In the summer of 1269, the southern Italian city of Lucera was under siege, and in the army outside the city walls a man was writing a letter. Decades earlier, in the first half of the thirteenth century, the Holy Roman Emperor Frederick II had expelled approximately 20,000 Muslim inhabitants from Sicily in an attempt to quell religious unrest on the island. The uprooted Muslim communities resettled in southern mainland Italy, and many of them ended up in Lucera. After the forced resettlement, Frederick II made what he probably believed was a very fair deal with his displaced subjects: they were permitted to practice their religion in exchange for taxes, military service, and support against the his enemies.
That arrangement lasted about forty years, until Frederick died and Charles I of Naples (brother of Louis IX, king of France) claimed, with papal support, the Kingdom of Sicily for himself. (The Kingdom of Sicily at that time included the island of Sicily and just about all of mainland Italy south of Rome.) Lucera resisted, and Charles' army laid siege to the city in 1269. Eventually the city surrendered, and Charles maintained an arrangement with Lucera very similar to the one Frederick had made: taxes and military service in exchange for a sort of religious freedom. But it didn't last. Charles eventually died and his son Charles II took over, and one thing Charles II wanted to do with his father's kingdom was get rid of all the Jews and Muslims. He sacked Lucera in 1300, destroyed the mosque and converted, enslaved, exiled or killed the entire Muslim population.
The siege of Lucera in 1269 wasn't a particularly important event in history; it was business as usual for politics in the thirteenth century. But that siege holds a special place in the history of science, because serving in Charles of Anjou's army outside the city walls there was a man by the name of Pierre Pelarin di Maricourt, who called himself Petrus Peregrinus ("Peter the Pilgrim").
Very little is known about Peregrinus's life. He was probably a knight and a crusader, and possibly worked as some kind of a military engineer, but that's all we know about him. The only reason anybody remembers him at all is because of this letter he wrote to a friend while he was hanging out in camp during the siege of Lucera in August of 1269. (Or so the story goes. It's a personal letter from an otherwise unknown individual; the exact provenance is not entirely certain.)
Whatever crusading knights in camps normally wrote about when they wrote to their buddies back home--maybe they complained about the food? the sweltering southern Italian summer?--it's probably safe to say that Peregrinus was the only one who decided that "letters from siege camp" ought to mean "lengthy and detailed exploration of the properties of magnets and magnetism as determined by empirical observation." Or, in his own (translated) words:
Dearest of Friends,
At your earnest request, I will now make known to you, in an unpolished narrative, the undoubted though hidden virtue of the lodestone, concerning which philosophers up to the present time give us no information, because it is character of good things to be hidden in darkness until they are brought to like by application to public utility. Out of affection for you, I will write down in a simple style about things entirely unknown to the ordinary individual.
There are numerous surviving copies of the letter--at least one of which states that it was written outside of Lucera in 1269, hence the commonly accepted time and place of origin--and it was a popular text amongst scholars for a long time after Peregrinus himself was forgotten. Contemporaries such as the Franciscan scholar Roger Bacon approved of Peregrinus's empirical methods, and three hundred years later the letter was praised as "a pretty erudite book considering its time" by William Gilbert, the English scientist who founded the field of modern terrestrial magnetism and first proposed that the Earth itself is a magnet.
Peregrinus certainly wasn't the first person in history to study magnetism, but he was the first in recorded history to devise a series of observations and experiments with the goal of understanding how magnets and compasses work. In the thirteenth century, empirical investigations were a rather new approach to the natural world, as scholars had gradually come to realize that the Greek "learn geometry and think about it logically" method left a lot to be desired. Pure logic and philosophical intuition, alas, do not always lead to conclusions that match what careful observation reveals. (Just ask all the astronomers who insisted on seeing circular celestial orbits rather than admitting to philosophically unsatisfactory ellipses.)
The story this letter and its subsequent importance is just one example in which the science of terrestrial magnetism--the study of why compass needles point north and everything that entails--is in many ways a weird little sideshow in the history of science. It's a scientific field dealing with questions that people have been asking since early Greek history, but it's only within the last twenty years or so have scientists been able to develop a reasonably accurate picture of how the Earth's magnetic field works.
And that long history is what this book explores:
North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth's Magnetism by Gillian Turner (Awa Press, 2010)
I was predisposed to think ill of this book before I read it, not because I have anything against it in particular (although I do question the use of both "epic" and "great" in the subtitle), but because I've seriously considered writing a book on this very topic and have done a great deal of research with that goal in mind. (Which explains why, yes, I do in fact have a translation of Peregrinus's letter on hand.) I put that project aside when I decided to focus on writing fiction, and to be honest I was happy to see that somebody else had recognized this obvious gap in popular science literature and decided to fill it.
Aside from a few minor quibbles, I think it's successful. North Pole, South Pole is a straightforward history, tracing the study of terrestrial magnetism from its origins in ancient Greece (and, to a lesser extent, China), through the so-called scientific revolution of the seventeenth century and the later development of electromagnetic theory, through the earliest days of seismology and the twentieth century advancements in geology and geophysics. It begins with philosophers wondering why some funny black rocks can make iron needles point north, and it ends with the development of dynamo models so complex they make modern supercomputers curl up and beg for mercy.
I would have liked to see a bit more about the early history of the mariner's compass, which is very much tied to early studies of magnetism (nobody studied the Earth's magnetic field, or even suspected that it existed, until people who used compasses regularly began to notice how weirdly they behaved at different locations), because it's an interesting period and I felt the historical context could have been stronger. The narrative also stumbles a bit when racing through the advances in electricity and magnetism in eighteenth and nineteenth centuries. That's a time period that's well-covered in countless other texts, sure, but that's no reason not to make it interesting in this version.
Where this book really shines is in what follows: from the turn-of-the-century discovery by seismologists of the Earth's liquid outer core, the application of dynamo theory to planetary bodies, and eventually the complete overhaul of geology that came about with the concurrent, interconnected discovery of polarity reversals and the development of the theory of plate tectonics. I suspect part of the reason for the brisker, lighter feel of these chapters is that it didn't happen very long ago, and therefore quite a lot of the people who made those discoveries are still alive to be interviewed--and to be gossiped about by their colleagues, which provides a feeling of active back-and-forth between scientists that's missing from earlier chapters.
The only other criticism I have is that I would have liked for the other planets in our solar system to rate more than a few paragraphs at the end. This is partly because there are some fascinating unanswered questions regarding the magnetic fields of other planets, such as why tiny Mercury has one but Venus doesn't. (NASA's MESSENGER mission, currently in orbit around Mercury, has made a lot of progress answering the first question, but the second is still a mystery. Venus is a very difficult planet to study.) But more importantly, I think any discussion of Earth's internal processes needs to be understood in its appropriate context (and not just because "Planetary Magnetic Fields and You" is one of the great unwritten essays in my great unwritten essay collection Planetary Science For Dummies, and Also For Science Fiction Writers). The context is that we don't know how strange or how normal Earth is in the grand scheme of things because we don't have much to compare it to, but what we do know about Earth's formation and evolution influences how we interpret other planets. It's a missed opportunity to not spend at least a little bit of time discussing what that means.
In summary: A good exploration of the study of terrestrial magnetism from antiquity to the present, with a few weak spots and especially strong chapters on the development of paleomagnetism in the twentieth century--a step in scientific history that even geologists don't often learn much about.