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Thanks to Richard Reed for pointing to this Ramsden sextant from c.1783, on
the website of the National Maritime Museum-
http://www.nmm.ac.uk/collections/explore/object.cfm?ID=NAV1103&picture=1#content

That causes me to revise my opinions, and my words. I had written, on 12
March, "Going by memory, that was an astronomical sextant, not a navigator's
instrument.". That was clearly wrong. There was, as that link shows, at
least one such mariners' sextant, and what's more, one still in existence.
Which bears out what Peter Ifland had claimed (and I had questioned), in
"Taking the Stars", that "The earliest known application of the drum
micrometer to navigation instruments is found on a sextant by Jesse Ramsden,
ca. 1787, now in the Maritime Museum at Greenwich.", even if there may be a
few years' discrepancy between dates.

But let nobody think that this was a "micrometer sextant", that worked in
any way comparable to the 20th century instrument, and possessing any of its
advantages. As I explained, in relation to the astronomer's sextant, it was
no more than a way of making a calibrated interpolation to the next
whole-degree engraved mark, and relied, for that purpose, on aligning two
engraved marks by eye (with a magnifier), a similar process to that of
aligning the lines of a Vernier, and just as dependent on the acuity of the
observer's eye. So taking an observation was a two-stage process: when the
objects had been aligned, by moving the drum (say) clockwise, the drum
reading would be taken, but the main arc didn't then need to be read. Then
the drum would be moved on further (still clockwise) , counting whole turns
if necessary, until the index-mark reached the next major division, engraved
on the main arc. Then the drum angle would be read a second time, and the
difference converted to an angle (the pitch of the thread may or may not
have been chosen to make that arithmetic simple), and subtracted from the
whole-degree reading. Anticlockwise motion could have been used just as well
instead, but whichever diection, it had to be kept to throughout, to
minimise backlash.

Of course, the uncertainties of checking, or zeroing, an index-error, at the
zero end of the scale, had to be added in as well. Overall, a micrometer of
that type provided no advantage over a Vernier to a navigator, just
complicated the reading process. Which was, no doubt, why it was quickly
dropped.

Such a micrometer arrangement could be really useful to an astronomer who
was measuring small angles between stars, such as with double-stars, if the
pair lay within rabge of the micrometer adjustment. But there's no
navigational use for such observations.

By its nature, such a "micrometer drum" could only cover a very limited
angular range, because it was a straight screw-strut controlling motion
around an arc. So the actual angle moved through depended on the arc-tan of
a screw-length factor, and wasn't linear. Such devices were used in precise
astronomical instruments, such as transits, and presumably there were
correction tables provided, to allow for that non-linearity.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Richard Reed" 
To: 
Sent: Saturday, March 13, 2010 1:45 AM
Subject: [NavList] Re: Chasing tenths of an arc minute


"The screw adjuster 'micrometer' is an interesting idea, but I can't see how
it achieves better accuracy."

I think I can answer my own 'can't see'.  Here is the URL of the 1783
Ramsden sextant 'with vernier and micrometer'


Unfortunately, the vernier seems to be missing in the image, but there are
two similar pieces on the site that have them above the scale in the arm's
window.  The other pieces don't have 'micrometers'.

The text says that the vernier goes down to 30 arc seconds and the
micrometer goes down to 10 arc seconds.  I think that the touch is made, and
if the vernier isn't dead on a mark, the micrometer moves the arm until it
is.  The amount of a turn to make the vernier touch a mark is added or
subtracted.

Later, it appears that Ramsden took a different approach.  Other Ramsden
sextants in the collection have no micrometer and longer or finer verniers,
one having a magnifier for a vernier down to 5 arc seconds.

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