Chapter 7, ‘One of Geometry’s Nicest Applications’, relates the digging of the Deep-George draining tunnel (1771–1800), named after George III, King of Great Britain and Hanover. During the planning phase, surveyors designed this engineering project with a previously unknown level of detail. Jean-André Deluc, Fellow of the Royal Society and reader to Queen Charlotte, visited the Harz mines three times as the operations were under way. Deluc’s geological and meteorological inquiries led him to perform barometric experiments in these mines. He relied on practitioners’ data to test and calibrate his instruments, marvelling about their precision in the Philosophical Transactions and the Journal des Sçavans. Scholars and amateurs – from Goethe to Watt – but also merchants and their wives rushed to visit the project. Year after year, journals reported to the public how the various sections of the tunnel connected seamlessly. In the late eighteenth century, the two worlds of natural scientists and mine engineers were meeting one last time, this time around common issues of precision, data gathering, and instrumentation.