British National Antarctic Expedition, 1901-1904
CARY, PORTER, LTD. (manufacturers). An early 20th-century laquered- and oxidised-brass surveying station pointer, signed 'Cary , London' and further stamped 'ANT: EX: 1901' and 'R.A.N.H.O. [bench mark] 86' on the arms, the circle divided 0-180-0 in ½° increments, the two mobile arms with tangentially-adjusted vernier scales with locking screws, three extension arms with brass fixing screws (minor superficial chipping on arms, possibly lacking two accessories), contained in a plush-lined mahogany case with maker's trade label and printed calibration label with manuscript insertions titled 'Station Pointer No. 86' dated '31/3/25' pasted to the inside of the cover, the upper face of the cover with painted lettering 'ANT: EX: 1901' and incised lettering 'R.A.N.H.O. 86', the front face of the cover with painted lettering 'ADMIRALTY H.O. [bench mark] 86.' (varnish chipped and rubbed, corners lightly worn, lining worn with minor loss) -- the unextended station pointer 16¼in. (41cm.) long.
AN IMPORTANT NAVIGATIONAL INSTRUMENT FROM SCOTT'S FIRST EXPEDITION. The station pointer is an instrument which allows positions to be found without the use of a magnetic compass, and allows compass errors to be calculated. This is essential in the extreme latitudes of both the Arctic and Antarctic as the proximity of the magnetic pole makes the magnetic compass dangerously unreliable. The observer, using horizontal sextant angles, measures the subtended angles of three points of land. These angles are set accurately on the station pointer using the vernier scales. The three arms are moved on the chart to fit the chosen points of land, and the navigator then marks his positions as being at the centre of the station pointer. From this position the navigator can take magnetic compass bearings of the selected points of land. He then compares these bearings with the true bearings from the chart and the difference is his compass error. He can then apply this error to future compass bearings, to obtain true bearings, when the use of the station pointer is impracticable.
The present instrument, manufactured by Cary, Porter, Ltd., a firm of nautical and scientific instrument makers established in 1765, who were instrument makers to the Admiralty.