0137
   SOUNDING SEXTANT  
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  Surveying Instrument Collection 

Maker

Kelvin & Hughes Ltd (Division of Smith & Sons Ltd)

Model

SURVEY MK III

Serial Number

68116

Dimensions

Radius 180mm, Width 250mm

Image

Sextant

Description

This Sounding sextant is an important marine/hydrographic instrument typically used for measuring angles between coastal features for locating the position of a boat relative to the shore. It is characterised by its relatively large aperture telescope and the absence of filters. The instrument is graduated at one degree intervals between -5 and 140 and is similar in principle and design to the ordinary sextant. This is a relatively modern type of sextant and features a telescope with a wide field of view which produces a magnified image when the target is being viewed by the observer. Also attached to the instrument is a vernier with a clamp which allows a slow-motion movement along the arc for a more accurate reading of the graduations. The instrument includes two plane mirrors, which are considered to be larger than normal, and both of which are fastened on the frame normal to the graduation surface. The angle between these mirrors can be varied. 

The structure of the sextant is based on the theorem that a ray reflected by two mirrors is deflected by twice the angle enclosed by the mirrors.  

History & comments

The sextant is a free hand instrument and so its accuracy may be limited. It must be noted that the sextant measures angles, and not directions or bearings. It is a more accurate instrument than the compass, but not as accurate as the theodolite. The sextant, however, is lighter, more portable, and quicker to use than the theodolite. It can also be used from a moving platform, such as a ship or a small boat, where it is impossible to use a theodolite.

Conservation

In a fine box

Condition

Good

Remarks
  • Catalogued by T. Ko
  • Updated by F. Pall

Dates

Manufactured in 1964 (approx). Catalogued in 1997

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