William Bayly was appointed assistant to the astronomer royal Nevil Maskelyne at the Royal Observatory, Greenwich, in 1776. Having mastered observational astronomy, he sailed on HMS Emerald to observe the transit of Venus at Nordkapp, Norway, on 3 June 1769. On Maskelyne's recommendation, Bayly was appointed by the Board of Longitude, as astronomer, along with William Wales, to Cook's second voyage, sailing from Plymouth in July 1772. Wales and Bayly were instructed 'to make Nautical & Astronomical Observations, and to perform other Services tending to the Improvement of Geography & Navigation' (Journals, 2.724). Bayly sailed on the Adventure (Captain Furneaux) and Wales on Cook's Resolution. The astronomers had charge of the new longitude timekeepers -- Wales one by Larcum Kendall and one by John Arnold on Resolution, and Bayly two by Arnold on Adventure. Bayly's observations made on the voyage were published in 1777, edited by Wales.
With Cook's return on the Resolution in 1775 and the decision to send him out in July the following year to explore the north Pacific, Bayly was again appointed astronomer, sailing on the Discovery (Captain Charles Clerke), along with James King, second lieutenant and astronomer on Cook's Resolution. After Cook's death in Hawaii in 1779 Bayly transferred to the Resolution and left the voyage at Stromness in 1780. He was commissioned by the Board to prepare the observations made on the voyage by Cook, King and himself, which were published in 1782.
Bayly was appointed headmaster of the Royal Naval Academy, Portsmouth Dockyard, a post he held from 1785 until it was transformed into the Royal Naval College in 1807. He died at Portsea, Hampshire on 21 December, 1810.
Mark Beaufoy, Bayly's pupil and executor, to whom the present relic passed, came from Quaker stock, was the first Englishman to summit Mont Blanc (1787) and was a distinguished physicist and astronomer. He was elected to the Royal Society in 1790 and founder member of the Society for the Improvement of Naval Architecture in 1791. He died at Bushey on 4 May 1827 and Cook's relic subsequently descended in his family to the present owners.
The inscription on the silver capstan records that 'this magnifier was given by Captain James Cook ... to Mr Willm. Bayly'. It may equally plausibly have been acquired by Bayly at the dispersal of Cook's effects on board the Resolution. Cook's clothes and belongings were sold at auction to his officers in the Resolution's Great Cabin shortly after his death at Hawaii. 'Such a ceremony was a gesture of respect and support for the dead man's family (who received the money that was raised), while keeping his relics among his shipmates.' (A. Salmond, The Trial of the Cannibal Dog, London, 2003, p.420)
The relic is not recorded in M.K.Beddie (ed.), Bibliography of Captain James Cook, Sydney, 1970 (Relics, pp.617-648).