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Robert,

Most vernier octants seem to have been divided to read to 30 seconds and many sextants of the period as well, though 20 seconds for sextants was perhaps commoner. While some sextants of the period were divided to 10 seconds, I doubt that the vernier was consistently readable to this precision, and the observation was unlikely to have been accurate to this level. On 15 January, 1773, Captain Cook discusses a comparison of results of lunars using quadrants with and without telescopes, so we may perhaps conclude that up to this point telescopes were not regularly used, making an intrumental precision of 10 seconds moot. He wrote "We certainly can observe with greater accuracy with the Telescope when the ship is sufficiently steady which however very seldom happens so that most observations at sea are made without..." On this particular day his and Mr Wales's result for longitude agreed to within a handful of minutes with Kendall's chronometer, while the mean of six observers not using telescopes was more than a degree greater.

Bill Morris

Pukenui

New Zealand