Surveying Instrument Collection
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Maker |
A. Tornaghi (London) |
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Model |
N/A |
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Serial Number |
S. G. O. No. 71 |
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Dimensions |
Length 313mm, Width 45mm, Height 190mm |
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Image |
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Description |
This trough compass, unlike the surveyor's and prismatic compasses, generally does not form a complete surveying instrument, but is instead used with either a plane table or a theodolite. The needle of this trough compass consists of a long (30cm), narrow, magnetized bar of steel, pointed at both ends, with the usual agate bearing at it's centre. It is mounted in a narrow rectangular box carrying a pivot at it's centre, and this box is provided 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 5º on either side of the central zero mark. When used in conjunction with a plane table, the edges of the box must be sharply cut out to enable the line of direction of the magnetic north to be transferred to the paper. 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 Compass" used in the field by the surveyors. |
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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.) |
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Conservation |
In case |
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Condition |
Excellent |
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Remarks |
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Dates |
Manufactured in 1860 (approx.). Catalogued in 1997 |
