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Subject: Re: Bubble sextant instructions
From: J.Sutherland (jcs@XXX.XXX)
Date: Wed Dec 12 2001 - 19:04:28 EST
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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