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  Surveying Instrument Collection 


E. R. Watts & Son, London



Serial Number



Radius 40mm


Prismatic Compass


This Prismatic Compass is one of the two main kinds of magnetic compasses included in the collection for the purpose of measuring magnetic bearings, with the other being the Surveyor's Compass. The main difference between the two instruments is that the surveyor's compass is usually the larger and more accurate instrument, and is generally used on a stand or tripod. The prismatic compass on the other hand is often a small instrument which is held in the hand for observing, and is therefore employed on the rougher classes of work. The graduations on this prismatic compass are situated on a light aluminium ring fastened to the needle, and the zero of the graduations coincides with the south point of the needle. The graduations therefore remain stationary with the needle, and the index turns with the sighting vanes. Since the circle is read at the observer's (rather than the target's) end, the graduations run clockwise from the south end of the needle (0 to 360), whereas in the surveyor's compass, the graduations run anti-clockwise from north. 

The prismatic attachment consists of a 45 reflecting prism with the eye and reading faces made slightly convex so as to magnify the image of the graduations. The prism is carried on a mounting which can be moved up and down between slides fixed on the outside of the case. The purpose of this up-and-down movement is to provide an adjustment for focusing. The image of the graduations is seen through a small circular aperture in the prism mounting, and immediately above this aperture is a small V cut on top of the mounting, over which the vertical wire in the front vane may be viewed. When the V cut, the vertical wire and the station whose bearing is required are viewed in one line, the bearing is directly read off the graduated arc at the point immediately underneath the vertical wire.

The oblong mirror located in front of the forward vane slides up and down the vane, and is hinged to fold flat over it or to rest inclined at any angle with it. This mirror is used for solar observations, or for viewing any very high object, and is not a normal fitting to a compass. The two circular discs in front of the back vane are dark glasses which can be swung in front of the vane when solar observations are being taken.

History & comments

The 'Prismatic Compass' was invented by the maker Charles Schmalcalder and patented in 1812.


In a box



  • When the instrument is not in use, the prism and its mounting can be folded about a hinge to lie flat against the side of the case of the instrument. The front vane can also be folded about a hinge to lie flat over the lid of the case.
  • Catalogued by T. Ko
  • Updated by F. Pall
  • Instrument stolen in July 2000


Manufactured in 1900 (approx). Catalogued in 1997.

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