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    Re: Sadler
    From: Nicol�s de Hilster
    Date: 2008 Dec 23, 15:53 +0100

    frankreed@HistoricalAtlas.net wrote:
    > Jeremy, you wrote:
    > "the effect of tides on dip seems very academic to me. "
    >
    > Oh HELL yeah! It's completely academic. But it's at least worth noting that 
    folks do calculate these sorts of issues so that we can sort out trivial 
    things from important things. And this tidal business is definitely 
    trivial...  Two centuries ago, people even tried to calculate the difference 
    in dip in a north-south direction relative to an east-west direction due to 
    the oblateness of the Earth. Can you imagine?? They had no sense of scale.
    >
    >   
    On board of a vessel the whole issue is indeed academic, but let us not 
    forget that Sadler's original idea was "...to observe the dip, 
    accurately by theodolite, from fixed locations on the seashore ... using 
    the height of the tide to give varying heights above sea-level.". Tidal 
    ranges of more than 10 metres are not uncommon and can be very useful in 
    such an experiment, as this will produce a difference in dip of more 
    than 3 arc minutes (assuming the observer stands at 3m above high water 
    and thus 13m above low water). Sadler only pointed out that it was 
    difficult to determine the relation between the tide measured near the 
    shore with the one measured at the horizon, an unknown perhaps not 
    measurable with a sextant, but surely with a theodolite.
    
    
    
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