Papers and three letters, chiefly concerning Mitchel's improved quadrant and its use for the four most important operations in navigation: finding the variation of the needle at sea, finding the latitude at sea, finding the apparent time at sea, and finding the longitude by Jupiter's satellites or the moon. In the final letter of the series, dated 14 July 1788, Mitchel states that he had 'laid aside' his quadrant method and now proposed a method of finding longitude by the stars or by the satellites of Jupiter [RGO 14/38: 133].