A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2010 Nov 6, 22:33 -0700
George H., you wrote:
"Frank wrote, under "old question-need summary" his familiar apologia for Sobel"
Sobel's excellent and highly successful book needs no apologies. But I did feel it was important to point out (yes, again) that, contrary to your description, she did not invent or mislead in her description of the Shovell disaster and its contribution to the story of longitude except to the extent that her version repeated the well-established version WHICH HAD BEEN TOLD AND RE-TOLD FOR DECADES by historians of navigation and cartography. Of course the well-established version of the story is a mixture of legend and fact, but it's not her creation, and it is not ALL legend. The Shovell disaster was associated with the Longitude Prize from the very beginning, in part thanks to William Whiston's broadsheet which he published in London during the parliamentary hearings creating the prize for finding the longitude in 1714.
And you wrote:
"and criticism of May's paper about his study of the surviving logs from the Shovell disaster"
Yeah, May's paper is a great read, but his final theory of events is contrived, and his chart is misleading. I am astounded that anyone would read that article and treat it as anything more than a suggestion, one way things may have occurred, but surely not the last word or even close to it.
And you wrote:
"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."
Right. Exactly. And assuming that this reported sounding is factual and actually recorded in one of the logbooks (which it may not be!), then this clearly demonstrates that they believed they were a long way from land and nowhere near the Scillies. Again, this fits well with the evidence of the orders. Shovell was no fool though he was probably over-confident. He would not have ordered his fleet to sail east in darkness in bad weather unless he believed he was far from land. If he had confidence in a 90-fathom sounding, he probably believed the fleet was still a long way to the west of the entrance to the Channel. In short, he had made a big error in longitude.
"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."
Sorry, that wasn't my point. I merely meant that we can use soundings today in this fashion. Or indeed a century ago when depths were well charted this was an excellent means of determining a line (curve) of position. Back in 1707, it was much less reliable. No matter what actual depth Shovell's men had found, the ability to use that for navigation was much more problematic when the charts themselves suffered from the inability to determine longitude with any accuracy.
"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"
I wouldn't call that "evidence". That's May's very specific theory. What we know is that Lenox and a couple of other vessels were ordered to separate from the fleet and meet up with the merchant vessels waiting ahead at Falmouth. Once Shovell and his fleet were in soundings and approaching safer home waters, Lenox and the others no longer needed to remain with the fleet. Once separated from the fleet, they were free to set their own courses, find the best wind, etc. According to May's little chart, Lenox and the other vessels only sailed on a northeast course for about eight nautical miles (I am estimating from the plotted track on his chart) and then they turned east. A mere eight mile leg to the northeast is not particularly consistent with May's theory! For his theory to make sense, they could have sailed for thirty or forty miles on that course. This short eight-mile jog followed by a course nearly east for the remainder of the afternoon and evening IS consistent with a belief that they were well out to sea to the west of the Scillies. And indeed May admits that they could have thought they were around 100 n.m. further west if they were using "Colson" for longitudes, though this point is minimized by the way he's drawn his chart.
By the way, before someone takes this the wrong way, I'm not suggesting that May was being deliberately misleading by drawing the chart like this, but once he had a theory in his head, he would have a natural tendency to draw his diagram to support that model and minimize other options, even though he explicitly described those other options in the text.
"The disaster can't simply be ascribed to failure in knowledge of latitude or longitude, but in a failure of both"
Yes, I agree with that. But which source of error was more significant? We may never know with anything approaching certainty, but an error in longitude would have put them well out to sea and safe to sail in darkness in a gale.
And you wrote:
"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."
Yes, absolutely. I've compared Shovell, metaphorically, to Smith on the Titanic. They were both highly experienced, and they both could have saved themselves, their vessels, and thousands of lives if they had simply had the wisdom to sit and wait. The immediate causes of the disasters were naturally very different, but the same simple caution could have avoided both disasters, and both led to major changes in perceptions and practices of navigation.
"In 1707, a navigator was expected to manage without longitude, and get his ship to its destination."
This is a modern point of view. If they were expected to "manage without longitude", well then, why did they go to such great lengths to calculate it?? Just "make-work" for the younger officers? Why did they record longitude each and every day?? The modern conception is that dead reckoning was damn near useless and longitude was a pure unknown, and further that latitude sailing was practically the law of the sea, but many navigators absolutely relied on dead reckoning longitudes much more than a century after the disaster in the Scillies. Navigators resolutely believed that they could do dead reckoning correctly if they were very careful, and in fact, if they were in regions with minimal currents or if they applied other navigational information like soundings, the appearance of the water, and so on, they could do very well much of the time (using other information, by modern formal definitions, means that this isn't pure dead reckoning, but that purity doesn't seem to have bothered navigators traditionally). There's no question that Shovell and the other officers in that fleet in 1707 had calculated and estimated their longitude, and more likely than not, many of them had confidence in their estimates --wasn't that a skilled navigator's job? Shovell believed in his estimate of longitude and his latitude, too, and that's why he willingly ordered his fleet to sail into the dark.
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