14 1/2 Inch vernier-type sextant,1770
Jesse Ramsden, London
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This very early type of sextant was tested by James Cook during his third voyage, and maybe already during his second voyage (1772-1775).
He concluded afterwards that the sextant used in combination with a chronometer was more accurate than the lunar distance method for determining longitude at sea.
This sextant was still hand-engraved.
Very early sextant, constructed in brass, signed on the lower cross bar Ramsden London, scale divided to 130º direct to arc, braced index arm, index mirror with Maskelyne flap, sighting tube without lenses, six shades, secondary mirror on rear-mounted adjustment lever, wooden handle and brass feet -- 17 x 17in. (43 x 43cm.) overall, no box, Sighting tube lacks lenses; (erect image), some dents to edges, traces of original finish behind.
This is the same pattern of sextant used by Captain Cook (1776-79) on his third voyages of discovery.
George Vancouver (1757-1798) had a 12½in. radius version when he joined the second voyage and disputed its accuracy with Ramsden who vigorously defended himself at the Board of Longitude.