A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Henry Halboth
Date: 2014 Nov 19, 14:28 -0500
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
<<I would be interested in hearing for what the wires in the inverting telescope eyepieces were used.>>
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).