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  Surveying Instrument Collection 


A. Tornaghi (London)



Serial Number

S. G. O. No. 71


Length 313mm, Width 45mm, Height 190mm


Trough compass


This trough compass, unlike the surveyor's and prismatic compasses, does not form a complete surveying instrument, since it ony establishes the direction of magnetic north. It is used with either a plane table or a theodolite. The needle of this trough compass consists of a long (30 cm), narrow, magnetised bar of steel, pointed at both ends, with the usual agate bearing at its centre. It is mounted in a narrow rectangular box carrying a pivot at its centre. This box features with a tongue or lever, worked from a small stud or handle outside, which enables the needle to be lifted off and kept clear of the pivot when the instrument is not in use. At each end of the box, which is closed by a glass cover, is a block of metal, the top of which is at the same level as the tip of the needle, and on which is engraved a zero line, the magnetic meridian, and a very short graduated arc extending about 5 degress on either side of the zero mark. The instrument can be screwed onto a tripod and levelled by four levelling screws. It is used in navigation and surveying to assist in locating and describing stations and in orientating reconnaissance sketches. This trough compass carries the inscription "Standard Needle", S.G.O. (Surveyor General's Office) and was used for calibration of the "Box Compasses" used in the field by surveyors.

History & comments

First instruments are dated 1030-1093 ( Encyclopaedia of Shon-Kua China ). The Compass was introduced to Europe by Marco Polo in 1260. (See also Cat. No. 0124.)


In case




  • TORNAGHI, Mathematical Instrument Maker, 28 Bridge Street, London.
  • Inscription "Standard Needle / S. G. O. (Surveyor General's Office) No:71"
  • This Trough Compass is one of two in the collection (see also 0124)
  • Catalogued by T. Ko
  • Updated by F. Pall


Manufactured in 1860 (approx.). Catalogued in 1997

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