Museo Galileo
Virtual Museum

Instrument so named by Philippe Danfrie (c. 1532-1606) in Traicté de l'usage du Trigometre (Paris, 1597). The device was, however, already in used as an accessory to more complex instruments such as Abel Foullon's holometer and Baldassarre Lanci's (1510-1571) distanziometro [distance meter]. The basic design consists of two graduated arms pivoting at the end of a fixed base. In Danfrie's version, each arm carries a graduated arc for measuring the angles formed with the fixed base. The distance to a point can be computed using the similarity of triangles or the sine theorem.