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Frank wrote, under "old question-need summary"  his familiar apologia for
Sobel, and criticism of May's paper about his study of the surviving logs
from the Shovell disaster-

"That's the status of the dead reckoning alone, but Shovell had more to go
on than "pure" dead reckoning. He also knew that they were in soundings and
presumably he had even seen the mud pulled up stuck to the tallow. Those
soundings, while quite deep, pin down the longitude considerably better. On
a modern chart, we can follow the contours of the soundings and mark the
position of the fleet with some accuracy. If we knew where Shovell believed
that the 80/90 fathom band was located, based on his nautical information
back in 1707, then we could narrow down better his estimation of the
fleet's position. But this was highly uncertain information. The surveys
themselves suffered from the same navigational uncertainty, especially with
respect to longitude, as did the navigators of Shovell's fleet."

I wasn't aware that such sounding information existed, but in the paper
that Frank and Douglas both referred to, which can be found at-
http://www.hmssurprise.org/Resources/SIR_CLOUDESLEY_SHOVELL.html
these words appear-
 "On the 21st of October the Admiral made an observation, probably the
first he had been able to take for many days. The next day, having
soundings at 90 fathoms, he brought to and layby about 12 o'clock 18 and
summoned all the sailing-masters of the various ships on board the
Association, and consulted them as to the fleet's actual position "

reference 18 is to Isaac Shomberg's "Naval Chronology", vol 1, page 132,
which is available Googled at-

http://books.google.com/books?id=5rFCAAAAYAAJ&dq=inauthor%3A%22Isaac%20Schomberg%22&pg=PA132#v=onepage&q&f=true 
,
and states "on the 23rd October the Admiral struck soundings at 90 fathoms,
the wind then blowing strong from the SSW, with hazy weather, he brought
the fleet to. At six in the evening he made sail again under his courses,
whence, it is presumed, he believed he saw the Scilly light ; soon after he
made the signals of danger"

May described the event as follows-
"After describing the despatch of the two frigates in the morning of the
22nd, he says "No Sun was visible on that day and around 4 pm [by which
time I presume that the nautical-date had changed at noon to the 23rd,
which may explain some of the date-discrepancies] the fleet hove-to for
about two hours to obtain soundings; then, satisfied they were in the mouth
of the Channel and clear of all danger,the ships ran to the Eastward before
a favourable gale. Less than two hours later the leading and more northerly
ships found themselves among rocks and fired guns..."

But the crucial piece of information that's emerged is that depth, recorded
by Schomberg, of 90 fathoms, or about 160 metres on a modern chart. I
wonder if that statement is backed by evidence from elsewhere. Perhaps
Douglas can tells us if his studies provide any clues. May doesn't state
any depth. Frank suggests- "If we knew where Shovell believed that the
80/90 fathom band was located, based on his nautical information back in
1707, then we could narrow down better his estimation of the fleet's
position." But we need to ask more than that. How did he make such a
hopelessly inaccurate sounding? Because inaccurate it must have been, as
events showed.

Attached is a corner of just such a chart as "Association" is likely to
have carried at that date, 1707. It's from a survey, "Great Britain's
Coasting Pilot", made by Greenville Collins "by order of the King", and
first published in 1693. Publication, by Mount and Page, continued right
through to 1753, from which this copy was taken (and a bit further still).
Few updates or amendments were made over that long life; not even a
correction of the erroneous latitude of Scilly. The fleet's flagship is
highly likely to have carried a copy of that pilot. And comparing it with a
modern metric chart, there is remarkably good agreement between the two, in
terms of depths of these Western approaches to the Channel, in general if
not in fine-detail.

That chart, which has unnumbered longitudes, and my modern metric charting,
both extend to nearly 8 degrees West, and neither shows water as deep as 90
fathoms anywhere near striking distance of the Scilly rocks. Frank wrote-
"On a modern chart, we can follow the contours of the soundings and mark
the position of the fleet with some accuracy." I challenge him to do so;
there's simply no such 90-fathom contour, between the latitudes of Ushant
and Scilly, within the confines of that chart. Possibly, Schomberg's
reported sounding depth may be mistaken. Or perhaps the depth observation
itself was at fault. I can suggest one possibility. Sounding at such depths
called for the deep-sea lead, a 56 pound weight. Although shallow-water
soundings could be taken under way, and piano-wire soundeing machines
allowed even deep-sea soundings in the 19th century, in Shovell's day
taking a deep-sea sounding required a vessel to heave to, to allow the
weight to drop down straight rather than trail at an angle. Yet, the
accounts agree in implying that first, the sounding was taken (when
reaching in a near-gale), and next, the fleet lay-to to discuss its
implications. That's the wrong order, and asking for trouble, in
overstating the true depth.

After passing East of Scilly, there's still Lizard to round, before
latitudes should ever have been allowed to approach anywhere near 50ºN, and
passing Lizard can't be relied on until the soundings have dropped to 45
fathoms: not 90 fathoms. And yet, the evidence is that he must have thought
his fleet clear East of Lizard when dispatching his scout frigates to go on
ahead, because these had been sent off in a Northeasterly direction: so
much so, that in the event they found themselves on the North side of
Scilly, doubtless to their great surprise. How that view could ever be
squared with a 90-fathom sounding is a mystery.

The disaster can't simply be ascribed to failure in knowledge of latitude
or longitude, but in a failure of both, together with carelessness about
sounding information. Dead-reckoning for latitude, from an observation
taken at noon on the previous day, taken with proper assessment of
soundings, and caution about blundering on in the dark, should have been
enough to keep Shovell's fleet off the Scilly rocks. Lead, log, latitude
and lookout, were the rules that a navigator was expected to follow. In
1707, a navigator was expected to manage without longitude, and get his
ship to its destination.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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