A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robert Eno
Date: 2015 Feb 5, 10:10 -0700
Good Morning Bill, List
Thanks for that clarification on the term “firefly” and “dragonfly” to describe the old C.Plath (SOLD pattern) bubble attachment. In my own defence, I only heard that term for the first time about one year ago on this forum and without really questioning it, adopted the term as everyone else appears to have been using it.
On a related note, I had a look at your instructions for re-filling the unit and while I do not disagree with soundness of your method, I have found an easier way to do this via the air chamber. I have successfully re-filled several of these instruments – my own and others’- quite a few times using my own method. The reason why I prefer doing it through the air chamber is because it obviates the need to muck around with the delicate concave glass lens and also the risk of cracking it as one tightens down the retaining ring. Me personally, I have a bad habit of over-tightening things. I have only had to open up that chamber a few times and that was on old instruments to clean out debris (the paint on the reticule tends to flake off on older instruments). Otherwise I avoid opening this up if I can. By the way, I have taken careful measurements of the concave lens on the top of the bubble chamber so if anyone out there has inadvertently broken theirs, I can provide the measurements that would allow an optician to produce a replacement. I use hexane to refill my bubble attachments because it will not freeze or become sluggish in sub-zero temperatures which is an important consideration for my location.
Re-filling through the air chamber is somewhat finicky and you have to play around with the adjustment knob to get the right amount of fluid into the unit but with a bit of practice, it is a breeze. Regrettably, I have never taken the trouble to write down the step by step instructions. Next time I refill one of these I will do so.
With respect to accuracy, I agree with you that they are of dubious value at sea. Trying to get that bubble to sit still, even in a gentle sea, is like running behind a moving vehicle and trying to thread a needle attached to the bumper. I have, nevertheless been somewhat successful in taking observations in a flat calm sea while in a protected harbour. On land, I have been able to consistently nail down my position to within less than one nautical mile. I could not do this today as it has been a while since I have used my sextant.
I also agree with you on the gold standard of bubble sextants being the RAE Mk IX. In my opinion, this is THE best bubble sextant ever made. I have always wanted to take the bubble unit out of a MK IX and modify it to attach to a marine sextant but I have neither the skill to fabricate such an attachment, nor the desire to cannibalize any of the MK IX’s in my collection.
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Bill Morris
Sent: February-04-15 7:22 PM
Subject: [NavList] Re: Plath bubble sextant on ebay (C&P versus Plath)
I don't wish to disappoint Paul...
I have described the C Plath bubble horizon unit here: http://sextantbook.com/2012/06/ and a version of the SOLD bubble sextant, which used practically the same bubble unit here: http://sextantbook.com/2013/11/. I have also written a blog post on how to refill the Plath bubble horizon unit here: http://sextantbook.com/2014/09/.
Just by the way, members have written of the "Firefly" and of the Dragonfly" unit, presumably because some German-English dictionaries give "dragonfly" as the translation of "Libelle". However, its meaning in technical German is of course "spirit level" and this is what Google translate now gives as the meaning of "libelle", so maybe we should simply describe it as I have above, as a bubble horizon unit. Before anyone attempts to engage me in a discussion of the German language, let me add that I was, probably correctly, not considered clever enough at high school to learn German and I have only a rudimentary, self-taught, working knowledge of some aspects of the technical language.
There are both naval and air-force versions of the SOLD and it seems that in the 60s German naval units were issued with bubble units to go with their C Plath marine sextants. This seems a bit odd, as it is generally agreed that bubble sextants are practically useless at sea, but it may be that in arctic waters with obsured horizon or unpredictable dip the unit could have been of some use. A former submarine commander of my acquaintance described the bubble sextant that could be attached to a periscope as "useless". On war patrol in WWII, the SOLD sextant with its integrating device may have given results in darkness that were better than none at all. On land, I found it at least as good as my gold standard, the British Mark IX BM, though less easy to use, both giving position lines correct to within 2 miles when used with their averaging/integrating devices over 2 minutes.
Using a bronze-framed C Plath sextant of roughly the same vintage as the attached original C Plath bubble unit today, a single sun shot gave a position line 14 miles in error. I am sure I could improve on this with time and dedication that I do not have at the moment. With bubble unit attached, the sextant weighs 2.3 kg (about 5.6 lb), so the combination is no lightweight.
As for the A10-A adaptations, details can be found here: http://sextantbook.com/?s=sextant+bubble+horizon. I doubt that their performance would be much different to the C Plath unit, provided the bubble is kept to the vertical midline. Plath have a square in the centre of the field of view and one is advised to keep the bubble there when making observations, but provided one does not stray too far from the vertical midline, tilt errors will be small. I would suggest that many shots should be taken and averaged, bearing in mind that 4 will halve random errors, 16 will reduce them to a quarter and you will need to take 25 to reduce them to a fifth.