The elements of navigation [electronic resource] : containing the theory and practice. With the necessary tables, And Compendiums for finding The Latitude and Longitude at Sea. To which is added, A treatise of marine fortification. Composed for the Use of The Royal Mathematical School at Christ's Hospital, The Royal Academy at Portsmouth, And the Gentlemen of the Navy. In Two Volumes. By J. Robertson, Late Librarian to the Royal Society, and formerly Head-Master of the Royal Academy, at Portsmouth. The Fourth Edition, with Additions. Carefully revised and corrected by William Wales, Master of the Royal Mathematical School, Christ's Hospital, London.