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Re: Two reckonings
From: John Huth
Date: 2010 Dec 30, 20:10 -0500
From: John Huth
Date: 2010 Dec 30, 20:10 -0500
George -
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Keeping up with the grind
Thanks for the correction. It's been a long day (for other reasons). Let me digest your reply.
Best,
John H.
On Thu, Dec 30, 2010 at 8:07 PM, George Huxtable <george@hux.me.uk> wrote:
John Huth quotes from The Haven-finding Art", which is by Eva Taylor
(usually quoted as "E G R Taylor"), not "Eve Lawrence", as he wrote.
Columbus had his own special reasons for keeping a true account and a
fictitious one. I do not think that the quoted passage was suggesting that
two separate reckonings be kept, but simply that in the case of any doubt
it was wisest to choose the higher estimate of ship-speed rather than the
lower, to avoid the vessel reaching the far-shore of the ocean
unexpectedly, and perhaps at night.
A similar practice was advocated over following centuries, when ship-speeds
were determined more scientifically by counting the knots that were paid
out from the line of the English Log, over a fixed time, usually 30
seconds. The knot-spacing was usually recommended to be adjusted so as to
read high speed rather than low, for that same reason.
In our account (Huxtable and Jackson), "Journey to Work: James Cook’s
Transatlantic Voyages in the Grenville 1764–1767" (Journal of Navigation ,
April 2010), we comment that, to our surprise, on every one of his 7
Atlantic crossings, Cook ended up "ahead of his reckoning", as follows-
"However, a rather surprising feature of each of the westbound crossings is
that, towards the end of the voyage, the Grenville was much closer to the
coast of Newfoundland than Cook’s dead reckoning had estimated her to be,
as shown by the dotted part of the track lines of Figure 2. This indicates
the discrepancy between the position at landfall and the calculated
dead-reckoning up to that moment, and each year, Grenville was consistently
‘ahead of her reckoning’. It was normal in that period to try, by
adjustment of the knots on the physical log cast astern of a vessel, to
ensure the opposite, so that the navigator did not approach landfall
without adequate warning, especially at night.
In the appendix to his British Mariner’s Guide of 1763, Nevil Maskelyne
noted that Richard Norwood had calculated, a century earlier, that:
‘the length of the knots of a log-line for a half-minute glass should be 51
feet, which is the samepart of a geographical mile, or 6120 feet, as half a
minute is of an hour, namely 1/120th. But he [Norwood] adds, because it is
safer to have the reckoning before the ship, than after it, therefore 50
feet may be taken as the proper length of each knot …’(Maskelyne, 1763)."
Further on, in dealing with Cook's Eastbound passages, we note an exactly
similar state of affairs on each one.
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.
----- Original Message -----
From: "Apache Runner" <apacherunner@gmail.com>
To: <NavList@fer3.com>
Sent: Friday, December 31, 2010 12:02 AM
Subject: [NavList] Two reckonings
I wanted to check with the history savants on this particular quote - I dug
this from Eve Lawrence's book "The Haven Finding Art", she doesn't cite
the
reference. She says this comes from a Portuguese navigation treatise from
1519, and a passage that she gives a translation:
*-------*
*Sailors reckon how far they have gone along the east-west line with the
help of the hour-glass, counting what the ship has done each day and each
night according to the way it makes for each hour of the glass. And for a
good reckoning one must judge by pacing what the ship’s way is. But
because this is a matter of judgment, the reckoning is uncertain. For
safety’s sake therefore it is better out of two reckonings to take the
highest number of leagues rather than the lowest so that you do not come
upon land before you expect it. You should shorten sail and keep a good
watch at night*
*--------*
I liked this quote and was curious if anyone knew of similar quotes or of
this practice. In particular, I was reminded of Samuel Eliot Morrison's
comment that Columbus kept two logs - one for the sailors and one for him,
but the one for the sailors ended up being more accurate - words to that
effect. If it was really standard practice in the late 15th and early
16th
century to keep two reckonings, that's an interesting piece of information.
Thanks in advance for any wisdom.
Best,
John Huth
--
Keeping up with the grind