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    Re: The development of bubble sextants
    From: George Huxtable
    Date: 2009 Aug 18, 11:44 +0100

    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{at}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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