In fact, the problem grew so complex that it remains largely unsolved today.