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    Re: The shipwreck of Admiral Shovell
    From: George Huxtable
    Date: 2007 Aug 30, 10:26 +0100

    | Gary wrote:
    | >>Shovel's fleet also hove to "for to take soundings" so what went wrong
    | >>with the soundings? are they ambiguous in the vicinity of the
    | >>Scillies?|
    
    and Peter Fogg responded-
    
    | All of the English Channel is fairly shallow, so soundings may not have
    | given a clear indication of position. As recently as about 12,000 (?)
    years
    | ago the British Isles were part of the continent. At that time, so I've
    | heard, the Thames was a tributory of the Rhine.
    
    from George-
    
    I can detect no relevance in Peter Fogg's comment.
    
    It's only when waters are "fairly shallow" that soundings become possible at
    all. It's only when the shallowness is uniform, that soundings become
    uninformative. And that is far from the case, in the Western approaches to
    the English Channel, that concern us (and concerned Shovell).
    
    It's true that in quite recent geological time, the Dover strait was closed,
    and the waters of the Thames, Rhine, etc., had to pass Northward through
    what's now the North Sea, between Britain and Scandinavia (as, mostly, they
    still do). Did that make the Thames "a tributary of the Rhine"? No matter,
    that concerns the other end of the English Channel; nothing at all to do
    with the Shovell disaster.
    
    In the Western approaches, the soundings decrease, from ocean depths, to
    continental-shelf levels, and then further to a 100-metre (50 fathom)
    contour, which lies roughly between Scilly and Ushant. That shoaling
    continues as you go further East, so it gives a mariner a rough indication
    of where he might be. There are indeed local anomalies and irregularities,
    as Gary suggested, which could mislead him. But the problem was in
    interpreting a sounding.
    
    The equal-depth contours run approximately Northwest to Southeast, so the
    depths are reducing as you go from Southwest to Northeast. All a sounding
    can tell you is if you are on a particular depth contour, nothing about
    where you are on that contour.
    
    If a mariner was sailing along a known line of latitude (needing clear sky
    at noon), which was navigational practice in those days around 1700, then
    taking a sounding would tell him where that line crossed the depth contour,
    and he would know his position, if only roughly. But if he had got his
    latitude wrong, as Shovell seems to have done, then his crossing of the
    depth contour would be at quite the wrong longitude, and the interpretation
    of any soundings would be highly misleading.
    
    Charts existed then to provide useful information to a mariner seeking such
    a landfall. Mount and Page published a chart of the Channel, in two sheets,
    in or about 1702, the chart that carried the results of Halley's recent
    survey of the Channel tides. That was one of the charts that showed Scilly
    and Lizard as nearly 10 miles too far North. We don't know whether copies of
    that chart were carried on board. In those days, officers had to provide
    their own. Depth contours were not shown then, and no proper surveys had
    been made, but spot soundings were liberally sprinkled over the area in
    question, rather like they are today, and seem to correspond rather well
    with a modern chart. Extra clues were provided, sparsely here and there, in
    terms of the sort of bottom that would be stuck to the tallow "arming" of
    the lead when it came up, such as "shells with fast black stones", and
    "ousey sand". A pilot might judge where he was from these appearances, taken
    with his previous experience. He could also use smell and even taste, but
    that may have been more for show; I doubt if the taste would differ much
    from that of salty tallow.
    
    An anonymous broadside was published, almost certainly by Halley, in 1701,
    which advised mariners "... as they come in, out of the Sea, on a parallel
    not more Northerly than 49 deg 40', which will bring them fair by the
    Lizard." If that had been carefully heeded, there would have been no Shovell
    disaster.
    
    The difficulties of taking soundings in those depths should not be
    underestimated. First, all the way had to be taken off the ship, by heaving
    to, so that she was moving with the tide only. Also, it was common for the
    tidal flow rate to differ with depth, which tended to angle the line. The
    line would be hemp, which is floaty, and quite thick stuff, to carry the
    56-pound dipsey (deep-sea) lead. It needed a team, working over the
    gunwales, to pass the necessary coils of rope, and haul up the heavy lead
    again after. And it took skill and judgment to decide when the tension
    slackened as the lead reached the bottom. Altogether, a two-hour job, which
    would be safe to do at night in a fleet sailing in close formation.
    
    It was only in the late 19th century that deep soundings under way became
    possible, using piano-wire.
    
    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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