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Richard Reed wrote-

"In 2005, George Huxtable found reference to a sextant by Jesse Ramsden,
around 1787.  That, as I understand, had a micrometer-like shoe that could
clamp at arbitrary positions and add or remove fractional amounts from
readings on the arc vernier."

Going by memory, that was an astronomical sextant, not a navigator's
instrument. That is, it was large instrument pivoted on a universal joint on
the ground, and had two movable telescopes, rather than a single telescope
with pivoted mirrors. The micrometer scale could read-off a small angular
difference, to the next division mark on the arc, so it relied on visual
assessment by eye (aided by magnifier) of the alignment with that mark. In
that respect it was really no more precise than a Vernier scale would have
been. Where it could score was in assessing small star-to star angles, with
respect to a reference star, if that angle was within the small range  of
the micrometer. That was the sort of sky-survey work that such astronomical
sextants were often used for.

I wanted to correct what I saw as one of the few errors in Peter Ifland's
"Taking the Stars", when he wrote, on page 63-

"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."

Such mis-statements tend to take up a life of their own, and I have seen
that one quoted elsewhere.

However useful such a land-based  micrometer might have been in measuring
small angles between stars, that's never a requirement for a sea instrument,
which always needs to measure a full angle from a self-corrected zero
position. At sea, then, the micrometer rack has to extend over the whole arc
of the instrument, a feature which was not implemented until Plath in 1907,
and (surprisingly) not in London instruments until 20 years later.

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.

   

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