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On 2020-03-09 9:02, Frank Atkins wrote:
> I have just picked up a Kelvin Hughes sounding sextant. Ex Royal Australian 
navy. Last active check was in 1981. It has no shades just the two mirrors as 
made.

This sounds like a sextant for hydrographic survey fixes via the
measurement of angles between points on shore. The procedure is
described in Bowditch and also in "Hydrographic Manual," Special
Publication No. 143 of the US Coast & Geodetic Survey. The 1928 edition
may be downloaded here:

https://library.noaa.gov/Collections/Digital-Collections/USCGS-Special-Pubs

(A 1942 edition is listed too, but it's only one chapter of the
publication.)

"Position determination by sextant angles taken on board the sounding
vessel is the most satisfactory and most commonly used method for
coastal work. Two observers observe angles simultaneously, one between a
right-hand object and a center object and the other between the center
object and a left-hand object. The position of the vessel is then
plotted by using a three-arm protractor to effect a graphic solution of
the three-point problem.

"Hydrographic sextants resemble those used for navigation but are of
lighter construction and are read only to minutes. They can be used to
measure angles up to about 140°. The telescope generally has a
bell-shaped tube, so as to include as large a field as possible."

Also of interest in that book is the special procedure for obtaining
celestial fixes of the highest accuracy.

   

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