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    Re: Children's land-locked "Sextant"
    From: Mike L
    Date: 2007 Nov 29, 06:56 -0800

    Whilst you may be right, if you are then someone has invented a shiny
    looking liquid to use in the middle of thermometers that I bought from
    ebay a month or so ago!
    
    
    But back to the subject. The sun shon for a few minutes and so I had
    the opportunity to try out a few ideas:
    
    I've been impressed by some of the responses, particularly the one to
    use printed paper as this would save hours carefully marking out the
    scale.
    
    I'm less enthusistic about encouraging children to look through a
    device into the sun unless I'm certain the filters come from a very
    reputable source, which basically means a working Sextant.
    
    0. Plumb line
    
    I don't have good experience using a plumb line to measure altitude.
    My own experience of a simple device to measure slope angle was that
    the plumb line never stayed still even when the wind stopped blowing
    it around (it was a survey for a windfarm!) The problem is that they
    are natural pendulums and once they start swinging they never in
    theory come to a stop unless you physically constrain them, in which
    case you may well alter the reading!
    
    
    1. Using shadows.
    
    Although the shadow has a 30' shaded edge, if the distance is chosen
    correctly so that the object is slightly more than  30' when viewed
    from the plain on which the shadow falls, the middle has a fairly
    distinct black area which I guess is around 2-4' which should allow a
    1' measurement.
    
    2. Lenses.
    
    I tried some supermarket 1.25dioptre lenses and at around 80cm they
    project a very decent image of the sun without the paper going up in
    smoke. Given the sun is only around 10', I'd be interested if someone
    living nearer the sun could try this with a midday sun and see whether
    it is safe! Although the sun was around 8mm wide, I doubt it will be
    as good as a simple shadow as the image tends to flare if the axis is
    not perfectly in line with the sun so it is quite possible the reading
    will be off a few minutes.
    
    3. False Horizon - Double shadow.
    
    I'm toying with the idea of using both a direct path and reflecting
    the sun onto the "gnomen" (shadow making thing) in order to create two
    shadows on the imaging plain and then measuring the distance between.
    Whilst theis may improve the resolution and remove the need to
    calibrate the horizon, it does mean that I will be both be looking for
    a day with sunshine, AND a day without wind. So, I might incorporate
    this as a form of calibration rather than part of the measurement.
    
    
    
    
    On Nov 29, 4:17 am, Alexandre E Eremenko 
    wrote:
    > Mercury is not very dangerous
    > (if you don't drink it).
    > However it is extremelly harmful for the environment.
    > Even a small amount can poison all fish in a lake.
    > That's the main reason why its use is severely restricted.
    > For example, beginning January 2007 you cannot buy
    > a mercury barometer on e-bay. The last internet store
    > in the US that was selling them does not do it anymore
    > since the beginning of 2007.
    > Some states in the US prohibit selling buying and
    > manufacturing of mercury barometers and termometers.
    >
    > Alex
    >
    > On Wed, 28 Nov 2007, Geoffrey Kolbe wrote:
    >
    > > Gary LaPook wrote:-
    >
    > > >With use it develops a dross floating on the surface which can be
    > > >removed by filtering it through a piece of cloth like an old t-shirt.
    > > >You have to twist the cloth to force the mercury though the cloth and
    > > >it comes though in shiny little balls leaving the dross in the cloth
    > > >which I then dispose of. You should probibly wear gloves when handling
    > > >the mercury like this.
    >
    > > In the 1960's the old Royal College of Science in London was pulled down to
    > > build Imperial College, the British attempt to emulate MIT in the United
    > > States.
    >
    > > The Spectroscopy labs in the RCS had long been plagued with the problem
    > > that continuum spectra always had absorption lines of mercury on them. On
    > > taking up the parquet floor in the lab, a veritable lake of mercury was
    > > found underneath! As far as I know, all the researchers of that era lived
    > > to a ripe old age, despite working for many years in an environment where
    > > the mercury vapour in the air was probably at saturation point.
    >
    > > Not that I am advocating that we should not take suitable precautions when
    > > using poisonous substances like mercury - its just that people somehow seem
    > > to be more susceptible to such things than they used to be in times
    > > past.... or, at least, that is the perception.
    >
    > > Geoffrey Kolbe
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