Instrument made by James Kynvyn, comprising a round plate divided into four quadrants, two with the sine grid, known as the reduction quadrant, one with the shadow square, and one with the quadrant for determining the longitude. At the center is a rotating graduated alidade; the circumference is engraved with the degree scale. On the back is a cylindrical section for fitting the instrument on a support. Provenance: Robert Dudley bequest to the Medici collections.