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I wrote:

> Do you have a link for the Sextants mailing list?

Disregard -- I finally located (and joined) the Yahoo Group.

--
GregR


--- "Greg R."  wrote:

> 
> George wrote:
> 
> > I had asked, on three mailing lists (NavList, Sextants, and rete)
> 
> Do you have a link for the Sextants mailing list? That one sounds
> interesting, but couldn't find anything on a Google search.
> 
> And I assume that "rete" refers to the Museum of the History of
> Science list
> about the history of scientific instruments:
> (http://www.mhs.ox.ac.uk/rete/index.htm?text)?
> 
> --
> GregR
> 
> 
> ----- Original Message ----- 
> From: "George Huxtable" 
> To: 
> Sent: Friday, October 13, 2006 2:41 PM
> Subject: [NavList 1411] Re: Book about Plath.
> 
> 
> 
> I had asked, on three mailing lists (NavList, Sextants, and rete)
> about the earliest introduction of the true micrometer to a sextant,
> and have had a number of useful responses, so will post this message
> to all those lists for anyone interested, with thanks to all
> respondents. It surprises me, somewhat, that such an important
> innovation has not been better documented.
> 
> Herbert Prinz, on NavList, and Richard Dunn, on rete, have pointed to
> the lack of such technical information about instruments in the 1987
> Jerchow work that I asked about. Herbert has suggested an earlier
> Plath anniversary publication from 1962, by Schaafhausen, Hoffmann
> and
> Kaltenbach. This has sent Wolfgang Koeberer to his extensive
> bookshelves, and I extract the following from his useful reply, sent
> off-list-
> 
> ==================
> "The 1962 book about Plath contains a part about the history of
> sextants by
> Heinrich Hoffmann and Peter Kaltenbach "Zur Geschichte des
> Sextanten".
> There - on p. 107 - they say that C. Plath developed and first
> offered
> for
> sale in 1908 a "Trommel-Sextant".
> 
> My search for a patent for that instrument had no results. The
> earliest
> Plath patent with respect to a micrometer sextant I found dates from
> 1922... It claims the patent for a sextant
> combining a micrometer drum and a vernier scale - just like the late
> 1920s
> Heath instrument shown on p. 63 in Peter Ifland's superb book.
> 
> A quick search of my - not complete - collection of German navigation
> manuals and handbooks of that period shows that the "Trommel-Sextant"
> was
> first treated in the "Lehrbuch f�r den Unterricht in der Navigation
> an
> der
> Kaiserlichen Marineschule" published by the "Reichsmarineamt" (Berlin
> 1917).
> The interesting thing there is that it shows the same illustrations
> as
> Cotter in his "History of the Navigator's Sextant", p. 164f.; It is
> pretty
> obvious that he has taken them from that book. From the illustration
> you can
> take another lead: it says "DRGM 274505" on the sextant, meaning
> "Deutsches
> Reichsgebrauchsmuster 274505" which was a kind of a lesser patent
> protecting
> your invention just for a few years. I have not been able to find out
> more
> about this, but I can imagine that you could ask for more information
> at the
> "Deutsches Museum" in M�nchen."
> ======================
> 
> So that seems firm evidence, then, that the first Plath
> "Trommel-sextant", or drum-sextant, dates back to 1908. Unless Heath,
> or another maker, got there earlier, that's the date of the
> introduction of the true micrometer sextant. And it seems unlikely
> that anyone did, because, (as Nicolas de Hilster has pointed out on
> the rete list), Peter Ifland, in "Taking the Stars", states-
> 
> ======================
>  "(on page 62) it was Hezzanith (the trade name for
> Heath, London) in 1909 who '...patented an improved system for making
> fine adjustments to the index arm [...]. A helical gear was cut in
> the
> back of the arc of the frame. A worm screw fastened to the index arm
> fit
> into the helical gear so that turning the screw advanced the index
> arm
> slowly along the main scale. A spring loaded lever (Hezzanith
> Patented
> Quick-Release) disengaged the worm screw from the helix gear so that
> the
> index arm swung freely to make gross adjustments'.
> 
> The text is accompanied with a picture of a 1920 Heath sextant that
> shows this mechanism, but has a plain drum, not an engraved/divided
> one.
> The '...fine adjustment screw moves the index arm by 5 seconds of
> arc'
> he writes. You can find a picture here:
>
http://research.microsoft.com/~gbell/cybermuseum_files/bell_artifacts_files/images/B69.80_Sextant.JPG
> "
> ====================
> 
> That Heath improvement related to the endless tangent screw, for fine
> adjustment, as I understand it, and not to a calibrated drum. Ifland
> shows, on page 63, a  calibrated drum on a Heath sextant, which he
> dates to the "late 1920s".
> 
> So it seems that credit for the introduction of the micrometer
> sextant
> must go to Plath in 1908, with Heath following somewhat later; exact
> date still undetermined.
> 
> ==========================
> 
> A number of respondents, on-list and off-list, have pointed to the
> instrument referred to by Ifland, on page 63, in these terms-
> 
> "The earliest known application of the drum micrometer to navigation
> instruments is found on a sextant by Jesse Ramsden, ca. 1787, now in
> the National Maritime museum at Greenwich." A description has been
> published by Alan Stimson.
> 
> But we have to distinguish here between different types of
> micrometer.
> For astronomers, particularly when an instrument was fixed to an
> equatorial mounting, a micrometer readout for small changes of
> reading
> was indeed useful, for measuring the offset between a star being
> studied and a reference star. For such purposes, the micrometer
> doesn't need to cover a wide range of angles. It seems to me that the
> micrometer fitted to the Ramsden instrument was of that type, a short
> screwed strut providing an adjustable chord, for which, no doubt,
> some
> small  numerical adjustment was needed to conform to a true angle.
> That could, indeed, be used for precise measurement of small angular
> gaps between two sky objects, and also for interpolating between
> whole-degree marks on the scale, when used over a wider angular
> range.
> But in that latter case, the precision was determined, not just by
> any
> error in the micrometer interpolation, but by an additional error in
> the visual  alignment to the nearest  degree mark, which would
> presumably be comparable with the errors involved in reading a
> Vernier. So, for wide angular differences, such a micrometer would
> provide little, if any, advantage in precision over the Vernier,
> although it might well avoid the need for such fine scale divisions
> around the arc.
> 
> For a mariner, that type of limited-angle micrometer adjustment seems
> to be of little use. All his measurements are wide-angle ones,
> altitudes measured up from the horizon, lunar distances that normally
> exceeded 20 degrees. As I see it, only when a micrometer screw could
> work against a complete rack, cut precisely over the full arc, would
> its high precision be of use to the mariner. I would be interested to
> discover if others have differing views on that matter.
> 
> For those reasons, I have discounted the Ramsden 1787 instrument as a
> contender for the first true micrometer sextant, at least where
> marine
> applications are concerned.
> 
> George Huxtable.
> 
> contact George Huxtable at george@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> 
> 
> 
> 
> 
> 
> > 
> 


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