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    Re: Lunars - Finding Bermuda in 1807
    From: Frank Reed
    Date: 2007 May 14, 02:51 -0400

    George H wrote:
    "That careful and interesting account was more that anyone has a
    right to expect from an 18-year old middie. It shows the degree of
    responsibility that could fall on one so young, and shows the care with
    which he discharged it, with the cooperation of the captured American
    And, also, this was his big chance to shine as a navigator. I'm sure he
    remembered it so vividly and recorded it so carefully because it was a real
    triumph of his early naval career. Imagine how felt as he was rowed out to
    HMS Leopard when she arrived at Bermuda a few days after the Erin. I can
    imagine Hall thinking of what to say to his superiors as he was rowed out,
    letting them know what a skilled mariner he was without sounding like a
    jerk... "Oh yes, uneventful passage. We arrived here two days ago. Had to
    shoot some lunars --even though the Sun wasn't in distance-- and sail back
    west to Bermuda... Remarkable set from the Gulf Stream this past week, dont
    you think? But it was nothing more than applying what I've learned." Well,
    at least, that's what I imagine him saying. :-)
    You wrote:
    "I ask Frank to give us references to that Basil Hall text, and his other
    works that he mentions. I haven't come across him before, and judging by
    that story, I need to put that right."
    If you google "Basil Hall" you will learn plenty about him. And if you go to
    google books and do the same, you will find the titles of some of his
    twenty-odd books. The quotable volumes were printed under the title
    "Fragments of Voyages and Travels". There were apparently nine books in
    three separate series under this over-all title. The volume with the story
    about the Erin is not available online and printed copies are extremely
    scarce so I assembled the account I posted from several sources.
    There is one compilation based on"Fragments" (printed twenty years after
    Hall's death) which has been scanned and transcribed in several places
    online. That book is "The Lieutenant and Commander" (here again, we see an
    influence on Patrick O'Brian, I would say). There is a carefully edited
    version of this available through gutenberg.org. The zipped version is here:
    It is only 247 kilobytes. Replace ".zip" with ".txt" if you want to read it
    without downloading it.
    You asked:
    "I wonder how Frank produced that nice email text for us. Was it transcribed
    longhand, which would have been quite a lot of work. Or has it already been
    converted to machine-readable form, perhaps by OCR?"
    In this case, I keyboarded it. I've been assembling this story from various
    pieces over the past two months, so the effort to type up the missing bits
    for the post yesterday was not great.
    You also wrote:
    "there's just one aspect that puzzles me. His first round of lunars was
    taken just after sunset, until the horizon became too faint to admit of the
    altitudes being taken with accuracy, he tells us. Then he worked out those
    lunars to get his longitude, discussed and cross-checked them with the
    captain, went back to take another round, worked out and cross-checked his
    lunars once again. So, for that second round, how did he get the necessary
    Moon and star altitudes to clear the lunar distance, as by then the horizon
    had long gone?"
    I noticed that, too. A student taught to take lunars with the stars back
    then would have been told that the horizon is essential. So he might have
    rushed his work during evening twilight because he believed what he had been
    told. But a few hours later, he may well have discovered what anyone would
    discover when the Moon is nearly full: you can see the horizon when the Moon
    is bright (I've taken lunars at midnight myself --it really does work). The
    altitudes may be uncertain by a few minutes of arc for various reasons, but
    that's not a problem for lunars. The altitudes don't have to be terribly
    accurate (unless the lunar distance is small). I don't think there's any
    reason to suppose that he computed the altitudes. It wouldn't have been
    necessary in good weather with a nearly full Moon.
    "The computations, between the two of them, must have been done very
    expeditiously, for those two rounds to have been worked out and a course of
    action settled in time for the new course to be steered around midnight."
    By 1807, there were several well-known fast clearing methods. There's no
    reason it would have taken them more than twenty minutes per set (counting a
    set as 'altitudes-before, four or five distances, altitudes-after'). If Hall
    took four sets, with two of them working, they would have been done in under
    an hour.
    You wrote:
    "Incidentally, there's a curious turn of phrase, to my ears, in the Hall
    writes that he "bore up", and "spanked along merrily toward the West". In my
    book, that would be bearing away, not bearing up. Do others agree?"
    Two centuries may not be long enough for the meaning to have changed, but
    it's probably enough for the meaning to have narrowed. I've got another Hall
    story that I intend to post. I think he uses similar phrasing there.
    And you wrote:
    "Well, at the latitude of Bermuda, a degree and a half is only about 76
    miles, so that statement shows an extraordinary self-confidence (or perhaps
    naivety) in presuming that such an error could not accumulate in 7 to 10
    days of dead reckoning."
    Yes, both confidence and naivete, I would say. I've seen this in other
    logbooks from this period. There are lots of comments fussing over the
    log-line and wondering about the sand glasses. Navigators back then
    *believed* in dead reckoning. They believed that the system worked if only
    they had the skill to apply it correctly. It was a genuine revolution in
    attitude when navigators began to trust the Moon and stars more than their
    own measurements right there on the surface of the sea. And in 1807, that
    revolution was in progress, but not yet complete. Lunars never really won
    out over that bias in favor of dead reckoning. It was the chronometer and
    the time sight that changed everything.
    In a similar vein, there is a logbook in the collection of Mystic Seaport
    from just a couple of years after Hall's story. I've discussed this logbook
    at all three of the little seminars I've done in Mystic over the past few
    years. A merchant ship, the brig Reaper from Boston, sails for Mocha to buy
    coffee and then heads on to Calcutta since Mocha is at war with the
    Wahhabis. The captain of the vessel takes fairly frequent lunars, and
    records the detailed calculations for most of them (!). In sight of Male,
    the principal island of the Maldives, his longitude and latitude are correct
    to within three miles. Unfortunately, it does him no good since the Maldives
    have been poorly charted at this date and he seems to trust the longitude of
    a fictitious island in his navigation manual more than his own lunar
    observation. He sails almost due east, getting ready to "turn left" (north)
    when he reaches 88 degrees east longitude. He sails on and on. When his
    lunar longitude shows him more than *five degrees* too far east, he cannot
    bring himself to ignore his dead reckoning. Not until he sights Sumatra
    ahead on the horizon does he admit in his logbook that the lunars were right
    all along. So he turns around and heads back to the longitude of Calcutta.
    It all ends well. In a single voyage, his profit was large enough to keep
    his family wealthy for generations.
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