Dip circle or dipping needle, made of brass with gold pivots encased in copper. The needle is 12 inches (30.5 cm) long, and the circular scale is graduated quadrantally to 30 minutes.

A dip circle measures the vertical component of the earth's magnetic field. This one is believed to have been used on Captain Cook's second voyage (1772-5), during which a number of observations of the earth's magnetic field were made. According to Rees's 'Cyclopaedia' (London, 1819): 'In the year 1772, Mr Nairne completed two dipping needles for the Board of Longitude, agreeably to a plan of the Rev. Mr. Mitchell, a gentleman eminently distinguished for his great knowledge in magnetics'. The same design is discussed and Illustrated in 'The Philosophical Transactions of the Royal Society', 62 (1772), pp. 476-80. A similar instrument is also described and illustrated in Constantine John Phipps, 'A voyage towards the north pole: undertaken by his Majesty's command, 1773' (London, 1774). For Cook's third voyage (1776-80), the instrument was redesigned to eliminate various defects that became apparent during the second voyage.