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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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