Theodolite consisting of a square plate engraved with a degree scale and windrose. At the center pivots a rotating disk that covers the windrose. The disk has four viewers and four indexes (one of which is missing) revolving on the degree scale. Through an opening on the disk, the user can see the wind direction lined up with the targeted location. The four corners of the plate are engraved with the letters A, B, C, and D. The instrument is signed Petrus ab Aggere. Provenance: Vincenzo Viviani bequest.