The station pointer consists of a graduated circle and three arms, one of which is fixed (at 0 degrees on the scale), while the other two can move around the circle and have fixing screws. The circle of this example has a diameter of 6.125 inches (156 mm) and is made of brass with a silvered degree scale. The scale runs from 0 to 360 degrees, with 30-minute subdivisions, while verniers on the movable arms allow reading to 1 minute. It is contained in a fitted mahogany case, which also holds extension pieces for each of the arms. The magnifying glass with which it would have originally been supplied is now missing.

This particular example has had a varied history, as the inscriptions on it testify. The maker’s name appears as ‘Potter Poultry London’, referring to John Potter, who worked at 31 Poultry in the City of London and became instrument maker to the Admiralty. On the central fixed arm is the British government mark (a broad arrow) beside the inscription ‘2 Polar Exp. 1875’. This indicates that it was part of the equipment taken on the Polar expedition of 1875-76 under the command of George Strong Nares (baptised 1831, died 1915).

The sailing orders issued to Nares made it clear that ‘the object of the expedition is for the advancement of science and natural knowledge’ but also added that ‘detailed surveys are unnecessary. The requirements of hydrography and geography will be provided for if the prominent features and general outline of the shores are sketched in as faithfully as circumstances will admit; and to ensure recognition by future explorers’. Nevertheless, the scientific and geographical information gathered included the charting of 250 miles of coastline, despite the expedition being cut short due to an outbreak of scurvy. This station pointer would have been used during the charting that Nares and his crew undertook on this expedition.

The instrument has a further history, indicated by the inscription ‘THAMES CONSERVANCY’ on one of the moveable arms. Until about 1980 it was used first by the Thames Conservancy, which was set up in 1857 to look after the Thames between Staines and Yantlet Creek, and then by the Port of London Authority (PLA), which from 1909 had taken over responsibility for the tidal Thames below Teddington Lock. The PLA’s duties still include vessel traffic management, maintenance of shipping channels and moorings and the location of buoys and markers. As part of these duties, PLA staff carried out surveys of the river, using equipment that included this station pointer.