This rectangular brass plate is engraved on both faces and framed by a hinged border which is also made of brass. One face bears the shadow square and the quadrant for calculating longitude. The other face bears the quadratum nauticum and the wind rose. The use of the instrument is difficult to establish with certainty. Undoubtedly it could be used for nautical purposes, as the markings and degree scales suggest.
It is signed by James Kynvyn, and originally belonged to the bequest left to the Grand Duke of Tuscany by the naval engineer Robert Dudley at the time of his death (1649).