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    Re: The development of bubble sextants
    From: Gary LaPook
    Date: 2009 Aug 18, 15:12 +0200

    In America there used to be a common expression (but I haven't heard it
    used lately) describing something as a "Rube Goldberg Machine." This
    came from cartoons by Rube Goldberg of incredibly complex ways to
    accomplish something simple. Interestingly, Rube Goldberg was mainly a
    political cartoonist.
    See cartoon at : http://dashboards.tv/images/rube-goldberg.jpg
    Also see: article at:  http://en.wikipedia.org/wiki/Rube_Goldberg_machine
    George Huxtable wrote:
    > Hanno included 3 attachments with his last message, but without accompanying
    > text.
    > First, as a general request, can I ask that when contributors provide
    > pointers or links for us to look at, a few words are added to the email text
    > to say what's likely to be found there, so only those with an interest will
    > be bothered to look.
    > In my case, Hanno's attachments were rather rewarding. I found three
    > diagrams from patent no 1912358 of 1929, by Vannevar Bush, a name that
    > Americans of a certain age may well recognise. It can be readily located,
    > from that number, within Google Patents. The patent referred to the use of a
    > mirror as an artificial horizon, and a proposal for stabilising it against
    > accelerations. It included several pages of text, omitted by Hanno, without
    > which those diagrams would make little sense.
    > An amazing proposal, which I greatly doubt could ever have been constructed.
    > It shows a horizontal mirror suspended on springs within a liquid-filled
    > transparent sphere, that floated on a bath of mercury within another
    > transparent sphere. Clearly, the author relished complication. It put me in
    > mind of those Eastern cosmologies in which the Earth was held by a comely
    > maiden, who stood on the back of an elephant, which was then supported by
    > the shell of a giant turtle, which swam in the ocean, and so on, combined
    > with the concentric crystal spheres that carry the planets in the Ptolemaic
    > universe. All with the idea of decoupling the mirror from the accelerations
    > that it was being subjected to.
    > Not for the faint-hearted to attempt constructing, then. It seems to have
    > got no further, which is unsurprising.
    > George.
    > contact George Huxtable, at  george@hux.me.uk
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > ----- Original Message -----
    > From: "Hanno Ix" 
    > To: 
    > Sent: Tuesday, August 18, 2009 4:57 AM
    > Subject: [NavList 9559] Re: The development of bubble sextants
    > Gentlemen:
    > Please refer to:
    > US Patent 1,912,358
    > V. Bush: Apparatus for establishing an artificial datum
    > Filed April 8, 1928
    > The inventor means an artificial horizon. Please refer to the attachments
    > for the drawings.
    > He has implemented a mechanical lowpass filter. We have discussed LP's
    > before. Better yet: This filter simultaneously works in 2 coordinates, roll
    > and pitch.
    > He also points out that the natural oscillating frequency of the mirror
    > should be as "low as possible" but he doesn't give any data. Since the roll
    > frequency of ships is rather low to begin with (judging from you-tube:
    > about. 0.1 Hz  = 1 period per 10 sec for an oil-tanker ) the filter has to
    > have a cut-off frequency of about 0.01Hz which amounts to 1 period per 100
    > sec. The filter also needs to be sufficiently damped. The reason is
    >  simply avoiding resonant oscillations of the filter/mirror in response to
    > the ship's movements. To build an LP of this kind is the challenge!
    > The inventor has made every effort to decouple the housing of the mirror
    > from the body of the sextant. So to speak, he created something like a
    > bubble level floating within bubble level.
    > With a lowpass of this kind, influences from pitch/roll would be reduced by
    > a factor of 100, possibly more. So, a 10 degree roll would create 0.1 degree
    > (6 arc-min) deflection of the mirror. Smaller ship will have higher
    > roll/pitch frequencies than oil-tankers, maybe 0.5 Hz. Accordingly, on
    > smaller ships 1 arc-min oscillation of the mirror might be possible. Is that
    > error sufficiently low given the circumstances? It is certainly much less
    > than I saw once in a bubble sextant.
    > Perhaps most importantly, this patent points out how
    >  to separate accelerations of the sextant which are instantaneous from
    > gravitation which is constant in time.
    > When I made my proposal with the tubular ring the other day I had similar
    > ideas in mind, however I had not fully understood the interaction between
    > bubble and spirit.
    > Regards
    > >
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