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Jeremy presented us with some interesting sextant pictures, "for the list to 
look at and comment", and I have taken up that invitation.

I have looked into that dealer's website, at (www.greatsouthbay.com) , but 
have failed to find my way to where Jeremy tells us that there are further 
details and prices to be found. A bit of more detailed guidance would be 
appreciated.

He wrote- "I was interested in seeing that the sextants didn't  have 
micrometer drums but rather magnifiers to read the tiny engraved  markings 
on the arcs." These are "Vernier" sextants (or "nonius" in some European 
languages). There are two scales, one on the main arc, the other on a short 
auxiliary arc that differs from the main scale, slightly, in the pitch of 
its graduations. What you have to look for is where the rulings between the 
two scales align. This provides a precise way of interpolating between 
divisions on the main scale. Plath in Germany started to supercede these 
instruments with micrometers in 1907, but micrometers didn't appear in 
English sextants until the late 1920's.

He mentioned "one quintant (175 deg total)", and  "I see that he  lists his 
quintant as a "pentant"." These a valid alternative names for the same 
thing; an instrument with an arc-length of a fifth of a circle, that can 
measure to 144�. However, I have considerable doubts about the "175 deg 
total". No reflecting instrument ever reached nearly such a large angle (in 
a single range) that I know of. Indeed, because a Vernier instrument always 
must have an added range on the main arc, to allow for the necessary overlap 
of the Vernier scale (commonly by 20 �), the working range as always 
significantly less than the calibrated arc-length. But usually, even before 
that point has been reached, the view in the index mirror, because of its 
tilt, has shrunk to a narrow letter-box slot, making the instrument 
unusable, or nearly so.

Looking at Jeremy's picture of that quintant, the angle of the 
thread-housing for the telescope mount looks a bit odd to me. Is it just a 
trick of the angling of his picture, perhaps? But it really doesn't look to 
me as if the telescope, when assembled into that holder, is going to point 
toward the horizon mirror, as it should.

The ebony octant looks to be in nice condition, as far as one could tell 
from the photo, with all of its joints seemingly tight, and its ivory scale 
uncracked. I wonder, though, whether the box it's in is original, or a 
modern construction.

That collection of instruments illustrates nicely what we have been 
discussing recently on this list; A wooden octant, intended for altitudes 
rather than lunars, so no special call for precision and stability: and 
brass sextants, stable and precise, and suitable for taking lunars, at 
angles over 90� when called for.

I appreciate that Jeremy was just taking a passing interest in these 
instruments, for our benefit, so don't expect him to be in a position to 
respond to any detailed scrutiny.

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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