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| 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@huxtable.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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