The Elements of navigation ; 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 scholl at Christ's hospital ; the royal Academy at Portsmouth, and the gentlemen of the navy. In two volumes / By J. Robertson,... ; The fourth edition, with additions. Carefully revised and corrected by William Wales,..