A computing instrument constructed in the form of an octant, with a curved index scale (90 degrees) and vernier. The right-hand limb is divided with s scale from 0 to 100. A second limb is set at right angles and can slide along the first. It is divided 10 to 70. The index is also divided 0 to 100. The instrument can be used to make calculations using sines, tagents and secants. The instrument is engraved with the words, 'Crow's Patent Seaman's Octant / London'.

An accompanying leaflet (NMM library PBF8050) decribes possible uses of the instrument for navigational calculations, which include: miles in a degree of longitude at any given latitude; meridional difference; difference of longitude; course and distance; plane, traverse and middle distance sailing; parallel, current and windward sailing.