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    Re: Plath bubble sextant on ebay (C&P versus Plath)
    From: Bill Morris
    Date: 2015 Feb 4, 18:16 -0800

    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.


    Bill Morris


    New Zealand



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