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Thanks Bill,

That Endeavor voyage sounds wonderful.  There may be some transcription errors 
in your quotation, most especially the minutes of longitude observed with the 
Kendall watch in comparison to Cooks and Wales' telescopic lunars.

My comment about the "Mate" was merely based on the number of "Mates" I used 
to see on auction on Ebay, when I tracked them conscientiously, compared to 
other types.  Those observations also encompass most of my knowledge of 
Hughes & Son sextants, so it is quite meagre.  Thank you for your detailed 
explanation & kind correction.

Personally, I prefer my "Mate" to my C+P.  It's more compact and better 
balanced.  But my most accurate and precise rounds of sights were with the 
C+P.  I haven't had much chance to try other varieties.

Fred

Fred Hebard
mbiew@comcast.net



On Nov 18, 2014, at 2:51 PM, Bill Morris wrote:

>  
> Fred, you wrote
> << Hughes also concentrated on producing the three-ring "Mate" model sextant 
during the war, which has a 6" arc, not as accurate as the larger open frame 
in your instrument and the one illustrated in David's (
> sic) link.>>
> All Hughes and Son micrometer sextants had the same arc radius of about 160 
mm, dictated by the 18 turns per inch pitch of the micrometer worm. Their 
late vernier sextants had a slightly larger radius, measured at the feather 
edge of the vernier, of about 170 mm. Practically all of Hughes' micrometer 
sextants were of the three ring patern - with the exception of those 
blue-grey-painted ones made for the Admiralty and bearing a National Physical 
Laboratory calibration certificate rather than an "in-house" one. Though in 
the 1957 edition of  AJ Hughes' "The Book of the Sextant" he illustrates a 
micrometer sextant with a curve bar pattern frame, I have never seen one.  My 
understanding of the "Mate" was that they simply had larger errors on 
calibration than the "Master" which was also of three circle pattern.
> 
> and you wrote
> 
>  
> <>
> 
> The short answer is that they served no purpose in the Admiralty pattern 
sextant, as there is no means of adjusting collimation of the telescopes. 
However, in those sextants where collimation can be adjusted, two stars at 
least 110 degrees apart were brought into coincidence at one wire and if they 
remained in coincidence when aligned with the other parallel wire, the axis 
of the telescope was correctly aligned parallel to the plane of the arc. This 
may have been important in the days of lunars and in attempting to rate 
chronometers using an artificial horizon in far-off places of known 
longitude. However, collimation errors have to be relatively great to make 
much difference to the observed altitude, except perhaps for high altitudes, 
and other, non-instrumental errors would tend to swamp the relatively small 
collimation errors.
> 
> On a stable vessel a x 6 telescope can be used for sun sights as it is then 
easier to judge tangency with the horizon, but on a rolling, pitching vessel 
anything over x 3 makes it very difficult to bring the sun down to the 
horizon without losing it. One might admire masters of old on sailing ships, 
using their telescopes of rather small aperture and field of view, but it may 
be that they made do most of the time with a "zero magnification" sighting 
tube. Indeed, Cook's journal for the Resolution for 15 January 1773 notes 
that even lunar distances were taken without telescopes. The mean of five 
observers gave the longitude as 39d 42m 12s and Kendall's watch gave 38d 41m 
30s. "...but Mr Wales and I took each of us Six Distances with the Telescopes 
fitted to our Quadts, which agreed nearly with the Watch, the results were as 
follows
> 
> Mr Wales ..................38d 35m 30 s
> 
> Mine ........................38d 36m 45 s
> 
> It is impossible for me to say whether those made with or without the 
Telescope  are the nearest the truth, circumstances seem to be in favour of 
both; we certainly can observe with greater accuracy with the Telescope when 
the ship is sufficiently steady which however seldom happens so that most 
observations  at sea are made without..." (Of course, from what we now know 
of Kendall's watch, the telescope gives superior results.)
> 
> Aboard a ship similar in structure to the Resolution, the HMB Endeavour, in 
September I attempted sun shots using a x 6 monocular. The vessel was rolling 
heavily and and it took me a couple of minutes to get the sun down to the 
horizon, and to keep it in view without being pitched from my perch was very 
difficult. It was of course easier when the bark was steadier and when using 
a x 2 1/2 telescope on some one else's sextant. This so impressed me that I 
have now made a "special", a  2 x 40 telescope for use on my favourite 
sextant during my next voyage on the Endeavour Bark (mid-February, 2015).
> 
>  
> Bill Morris
> 
> Pukenui
> 
> New Zealand
> 
>  
>  
> 
> 

   

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