0107
   BOX SEXTANT  
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  Surveying Instrument Collection 

Maker

N/A

Model

N/A

Serial Number

N/A

Dimensions

Diameter 75mm, Height 40mm

Image

Box sextant

Description

The English introduced this box sextant at the end of the eighteenth century, based on the same optical principle of the sextant ( i.e. used for angle measurements), but very different in appearance. The optical components of a sextant were miniaturised and placed inside a cylindrical box. It is much smaller and lighter than the nautical sextant, not much larger than a pocket instrument, and is the type of sextant used for ordinary land surveys rather than astronomical applications. The graduated arc is graduated to half-degrees from 0 to 120, the vernier enabling angles to be read to single minutes. The readings are read off the graduations at the top using the magnifying glass. The working parts of the instrument are contained in a circular brass case of about 75mm in diameter, the index arm and mirrors being protected when the instrument is not in use by a lid which can be removed and screwed on the bottom of the case to form a handle when observations are being taken. The larger of the two milled heads shown in the illustration operates a pinion, which engages with a toothed circular segment carrying the index glass and index arm. By this means a controlled fine motion can be given to both index glass and index arm. The smaller milled head is a key which can be used to operate adjusting screws for eliminating index error and for adjusting the horizon glass. A small removable telescope is provided for long sights, but ordinary observations are taken through the peephole in the case.

History & comments

The instrument, when in use, is held in the left hand and the index glass turned by rotating the large milled head by means of the right thumb and forefinger. It's main advantage (when compared to the nautical sextant), lies in the fact that all the parts are very well protected from accidental damage and that in the smaller sizes it is very compact and portable.

Conservation

In a case

Condition

Good, mild rust on brass surface

Remarks

  • Donated from the Estate of Mr. Patrick Walsh, F. I. S. (Australia) October, 1977.
  • Catalogued by T. Ko
  • Updated by F. Pall

Dates

Manufactured in 1900 (approx). Catalogued in 1997

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