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    Re: The shipwreck of Admiral Shovell
    From: George Huxtable
    Date: 2007 Sep 9, 22:50 +0100

    Coralline Algae wrote-
    
    | Let me start by saying that I am not knowledgeable about the details of
    the
    | historical incident, except that I seem to remember that the accident
    | prompted the Admiralty to give much more attention to improving
    | both charts and the practice of navigation.   I believe their efforts were
    | successfull in addressing both concerns.
    
    Well, knowledgeable or not, Coralline Algae has most things right.
    
    But it was a long time before the Admiralty did much about the charts,
    though they would be best placed to improve them. Chart production was in
    the hands of private enterprise, and some hopelessly erroneous stuff was on
    sale, right through the 18th century.
    |
    | After reading some of the posts in this thread, it seems that the state
    | of the art in angle measurement was either crossstaff or backstaff.
    
    Yes, that's right, for angles measured at sea. Good to perhaps 10 minutes,
    in the right hands, and in suitable weather.
    
    | I am not familiar with either instrument but accounting for the
    | capabilities of the tools what errors were likely in measuring
    declination?
    | It seems that there were tables of declination of good accuracy at
    | that time.
    
    Declination of the Sun was measured, rather precisely, by astronomers on
    land, to a fraction of an arc-minute, mostly by some sort of fixed quadrant.
    Precise tables of declination had been produced by the Greek Ptolemy (and
    earlier, Hipparchus), then refined and built on by Muslim astronomers, but
    precise observations were not made in Europe until Tycho, just before 1600.
    
    A useful publication in English was William Bourne's almanac with a
    four-year cycle in "A Regiment for the Sea" published in different versions
    in the 1570's, and giving daily Sun declinations to the nearest minute, with
    reasonable accuracy.
    
    So I would generalise by saying that Sun declinations have over the years
    been known, more accurately than mariners could use with the instruments at
    their disposal. Whether that information was available, on board, was
    another matter.
    
     | Also in the thread it seems that the charts of the time placed the
    | scilly islands further north by 10 miles than was the case.   I am
    | wondering though, when the charts in use at that time were actually
    | produced; as it is not unusual today to use charts with data over
    | 50 years old ( particularly soundings).   Was it possible that more
    | accurate information could have been available to the fleet had
    | there been a system like todays notice to mariners?
    
    Yes, it certainly could, if there had. Once a chart had been engraved, that
    plate would be used for many years. Mount & Page, who had a near-monopoly of
    charts, had engraved plates of the English Channel before Halley's time,
    based on survey work by Greenville Collins in the 1680s. This showed the
    Scillies (and Lizard) nearly 10 miles North of their true position; a
    wickedly dangerous state of affairs. Halley had issued a pamphlet warning
    mariners of those dangers, in 1701. Nevertheless, when data from Halley's
    1701 tidal observations was placed on a chart, which became known as
    Halley's Chart, it was superimposed on that same erroneous plate; that same
    chart, with the same errors, was reprinted by Mount & Page, right up to
    1753. I have mentioned these matters before, but they bear retelling,
    because they show how bad things had got, even in home waters. It shows a
    dangerous level of complacency, a couldn't-care-less attitude, on the part
    of chart suppliers.
    
    Yes, the Admiralty were in a position to do something about it. They could
    have despatched observers to the important headlands, to establish precise
    latitudes at least. Longitudes were a much more difficult matter, and charts
    usually lacked a scale of longitude until well into the 1700s. But they
    didn't appoint an official Surveyor until the time of the two Mackenzies, in
    the 1770s. (Collins had been appointed directly by the King, not the
    Admiralty). And there wasn't an official Hydrography Department until near
    the end of the 18th century.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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