NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Old Sextant pictures
From: George Huxtable
Date: 2008 Jul 22, 20:34 +0100
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 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. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---