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    Re: Making an artificial horizon
    From: Gary LaPook
    Date: 2011 Jan 21, 00:58 -0800
    I have only 8 ounces of mercury and it makes a puddle about two and a half inches in diameter. I placed a cardboard stop over it which cuts the viewing area to two and a quarter inches. Due to the small size the field of view is narrow and it helps to be close to the bowl, say about a meter, and it is good to have something to rest the back of your head against to keep it in the correct position.

    You can make a bigger enclosure for an artificial horizon which I was planning to do to make it more convenient to take sun sights with a bowl of water. I bought two larger pieces of glass, 8 by 10, for about eight dollars each and may make a bigger one in the future.

    gl

    --- On Thu, 1/20/11, George Huxtable <george{at}hux.me.uk> wrote:

    From: George Huxtable <george{at}hux.me.uk>
    Subject: [NavList] Re: Making an artificial horizon
    To: NavList@fer3.com
    Date: Thursday, January 20, 2011, 2:13 PM


    Patrick wrote-

    | But I agree: even if we blink the concerns about safety, Hg is too
    | expensive.  I am considering cutting a small piece of optical grade front
    | surface mirror to a size that would fit without constriction into the
    Davis
    | artificial horizon, backing it with a thin sheet of cork or other buoyant
    | material, and then putting it afloat in the artifical horizon.  Can
    anyone
    | see a problem with this idea?
    |

    Mercury is VERY dense (over 13) so an ounce of the stuff won't go far;
    occupying about 2 millilitres. My guess is that  around 10-15 ml would be
    required in the trough of a sensibly-sized art. horizon, to make it easy to
    use without having to be over-careful about levelling. That would
    correspond to 5 to 8 ounces. Maybe it would be possible to penny-pinch and
    get way with somewhat less.

    If the US allows trading in Mercury, its citizens are fortunate. That's
    illegal in Europe.

    Patrick's proposal for a floating mirror on water looks plausible in
    theory, but I doubt if it could be made to work to sufficient precision in
    practice. Any such float-raft would need to be made with great uniformity
    to achieve a level to half an arc-minute, necessary to provide answers to
    the nearest minute. It would call for symmetry to one part in 7,000, or so,
    across any diameter. Cork is not a suitably uniform material; what
    alternatives exist?

    Perhaps a cut-and-try procedure could be adopted, after measuring for any
    tilt, with a tiny blob of movable ballast to be attached at a determined
    spot.

    How can the raft be prevented from drifting to one side, and trying to
    climb up the meniscus at the edge of the trough? Perhaps by tethering it
    near the centre, with limp threads, such as the finest silk. Which would
    introduce another set of problems.

    And then there are the surface tension forces around the edge of the raft.
    How can the surface condition be controlled to ensure that they are kept
    exactly in balance?

    Finally, how is steaming-up of the windows to be avoided?

    All practical problems, which need to be overcome.

    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.




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