14 1/2 Inch vernier-type sextant, 1770-2
Jesse Ramsden, London
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Op zijn derde reis (1776-1779) testte James Cook dit zeer vroege type sextant. Hij concludeerde nadien 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 (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.