By the early decades of the nineteenth century, with surveys established as the weapon of choice for the fiscal military state, their instrumentation provided a focal point for radical attacks on political establishments. This paper considers a notorious dispute over mastery of iron in the instrumentation of magnetic surveying that took place in the 1830s between an Admiralty committee and the Reverend William Scoresby, a whaler-turned-clergyman. Scoresby staked his claim by drawing on the labour law of the whaleboats, a culture peculiarly preoccupied with the properties of bone and blubber, ink and skin, parchment and iron, where magnetism was forged in the 'combinations', as Scoresby put it, of such specific materials. The enterprises of his most avid reader, peer and fellow labour rights activist, Herman Melville, showcase the salience of Scoresby's struggle with Admiralty authority. The eminent Australian scholar Greg Dening's approach to ethnohistory proves the appropriate instrument with which to analyse such an encounter between traditions, negotiated through material forms. In the fraught exchange between whaler and maritime state, the combination laws that helped prompt the threat of revolution in early nineteenth-century Britain were translated into Scoresby's iron. Extant material and archival collections in Greenwich and Whitby offer traces of a battle between ways of knowing this protean metal: 'not down in any map; true places never are'.