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    Re: Two reckonings
    From: John Huth
    Date: 2011 Jan 2, 10:56 -0500
    George - 

    The local curator of the Museum of Historical Scientific Instruments told me that the sand-glasses were 28 or 29 seconds, rather than 30.   Could this be to also create an in-built over-estimate of distance run?

    I guess this doesn't completely cover problems like what Cook encountered.

    John H. 


    On Thu, Dec 30, 2010 at 10:41 PM, Gary LaPook <glapook---.net> wrote:
    I have attached a page from Columbus' log in which he states why he reported a lesser distance covered to the crew than was actually traveled, he did this on 23 out of 33 days.
    gl

    --- On Thu, 12/30/10, George Huxtable <george{at}hux.me.uk> wrote:

    From: George Huxtable <george{at}hux.me.uk>
    Subject: [NavList] Re: Two reckonings
    To: NavList@fer3.com
    Date: Thursday, December 30, 2010, 5:07 PM


    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{at}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---.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







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    Keeping up with the grind
       
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