14 1/2 Inch vernier-type sextant,1770
Jesse Ramsden, London
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Dit type zeer vroege sextant werd door James Cook op zijn eerste en tweede wereldreis (1776-80) getest op de Stille Oceaan. Hij concludeerde na zijn tweede reis dat de sextant mét chronometer nauwkeuriger was dan de ‘Maansafstand-methode’ om de lengte op zee te bepalen. Deze sextant werd nog met de hand gegraveerd.
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 (1728-1779) on his first and second 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.