Petrus Peregrinus opens his 1269 letter not with abstract theory but with a hands-on procedure: marking the poles of a round lodestone by floating it on water and observing its alignment. This practical starting point sets the tone for a work that blends meticulous experiment with cosmological speculation. The letter is divided into two parts: the first examines the lodestone's properties—polarity, attraction, and the way iron acquires magnetic virtue—while the second describes instruments that exploit these phenomena, including a floating compass and a device for measuring azimuth. Throughout, Peregrinus insists on repeatable tests, such as balancing the stone on pivots and checking its motion at different hours, yet he also invokes the heavens as the ultimate source of magnetic virtue.

Polarity as the Organizing Principle

Peregrinus’s first major claim is that every lodestone has two poles, one seeking north and the other south. He demonstrates this by floating the stone: it rotates until the poles align with the world’s poles. The language is precise: “the northern part of the stone turns to the north, and the southern to the south.” He then shows that opposite poles attract, like poles repel, and that a piece of iron touched by one pole acquires that same pole’s orientation. This polarity is not merely a property but the key to all magnetic behavior. He even notes that breaking a lodestone produces two complete magnets, each with its own north and south—a observation that would later become foundational.

Celestial Virtue and Experimental Caution

Having established polarity, Peregrinus argues that the lodestone’s virtue comes from the heavens: “the poles of the lodestone receive their virtue from the poles of the world.” He supports this with a test: mount a balanced round stone on pivots, align it with the meridian, and observe whether it follows the celestial motion. If it does not, he warns, “ascribe the failure to your own lack of skill rather than to a defect in nature.” This sentence reveals a careful experimenter who acknowledges human error. He also notes that the needle points to the celestial pole, not the pole star, because the meridians intersect at the poles—a correction of common belief.

From Theory to Instrument: The Floating Compass

Part II shifts to construction. Peregrinus describes filing a lodestone into an elongated shape, enclosing it in two sealed wooden capsules, and floating it in a vessel of water. The vessel’s rim is marked with the four cardinal points, determined by a meridian thread. A smooth strip of wood is placed across the capsules as a diameter; when aligned with the meridian, it is fixed, and the capsules are engraved with 360 divisions. This device, he says, will “indicate forever the meridian of that place.” The instructions are detailed: the capsules must be light, waterproof, and cemented carefully. The result is a portable compass that can also serve as an astrolabe for finding the ascendant.

The Astrological Horizon and the Limits of Evidence

Peregrinus’s instruments are not purely navigational; they are designed for astrological reckoning. With his floating compass, he claims, “you will need no timepiece, for by it you can know the ascendant at any hour.” The letter’s final chapters describe measuring the azimuth of the sun, moon, or any star, using the compass’s graduated circle. This fusion of magnetism and astrology reflects the 13th-century worldview, but the excerpts break off before the full azimuth procedure is given. What remains clear is that Peregrinus sees no conflict between empirical testing and celestial influence: the lodestone’s behavior is both a natural phenomenon and a link to the heavens.

Readers should approach this letter as a working document: it is part manual, part philosophical treatise, and part promise of future instruments. The translation by Brother Arnold preserves the direct, instructional tone, though the introductory notice by Brother Potamian provides useful historical context. Because the excerpts are incomplete, especially toward the end, the full scope of Peregrinus’s azimuth instrument remains unclear. What survives is a vivid snapshot of medieval science in action—experimental, speculative, and deeply practical.