William Gilbert's On the Magnet (1600) opens with a methodological manifesto: the author insists that trustworthy experiments and demonstrated arguments surpass the probable guesses of ordinary professors of philosophy. This 1900 translation by Silvanus P. Thompson preserves the combative, first-person voice of Gilbert, who presents himself as a physician of London and a pioneer of a new physiology of the Earth. The catalog subject 'Magnetism -- Early works to 1800' accurately reflects the work's content, but the excerpts reveal a text equally concerned with refuting contemporaries and predecessors—Pedro de Medina, Giambattista della Porta, Livio Sanuto, and Simon Stevin are all named and corrected. Gilbert's experimental claims are interwoven with a rhetorical strategy that positions his own observations as the key to unlocking the Earth's interior.

A Polemical Experimental Philosophy

Gilbert's preface frames his project as a corrective to centuries of ignorance: he will begin with the common magnetick, stony, and iron material, then proceed with demonstrable experiments to penetrate the innermost parts of the earth. This progression from the tangible to the hidden is explicitly compared to geometry's ascent from easy principles to the most difficult. The author's tone is confident and dismissive of rivals. In Chapter IX, he attacks the notion that magnetic variation can yield terrestrial longitude, calling B. Porta's hope 'vain' and his opinion 'fruitless.' He similarly dismisses the 'crowd of philosophizers and sailors' who believe the meridian through the Azores marks the limits of variation. These polemics are not incidental; they structure the argument by clearing ground for Gilbert's own theory. The excerpts show a writer who treats error as a obstacle to be removed, not a position to be debated neutrally.

The Earth as a Great Magnet: Evidence and Limits

The catalog subject 'Magnetism -- Early works to 1800' is accurate but narrow. Gilbert's central claim—that the Earth itself is a great magnet—is supported by analogical reasoning from lodestone experiments to terrestrial phenomena. He asserts that the Earth's magnetic properties are 'conspicuous and exalted powers' that have been 'hitherto quite unknown.' However, the excerpts do not provide the full experimental chain. What is visible is Gilbert's reliance on the behavior of a 'versorium' (a pivoted magnetic needle) and his detailed critique of others' data. For instance, he refutes Stevinus's table of variations by pointing out that the variation changes with latitude, not just longitude: 'on the meridian passing through Plymouth in Latitude 60 degrees the North-easterly variation is greater: in Latitude 40 deg. much less.' This attention to quantitative detail suggests a careful empiricist, but the excerpts also show that Gilbert's conclusions are sometimes based on limited geographic evidence (Europe and the Atlantic).

Navigational Controversies and the Search for Longitude

A significant portion of the excerpted text concerns the practical problem of finding longitude at sea. Gilbert engages with several navigational writers, including Pedro de Medina, whom he accuses of disfiguring his Arte de Navegar with errors. The core debate is whether magnetic variation (the difference between magnetic north and true north) follows a predictable pattern that could serve as a longitude index. Gilbert argues it does not: variation is 'always uncertain, both on account of longitude and of latitude, and because of the approach toward extensive tracts of land.' He provides a counterexample: on the meridian of Corvo, variation is zero near the island but becomes northwest in higher latitudes and east in lower latitudes. This refutation is grounded in specific data points, though the excerpts do not reveal how Gilbert collected or verified these observations. The discussion underscores that Gilbert's work is as much a critique of existing navigational theory as a positive exposition of magnetism.

Readers approaching this edition should note that Gilbert's text is not a neutral survey but a partisan intervention in early modern natural philosophy. The translation by Thompson, a physicist and historian, likely shapes the technical vocabulary (e.g., 'versorium,' 'chalyboclysis'). The excerpts suggest that Gilbert's arguments are most compelling when he engages with specific experimental or observational claims, and most tendentious when he generalizes. For a full assessment of his method, one would need to examine the experimental descriptions in the complete work, particularly his use of the terrella (a spherical lodestone) to model the Earth.