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George wrote:

> What surprises me, is that it took until 1837 for navigators to realise
> that a useful position line could be drawn from a single observation of the
> altitude of a body, even if it wasn't at meridian passage.
>
> Indeed, in an earlier era, before Mercator charts were in common use, and
> when navigators measured off a globe with dividers, that would seem to be
> an obvious conclusion. If you plot the geographical positiuon of a body on
> a globe, then draw a circle round it at a radius corresponding to the
> measured zenith angle, then that circle is a locus of your position. You
> must be somewhere on it. On a globe, it's visual, indeed obvious.

[...]
 > But it still surprises me: why, oh why, did it take so long for
intelligent
 > men to discover such a simple matter?


But you can't plot the GP of a body unless you have GMT, which of course
wasn't available in the period when navigators delighted in having their
portraits painted with dividers in hand, marking off a distance on a
globe. Sumner's realization that celestial LOPs could be drawn came only
a few decades after the accurate measurement of time became practical at
sea. It is that delay which needs to be explained, not the whole period
from Prince Henry.

So why didn't the use of LOPs follow swiftly after chronometers and
lunar distances?

For one thing, I'd take a guess that LOPs as such weren't understood for
what they are, i.e. loci of position. If nobody thought in terms of a
bearing of a seamark being a locus of their position, it would have been
quite a step to figure out that such an abstract thing could be created
out on the ocean just by a simple elaboration of the time sight
calculations that they were already doing. [It may be worth noting that
Sumner was as impressed by finding a bearing of the land (by which he
seems to have meant a bearing to a known landfall) as by getting an LOP
which could crossed with a second one to get a position.]

However, I suspect that the prime cause was that the 18th century mind
just did not well cope with the sort of linear problem solving which
most of us now take for granted. (That's not the fault of our
18th-century ancestors. We are the weird ones, applying a mode of
thought which was essentially unknown for all but the last 200 or so of
the 120,000 years, give or take, that our species has been around.) By
way of illustration, I am currently working through Tunstall's tome on
the history of naval signalling from 1650 to 1815. (Tunstall claimed it
as a history of tactics but his treatment of tactical thought is largely
limited to what he could glean from the evolutions for which signals
were provided.) The inadequacies of 17th-century systems (based on
assorted flags, with their meaning changing depending on where in the
rigging they were flown) troubled Admirals for a hundred years, while
there were multiple approaches to Popham's numerical system of 1803 over
decades, yet nobody before him seemed able to figure out that with just
ten numeral flags plus two substitutes, it was possible to send 1,000
different signals with only three flags per hoist. To the modern eye,
with the benefit of knowing the final answer, it all seems so bizarre.
Why couldn't anyone see the obvious? But they couldn't.

As another example, Sutherland's textbook of ship design, of 1711,
devotes a chapter to the design of spiral staircases, which topic is
addressed in terms that imply that the beauty and symmetry of the stairs
were essential to fighting efficiency. By Stalkartt's work of 1787, such
foolishness was left far behind, but there was still much attention to
the shape of the hances in the rails. Not until the rigours of the long
wars after 1794 did the Surveyors of the Navy figure out that it was
better to build up bulwarks to protect the gun crews on the forecastles
and quarterdecks of ships-of-the-line, rather than troubling over the
elegance of the sheerline. Not until then did people seem to realize
that they could manipulate cause-and-effect relationships to better
achieve their goals.

Of course there were remarkable individuals who thought in such ways
very much earlier but there were not very many of them. The French did
quite a good job of harnessing the brain power of their academicians to
maritime problems (a very much better job of it than the English
managed) but it needed communication between the people who faced a
problem and those who could solve it -- which is tough to set up when
the people who face the problem don't know that it can be solved, so
don't frame it in their own minds in terms of a problem, as distinct
from a burden to be endured.


Then came Sumner, who evidently was trained to think in our modern way,
who had to face navigational problems directly and who did find the
solution which is obvious to us. Hence my interest in what combination
of factors put him in command of a ship, such that he faced navigational
challenges, after he had been given the training to solve them. And,
since he evidently had what it takes to rise to command after once going
to sea, that comes back to a question of why he shipped out in the first
place -- and hence to just what it meant, in his specific case, to be a
"common seaman".



Trevor Kenchington





--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                     Science Serving the Fisheries
                      http://home.istar.ca/~gadus

   

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