Known as instrumentum triangulare, this triangulation device for surveying work was made by Joost Bürgi, a well-known craftsman specializing in clocks and scientific instruments. Bürgi worked in Kassel (from 1579 on) for the Landgrave Wilhelm IV of Hesse-Kassel, and in Prague (starting in 1604) at the court of Rudolph II. The instrument is unsigned but entirely similar—apart from the slightly longer arms—to a copy in the Museum of the History of Science at Oxford, which is signed “Ioost Bürgi F.” The arms are divided into 300 parts each and display measurement scales for the Prague foot and for weights of stone (“STEIN”), cast iron (“GOSSEN EISSEN”), and lead (“BLEI”). The sights are present, but of the third cross-arm only a fragment survives, still hinged to the cursor. The instrument was used for surveying work and to measure heights and distances. Provenance: Medici collections.