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    Re: Arificial Horizons and Tea
    From: George Huxtable
    Date: 2003 Jul 11, 20:05 +0100

    About Mercury artificial horizons, John Kabel wrote-
    >Mercury!! Were they kidding??  Bad enough in a barometer or
    >thermometer, where the glass is about as hazardous, but very bad in the
    >open.  I hope nobody is still using it, in this group.  A license to
    >die young.
    Can anyone supply any information as to the real level of hazard that would
    be involved in using mercury in an artificial horizon outdoors?
    I suspect that it may be less lethal than John Kabel implies.
    I base this opinion on my childhood at school, where I sat for physics
    classes at a laboratory bench. Every hollow and cranny in the wooden
    surface of that bench would contain droplets of mercury. When the lesson
    became boring, we would do our best to fish them out with a pencil, to
    collect enough to coalesce into a blob that was big enough to flick at our
    friends in the row in front.
    Our reversing-switches would involve electrodes that dipped into open pools
    of mercury, We would make barometers that dipped into similar open pools.
    Later, at university, we found a similar environment. We would use mercury
    in pint quantities for diffusion-pumps in high-vacuum systems.
    I suspect many physics students from my generation, the world over, could
    tell a similar story.
    Were we risking our lives? It didn't seems so at the time. I haven't
    collected mortality statistics. Were we risking brain damage? Perhaps
    that's starting to show, now...
    Now we have our teeth stuffed with amalgam. Does the amalgamation remove
    the toxicity? I wonder.
    Somehow I doubt that taking sextant altitudes by exposing for a short time
    a small pool of mercury, out in the open, where any vapour could blow away,
    presents a comparable hazard. But I am quite prepared to be convinced
    otherwise, if any real evidence exists. Is John Kabel being over-alarmist
    about the danger? Am I being over-casual?
    In other respects, mercury must be the ideal artificial horizon. There's
    room for a simple design in plastic for a shallow pool with a sealing lid,
    from which the mercury would never need to be removed, with a rim intended
    to trap any overspill, designed with a tripod base. This would be handier
    than the equipment the old explorers had to handle, in which mercury had to
    be transferred between a tray and an iron storage vessel with a screw-cap.
    But how available is mercury today? Are there regulations that prevent you
    and me from acquiring enough mercury to do the job? Do we have to collect
    antique barometers to do so?
    I'm not convinced about the virtues of floating a solid mirror on a
    disc-raft on liquid. The liquid and the solid would need to have a
    repulsive surface tension between them to ensure blobs wouldn't gather up
    the sides of the raft. That surface tension would require to be exactly
    even around the edges of the disc or the raft would be unbalanced. How
    would one prevent the raft from nearing the edges of the container, which
    would unbalance the surface-tension forces or give rise to friction which
    would constrain the self-levelling? There are serious problems here which
    would need resolving.
    George Huxtable.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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