him by his friend Edward Wright, and an explanation of the principle of Mercator's projection. Hondius, in 1595, published at Amsterdam a new chart of the world on Mercator's projection, in the preparation of which he utilised Wright's tables. Blagrave and Hood improved the astrolabe and cross-staff. Hues expounded various problems in navigation, and included in his 'Tractatus de Globis' (1594), a chapter by Heriot on the use of rhumbs. John Davis, the navigator, wrote 'The Seaman's Secrets' in 1594, and invented the back-staff or, Davis's quadrant, which rapidly superseded the cross-staff, and which, improved by Flamsteed, remained in common use until Hadley's reflecting quadrant took its place in 1731. And Doctor Gilbert of Colchester, in the last year of the century, followed up the previous works by Borough, Norman, and others, on magnetism, by propounding the theory that the earth itself is a magnet. Nor must the invention of the telescope be forgotten. It is due to Zacharias Janssen, of Middelberg, about 1590, and the instrument, quickly improved, soon became part of the sea captain's equipment.
BACK-STAFF, OR DAVIS'S QUADRANT.
(From John Robertson's 'Elements of Navigation,' London, 1742.)
Henry VII., unlike some of his fifteenth-century predecessors,
