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Fred Hebard said-

>There is a very nice Troughton circular sextant with pillar on ebay.
>It was just listed about 1 pm pst yesterday, and has gone from $100 to
>$1575 with 17 bids by 9:30 am pst today.
>
>There are numerous pictures.
>
> From what I can tell looking at the vernier, the instrument appears
>capable of measuring to 20 seconds of arc by direct reading.
>Interpolation could lower that to 10 or 5 I suppose.  The text says 5
>seconds.  From the text, it appears the author knows what he's talking
>about, so I suppose 5 seconds is about right.
>
>Fred

For a photo and description of a Troughton circle of about 1800, see
Cotter, "A history of nautical astronomy" fig. 8 and page 83. However,
Cotter doesn't show a pillar-mounting, which would add much interest and no
doubt increase the monetary value. I know little about valuing such
instruments, but my guess is that the price has some way to go yet. If it's
in good and complete condition, it would make a fine trophy for the
mantelpiece. I am no collector, but would covet such an instrument.

From Cotter I gather that Troughton circles normally carried three verniers
120 deg apart, and all three were averaged, which eliminated any centring
errors, and reduced some errors of scale division.

As I understand it, the Troughton was a simple circle, rather than the
Repeating Circle of Mayer and (particularly) Borda. The latter allowed a
number of separate readings of altitude to be quickly repeated and
automatically summed. The advantage of the Borda arrangement was this: each
repeated altitude measurement was made over a different part of the
circular arc. No matter how unevenly that arc was graduated, once turn
round the arc simply had to add up to 360 degrees. As long as enough
readings were taken and the sum averaged, any unevenness in the division of
the scale would average out. This system was popular in instruments from
continental Europe, in which scale divisions were much less precise than
English makers could achieve by engine-division.

The better English instruments could obtain sufficient accuracy in a simple
sextant, which was of course smaller and lighter than a circle. It seems
unlikely that the for-sale instrument is a "repeating" circle, but it might
be worth checking.

This pillar-sextant may have been intended for use by a surveyor to
determine longitude from lunar distances, having landed on a coast, in
which case the pillar would come in useful. At sea, a mounting-pillar would
only be an encumbrance and presumably it could then be removed. It would be
interesting to learn if the axis of rotation at the top of the pillar could
be tilted to point along the Earth's polar axis, useable over a wide range
of latitudes like some polar-mounted telescopes. Presumably, then, the
"sextant" part could be offset from that axis by adjustable angles in two
planes, so that the instrument could keep in view the Moon and the
other-body for a relaxed set of observations.

The ideal arrangement would be to find a location which provided a clear
view of sky and sea-horizon, in the general direction of the meridian, and
cut down a tree, leaving its stump at a suitable height as a firm base, to
mount on it the three adjustable feet of the pillar.

The main difficulty about using such an instrument is its size and weight,
making it difficult to hold up when observing at sea, and a hefty load to
carry on an inland expedition.

The instrument on sale may well have an interesting history behind it.

These observations are somewhat speculative, as I have not seen any
specification or pictures of the item on sale, only Fred's description.

It would be interesting to discover the final price that it's sold for. If
anyone finds out, please post it up.

George Huxtable.


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01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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