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Gary, you asked:
"Did we ever come to a consensus on whether it was a latitude or a longitude 
error that led to this disaster? I remember that had they been at the correct 
latitude they would have passed safely south of the Scillys and north of 
Ushant."

Well, from a certain standpoint, it's both, right? They hit a big rock. 

There's little doubt that among the logbooks analyzed by W.E. May, there was a 
much greater spread in longitude than latitude. The error in longitude is 
even greater than diagrammed in his paper since the longitudes were not 
measured from any standard meridian but rather from Cape Spartel, the last 
point of "departure" for their dead reckoning, so the actual error in 
longitude depends also in the error in the longitudes of "known" points. All 
in all, the error in longitude was five to ten times greater than the error 
in latitude. We would know better if we had the logbook from the Association 
(the flagship) but of course that was lost in the disaster so we have to 
theorize based on the remaining logbooks from the vessels which survived. In 
May's paper he makes a theory involving the actions of the few vessels which 
were detached from the main fleet earlier in the day concluding that it was 
therefore mostly an error in latitude. I don't find his argument particularly 
convincing. Incidentally, George supplied a copy of May's article which you 
can grab here: http://fer3.com/arc/img/Clowdisley_Shovel_1707_JIN_1960.pdf 
(this may have to be removed at some point).

What about back in the day? What did they believe in the early 18th century? 
In the official inquiries, blame was placed on "bad compasses" which reminds 
me of a certain saying about a carpenter and his tools and doesn't help much 
either way. Less than a decade later, Whiston and Ditton began soliciting 
their peers in the world of mathematics and natural philosophy to convince 
Parliament to offer a huge reward for a navigational solution they were about 
to release to the public. Now Newton and Halley and the others were intrigued 
by this, but they prefered to present it as a general prize to "find the 
longitude" since this was the well-known "Holy Grail" of geographic and 
navigational science. While drumming up support for the prize, Whiston and 
Ditton published a broadsheet describing the great benefits of this "prize 
for finding the longitude", and then they alluded to their "own method which 
would surely find the longitude" (I am paraphrasing here --I posted the 
actual text in a recent NavList message), and then they noted that "it" would 
surely have saved the fleet of Cloudesley Shovell. So it all depends on what 
the word "it" refers to... If "it" refers to 'finding the longitude' then 
they're saying that the fleet was lost due to poor knowledge of longitude. If 
"it" refers to their 'own proposed solution' then it's a different thing 
altogether. And as you may recall, the Whiston-Ditton solution, good in 
theory but unworkable in practice, would have given both longitude AND 
latitude to a ship entering the Channel, and so then it wouldn't matter 
whether Shovell's specific navigational error was latitude or longitude. I 
think that's what Whiston and Dutton meant. But from that broadsheet, which 
was apparently read in part as evidence in Parliament before the prize was 
approved with Newton nodding in agreement, the connection between the Shovell 
disaster and the quest for longitude was locked in place.

So what about Ushant? In the later 1990s after Dava Sobel's book came out, 
there were some eager revisionist readers who noted a flaw in the traditional 
version of the tale. Sobel's book repeats the story just as it was told by 
other navigational historians for the previous fifty years at least, and it 
includes the suggestion that they believed they were somewhere "off Ushant" 
which is off the coast of Brittany in France and therefore sailing a course 
to the east-northeast would lead them safely up the Channel. Those readers 
who noticed this "off Ushant" part of the traditional telling knew from any 
common chart that Ushant and the Isles of Scilly are mostly separated in 
latitude. So if you think you're off Ushant, and you wreck on the reefs of 
Scilly, then that is surely an error in latitude, overturning the whole idea 
that this was a classic case of the inability to find longitude at sea. And 
that all sounds great. But the trouble is that this, too, was part of the 
legend. They never thought they were "off Ushant". That part of the story was 
added in sometime in the late 19th or early 20th centuries. Any reasoning 
based on it is irrelevant.

What do we actually know? We know that there was moderate uncertainty in the 
latitudes. Most of the latitudes are closer to the northern side of the 
Channel, near Scilly, and nowhere near the latitude of Ushant. We also know 
that their longitudes showed great scatter, so we don't know what longitude 
Shovell was using, however, as May points out, we expect that he wouldn't 
have trusted it much. We also know that they were in soundings -- they had 
found bottom and presumably were busy interpreting the exact appearance of 
the mud and sand that they found. And finally we have the most important 
piece of evidence of all: Shovell's order to sail into the night on a course 
of about 70 degrees true. He and his officers must have agreed that there was 
no danger before them on that course for at least 70 or 80 miles... that they 
could sail in darkness with heavy seas and strong winds. So get yourself out 
a chart and imagine sailing for 12 hours on that course. Where could you 
believe yourself to be and leave a reasonable margin for safety? Shovell 
either believed his longitude was well west of the Scillies (and somewhat 
south) or he believed he was well south of the southern English shore. Either 
would work, but given that the latitudes in the surviving logbooks all 
cluster towards the north side of the Channel, which would seem to rule out a 
course that would parallel the general track of a coast with several 
dangerous headlands, it is at least not unlikely that they believed they were 
well out to the west. That would make it an error of longitude...

And yet, as I said at the top of this post, they hit a big rock. That rock has 
a specific latitude AND longitude. Change either one, and you miss the rock.

-FER





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