Print of a diagram showing the way a backstaff or Davis quadrant, a navigational instrument, is used to measure the elevation of the Sun. Measured at Noon each day, this would allow a mariner to find their latitude at sea when out of sight of land. It is taken from 'Seaman's Secrets', published 1607 and reprinted in the Hakluyt Society's volume series (Vol 69, p 334). This work deals with various aspects of good seamanship as well as the methods available for nautical navigation.
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