A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2021 Jan 7, 14:47 -0800
I had an inquiry a couple of days ago about "bridge sextants" which are listed in the collection at the NMM in the UK (see the first five hits from this search: NMM collections bridge sextant). Bridge sextant?? The name was new to me. I looked at the examples on the NMM website, and they seemed to have some common structural elements. This was my reply :
Those "bridge sextants" appear to be a class of "pillar sextant" with protective structure around the bits that stick out from the frame, including the mirrors and scope. The whole idea of a pillar sextant was to make an instrument out of relatively flimsy plates of metal which were secured by those rods or "pillars". Strong and light. It seems reasonable that some makers thought they needed yet more structure. Hence the boxy protective frame. Not a bad idea, but clearly not popular either. They didn't sell. Of course, museums are often graveyards of bad ideas! :) --especially when they look like fine art. :)
I believe that when these sextants are held and examined and seen in "three-quarter view", the frame would bear a rather strong resemblance to a little "truss bridge" (an actual roadway bridge, that is) attached to the frame. But this type of bridge is anachronistic for the sextants on the NMM website which are from c.1790s when pillar sextants were popular (especially from Troughton, thanks to patent protection). Iron truss bridges date from the mid-19th century at the earliest (I think). Since the name "bridge sextant" is completely alien to me, and since I can find no historical references to "bridge sextant" via hathitrust.com book search, apart from typos and accidents, I suspect that this is a name applied by the collection curators at some point in the past 150 years or so. And it stuck, locally, at NMM.
These are simply my educated guesses on this type of sextant. In any event, they strike me as minor variants on standard pillar sextants.
One clarification: these sextants with the bridge-like protective framework are not all pillar sextants, but the structure may make more sense with pillar sextants. At least that's how it seems to me. Anyone have any other information on "bridge sextants"?