Large and early brass sextant, 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 , no box, Sighting tube lacks lenses; 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.
In 1773-5, Jesse Ramsden of England invented the circular dividing engine, an instrument which had a profound impact on Western history. Prior to his invention, the division and inscription of scales on mathematical instruments was done by hand. So this sextnat is form before his dividing machine!
With this type of sextant, James Cook desided during his second voyage, that the chronometer method was more accurate than the Lunar-Distance method to determine longitude!