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Jim Martin wrote-

| Sirs,
|      I have recently purchased an Heath&Co Vernier sextant?.The
| Vernier has the usual 10' divisions,each with five
12"subdivisions.My
| problem is, that there are three 12"divisions to the left of 10',
and
| three to right of 0'.What is the purpose for this?.Incidentally the
| main arc reads to 150deg.,hence my query against the use of
sextant.The
| certificate is dated 1940

That method of division may be common, but not universal, even among
Heath Vernier instruments. An alternative arrangement was to have six
subdivisions to each minute on the Vernier, so the Vernier reads 10'
along its length, divided to sixths of a minute, or 10". The main
scale is divided, correspondingly, to sixths of a degree. The Vernier
spans across a 20-degree arc on the main scale; perhaps a bit less on
yours, because it's dividing into only 50 parts, not 60. I think the
"5ths of a minute" method of division that you have came in when
almanacs started to give angles to decimal parts of a minute, rather
than in seconds. Then, it became awkward to adapt the reading from a
"6ths of a minute" instrument, when calculating.

Because zero on the Vernier is at its left-hand end, the main scale
has to be extended (by at least 20 degrees in the case of my sextant,
maybe by a bit less than that in yours) beyond the maximim measurable
angle, to provide something to read the Vernier against. That explains
why the scale goes to 150 degrees. You will find that it really is a
sextant, not an quintant; there may be a few degrees extra, above 120,
but not far above that the index arm will collide with the horizon
mirror mounting.

I suggest that the extra few divisions on the Vernier at each end may
be to cope with the situation when measuring an angle that's nearly
exactly on some multiple of 10', so very near one end of the Vernier
or the other. If you thought it came near the top end of the Vernier,
and had moved the magnifier there and got down to reading it, it would
be a real nuisance to then find that the angle was just above the zero
and had to shift to reading the other end. Those extra few divisions
allow a bit of leeway in the overlap, but you have to be careful, off
the end of the range of the Vernier, to record the correct main-scale
reading that corresponds.

George.

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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