The instrument consists of a circular plate whose rim carries a degree scale divided into eight 45° sectors, the names of the winds, and, on a protruding portion, other two graduated scales. On the inner circle is the signature of the maker, Baldassarre Lanci, and the date, 1557 ("BALTHASSAR LANCÆVS URBINAS FACIEBAT A.N.D. MDLVII"). A magnetic compass is mounted at the center. Around it rotate the arched legs fitted with sights. One of the legs is fixed; the other is movable and has an adjustable viewfinder and a graduated clinometer with plumb bob. The clinometer carries the initials of the maker, "B.L." (Baldassarre Lanci). The instrument was used to measure the diameter of cannons and projectiles, distances, and heights, as well as for surveying.