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    Re: Bubble sextant intervention needed.
    From: Jared
    Date: 2012 Oct 19, 04:37 -0400

    Dear All,
    The A-10 and my A-10A bubble sextants' bubble-sizes are readily 
    adjustable, by their design and builds.
    Early on I got lucky and ordered from New Zealander Dr. Bill Morris 
    one of his series of simply outstanding bubble manuals, mine is 
    that for A-10 and A-10A. His tech writing is clear and economical 
    but leaves nothing out; his extensive color photos, just right in 
    number and coverage, give cripplingly clear familiarity with all 
    the body of these 70-year-old instruments, telling of their 
    strengths and few frailties, and how to use and fix the instruments.
    After reading a few early pages, it was clear I'd not be able to 
    make adult decisions how to select and use the instruments 'til I 
    read Bill's manual.
    Bill's URL is just below; You see the A-10s' and A-12 manuals there 
    along with those of several others.
    These bubble sextants got tens of thousands of planes on target on 
    time, then back to the radio ranges of home, with surely many 
    sights taking place in astral domes under tropic suns hot enough to 
    try turning the nav's brains into mayonnaise while warming the 
    black sextants pretty good.
    I ordered from the The Kiwi's (Bill's) webpage just below.
    Finally, I gotta say, my position-finding would still be stuck 
    using kemal, backstaff and GPS, without my A-10A and its fine manual.
    --Jared Sparks.
    On 10/19/2012 1:04 AM, Paul Dolkas wrote:
    > All-
    > I have a stupid question - or perhaps I just need somebody to stop me
    > from doing something stupid. I�ve been playing around with my A-12
    > bubble sextant for a few months now, and I�ve noticed an interesting
    > phenomena. The bubble cell, unlike some of the more expensive models,
    > has a fixed volume. So when it gets hot outside, the fluid inside the
    > cell expands and the bubble gets smaller. On at least one occasion, it
    > has disappeared entirely. Knowing the pressure that must exist inside
    > the cell to do this, this can�t be good. Luckily, I haven�t had it break
    > the cell window.
    > Which brings me to my point. I know some nautical sextants are painted
    > white to avoid getting hot and going out of alignment in the tropics.
    > However, I have yet to see any white aircraft sextants out there. Now,
    > this probably because models like the A-12 were made to be used in open
    > cockpit aircraft, where freezing was more of a concern than baking. But
    > for those of us who use them at sea level, wouldn�t it make sense to
    > paint them white - at least on the outsides?
    > So to check this out (and to embrace my inner science geek) I ran an
    > experiment where I took 2 aluminum blocks and painted one white & the
    > other black and left them out in the midday sun (solar elevation: 45o ).
    > Naturally the black one ran hotter, by about 20oF. I did some
    > calculations, and this modest temperature rise is enough to almost make
    > a small (1/16�) bubble disappear.
    > So my question is: aside from making what will probably be a truly
    > butt-ugly sextant, is there any reason I shouldn�t paint the outside of
    > the sextant white? (naturally, I would leave the inside black to
    > minimize stray light.) I kinda hate to go against tradition, but there
    > don�t seem to be any real downsides to doing this. Can anybody think of any?
    > -Paul
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