Earth's Magnetism in the Age of Sail - USGS Geomagnetism Program

358 Book reviewsdon over several decades, the English mathematician,Henry Gellibrand (1635), announced that magneticdeclination changes with time. Specifically, Gellibrandfound that the declination at London was decreasingby ∼ 0.13 ◦ per year. This observation was soonconfirmed by others, and, in 1676, Henry Bond, anEnglish mathematician and navigational teacher, proposeda dynamic magnetic longitude scheme based ona precessing dipole, with the magnetic poles slowlyand steadily migrating in a westward direction aboutthe geographic poles (Howarth, 2002; Jonkers, 2003,pp. 85–89). This model became deeply embedded inthe navigational literature, even though gross discrepancieswith data were almost immediately noted. Moregenerally, what was developing was a routine expectationof consistency between the models and the, nowquite obvious, secular variation.8. Time-dependent, multiple polesAfterwards, time-dependent, multipolar modelswere proposed. Most prominent among these werethose proposed in by Edmond Halley (Halley, 1683,1692), the English astronomer whose name is mostcommonly associated with the comet that bears hisname, but who also made substantial contributions togeophysics (e.g., Bullard, 1956). Extrapolating fromhis own observations on the effects of two or morelodestones, Halley modelled geomagnetic directionalityin terms of four poles, two in the crust and twoanchored to an interior “ball”, neither of which hadantipodal symmetry. Halley supposed that the interiorball was separated from the overlying crust, and thatit could rotate with respect to the surface, therebyproducing the observed secular variation of declination(Kollerstrom, 1992; Jonkers, 2003, pp. 90–95).Although different in detail from what we knowtoday about the Earth’s stratified interior, Halley’shypothesis was certainly remarkable for its time.These conceptual developments did not, however,occur in a vacuum. Halley was well aware ofobservations indicating that the global declinationfunction was both geographically complicated andtime-dependent. Furthermore, there is some evidence,the entire significance of which is difficult for historiansto ascertain, that Halley was strongly influencedby Peter Perkins, an English mathematician whoproposed, through unpublished, verbal presentationsgiven before the Royal Society of London, that declinationcould be modelled by four moving poles(Jonkers, 2003, pp. 90–95). More generally, Halleywas also influenced by rapid developments in thetheories of forces, which were beginning to be conceivedwithin a larger and unifying mechanical andquantitative framework (e.g., Thrower, 1990).9. Mechanistic theoriesJonkers makes very careful and sparing usage ofthe word “field”. This is for good reason: for prettymuch the entire historical window of time covered byEarth’s Magnetism, attraction and repulsion at a distancebetween magnetic objects was observed and geomagnetismhad begun to be mapped on a global scale,but these subjects were not explained with terms andconcepts that we would today recognize as belongingto a general field theory. Peregrinus and Gilbertmade what were essentially qualitative, descriptive accountsof magnetism, drawing analogies with livingbeings and seeing teleologistic intent. In a clear breakwith ancient dogmatic thought, Descartes (1644) advocateda scientific philosophy that was more practicalthan that of his predecessors. In his rigid rationalism,appealing to spiritual notions and divine purposewas unnecessary. Instead, scientific theories could beconsidered successful if they could make mechanisticpredictions of observations (Hesse, 1961; Jonkers,2003, pp. 80–81).To account for magnetic action at a distance,Descartes suggested that an aether, composed of interlockingvortices or “tourbillons”, supported thecontinuous flow of infinitesimal particles along magneticlines of force, with a channeling of the particlesalong the inside of magnetic objects. According tohis theory, a compass needle would turn so that itsmagnetic channels would be parallel to the path takenby the flow of Earth’s magnetic particles. Descartes’stheory is often described as corpuscular, but withsome accommodation for differences in vocabularyit also clearly possesses some of the properties of afield theory. Of course, a more precise elaboration requiredmathematics that were undeveloped at the timeof Descartes. The necessary quantitative foundationwould be established when the great English physicist