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G'day, Brad

On the front cover of John Thomson's biography of Frank Worsley "Shackleton"s 
Captain", there is a painting showing Worsley taking a sight with a ladder- 
frame sextant. Can you confirm that this is a correct representation? 

I have a Heath Hezzanith three ring framed instrument from the same period 
without calibration certificate, so cannot see whether it has maintained its 
accuracy. However, I also have a Hughes ladder-frame sextant with an NPL 
certificate from 1920. Like yours, it has zero correction throughout the arc. 
This of course does not mean literally zero error, as the absolute limit of 
reading is 10 seconds. Even then, for me there is often uncertainty as to 
which of two or three pairs of graduations line up, despite being  aided by a 
stero microscope at X 15 power. I will try to re-calibrate it over the next 
few days and will report back. Because of the uncertainty in reading the 
vernier, I would not attach too much importance to the results.

I would not expect a well-built and carefully maintained sextant to have much 
wear even after a life time of daily use. The bearing is lightly loaded and 
moves slowly over a small arc. A plain bearing in, say, an electric motor 
would be subject to  more wear under greater loading in ten minutes of 
running.

Bill Morris
Pukenui
New Zealand




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