Earth's Magnetism in the Age of Sail (review)

This book is a paean to interdisciplinarity. As Jonkers states at the outset (27), he explores "the rewards of merging history and geomagnetism," and he reaps all of the rewards (and many of the problems) of such a project. Jonkers attempts to show not so much that the two fields are interconnected as that each has much to offer the other: that historical documents can provide data essential for geomagnetical modeling and that successful models can provide information that facilitates historical interpretation. And indeed his most successful chapter is the conclusion, where he turns his model back on the raw data and analyzes centuries of attempts to gather information at sea. The risk, though, is that he has produced a book that both communities will mine to great effect, with neither being wholly satisfied with the result. I will limit myself here to the historian's point of view.

To set the stage, Jonkers provides a readable summary of the recent history of magnetic research and the continuing difficulties in modeling the earth's magnetism. He then returns to the age of sail, and spends the next hundred pages tracing the development of ever more complicated theories. While those interested in any particular theory would probably be better served by an analysis either more technical or more contextual, for the nonspecialist he provides a useful overview of the trends over the several centuries and much of Europe. While the bulk of the discussion is surprisingly schematic (especially his four phases of complexity), he does point out several commonalities driving research: the importance of expected applications in the sixteenth century, connections to natural philosophy and astronomy in the seventeenth century, and the growing emphasis on data compilation and scientific expeditions in the eighteenth century.

In the second half — In the Age of Sail — Jonkers turns from theory to application, tracing the practical effects of the attention paid to magnetism. In the process he gathers staggering amounts of information (largely from seventeenth- and eighteenth-century logbooks) on the magnetic data gathered by sailing ships and their attempts at using this data to find their location at sea, that is, to solve the problem of the longitude. He opens the section with a chapter on the basics of navigation and long-distance travel. Chapter 6 then discusses the use of the magnetic compass at sea, and the growing knowledge of the practical problems and opportunities due to magnetic declination. Chapter 7 focuses on the hydrographers and their attempts to gather and disseminate information in the seventeenth and eighteenth centuries.

The entire book, however, is but a prologue for the conclusion, "Quantifying Geomagnetic Navigation," where he draws together the data so carefully explained in the preceding 200 pages and uses his new geomagnetic model to evaluate data-gathering aboard ships. Unfortunately his focus here remains narrow; he produces figure after figure comparing the observations by various groups, on various routes, and in various centuries, but largely leaves to others the task of interpreting these results.

I could quibble with the coverage, though he is right that the logbooks he relies on simply do not exist in large quantities before 1600. Nonetheless, while England, France, and Holland are essential countries to consider in the age of sail, the inclusion of Denmark is not a satisfactory substitute for the almost complete exclusion of the Spanish and Portuguese empires.

In sum, Jonkers has amassed an impressive array of sources and synthesized much of the information into a series of tables. This will be an essential source for anyone interested in the gathering of information at sea and related issues of fieldwork and data assimilation. He also provides a chronological list of various geomagnetic hypotheses (241–48) though one made less useful by the lack of footnotes to the specific proposals, references to the text, or even a general bibliography. (This is one circumstance where...