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    Re: Old Sextant pictures
    From: George Huxtable
    Date: 2008 Jul 22, 20:34 +0100

    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 
    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.
    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. 
    Navigation List archive: www.fer3.com/arc
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