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Mr. Morris.  With reference to the Fairchild A10; you say:-

"A grub screw attaches the 
one-toothed clutch to the shaft of the counter and the latter is provided 
with a screw-driver slot to make the adjustment easier." 
------------

Many thanks for this.  I didn't look hard enough to realise (having just 
filled the bubble chamber and wanting to get on and play with comparisons of 
the A10 with my definitive Hughes MkIX BM; ...though I suspected there had to 
be some means of disconnecting the main drive shaft and worm unit from the 
counter to set the counter to zero (or whatever) then disconnect the counter 
and adjust the index prism.

I have to say I am not impressed with the accuracy achived in my comparisons 
with sun shots between the two sextants getting very variable results from 
the A10 up to plus or minus 15 or even 20 minutes of arc compared with the 
MkIX.  Though to most differences were around 8 to 10 minutes of arc.  That 
is still too much.

This of course is partly due to no telescope with the A10, with a relatively 
small image of the sun and bubble compared to the MkIX; but even so, the 
results are not good at all. I even wonder if the worm is distorted, though I 
have not done definitive checks yet to assesss the linearity of the worm 
screw thread and tangent sector gear.
I would not have been happy in a B29 Superfortress bomber mid- Pacific with 
that sextant to get me home,  as the Americans were doing in WW2.

In my opinion the Hughes MkIX is the definitive sextant of its type; 
approaching if not actually being the very best that can be achieved with a 
bubble sextant.

Douglas Denny.

Chichester. England.

P.S. Pic enclosed of the two sextants.
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