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    Re: Bubble sextant instructions
    From: JC Sutherland
    Date: 2001 Dec 12, 5:04 PM

    Cap'n Rodaway;
    I have used the Mk9 sextant for many years. In its
    unmodified form it is of
    little use at sea as it is generally impossible to
    keep the bubble steady
    enough, although I confess I have never tried it
    from the deck of a large ship.
    If the steadyness of the vertical is tolerable,
    then the vibration of the deck
    caused by the engines can be an advantage.
    These sextants are very old now and deterioration
    of the glass surface on the
    inside of the bubble chamber tends to make the
    bubble stick (especially if it
    is small) and the vibration helps in unsticking
    it. Adjusting the bubble size
    is a knack but it is surprising how many of these
    sextants have retained their
    liquid filling
    War surplus sextants were frequently modified by
    removing the bubble chamber
    and adding a prism to arrange a simultaneous view
    of the horizon. I have
    modified (one of the earlier ones that didnt have
    the averaging clock)  by
    drilling a hole through the front and replacing
    the 'horizon mirror' with a
    split mirror (to produce the familiar arrangment),
    while retaining the bubble
    for use on land. The biggest problem with this
    arrangement is that the glare
    filters are in the combined light path and only a
    strong horizon combined with
    a weak Sun gives ideal sights.
    The sextant Mk IX with the clockwork averaging is
    ideal for back yard
    navigation and accuraces of less than + or - 1
    minute can be achieved with
    practice. Without mechanical averaging it will be
    necessary to take up to a
    dozen sights and average the result by graphing
    the data.
    The small bulbs for illuminating  the bubble and
    the scales are very difficult
    to find and even seriously damaged sextants should
    be aquired if only to
    caniballise  these bulbs.
    Without them only Sun sights are possible
    The web site below is what you are looking forif
    it is still running. The
    reproduction is poor quality but if you have any
    trouble come back to me and I
    will photo copy my booklet for you.
    Have fun!
    Clive Sutherland
    Abingdon UK
    > http://www.physics.uq.oz.au/physics_museum/bblsex.html
    Cap'n MCL Rodaway wrote:
    > I was wondering if any one can point me to a web site or any other source
    > for a instructions on using a WWII RAF Bubble Sextant Mark IX A.  Also what
    > differences, if any, are there in working the sight with it, compared to a
    > marine sextant?
    > Yours Aye
    > Michael Rodaway, M.N.

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