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    Re: How good were chronometers?
    From: Frank Reed CT
    Date: 2006 Mar 14, 21:28 EST

    Jim, you wrote:
    "One book I read of late  regarding chronometers initially after Harrison's
    time commented that a  chronometer would cost about 500 pounds sterling, a
    sizable sum compared to a  ship at about 1500. So, chronometers were
    expensive.Then you need at least 3 if  you want a reasonably simple way to check how
    consistent the rates are holding,  so more expense."
    Just as an incidental comment here, there were other  methods of proving your
    chronometer's accuracy, even assuming you didn't do  lunars and couldn't
    afford multiple chronometers. Ships almost always traded  longitudes when they met
    on the high seas. Also, there are numerous small  islands that can serve as
    longitude checks (assuming we are late enough in the  19th century that plotted
    positions can be trusted).
    And you  asked:
    "So, that is a long winded approach to getting to the question. What  was the
    experience with chronometers like for the average seafaring navigator  when
    mechanical chronometers were the order of the day?"
    In America, the  dividing line is around 1835 [based on the evidence I have
    seen so far.  additional evidence might convince me to shift this date but I
    very much doubt  the shift would be larger than ten years in either direction].
    Before this date,  most vessels got their longitude by dead reckoning (not
    just a rote calculation,  but also including other factors) and their latitude by
    Noon Sun. On a long  voyage, the reckoning might be checked a few times every
    two weeks by lunars,  but the DR was the "principal" longitude, and
    navigators took it very seriously.  After about 1835, a majority of American vessels
    were starting to carry  chronometers. In whaling logbooks from the early 1840s,
    it is not uncommon to  see references to two chronometers being carried. They
    still used lunars in the  era, taking them for a few days in a row every two
    weeks as a check on the  chronometer-based longitude. By 1855 or so, lunars
    were essentially history, and  chronometers were considered completey reliable
    for practical use. As I have  mentioned previously on this list, there was
    apparently no "lunars era" when  lunars were the principal method of getting
    longitude. They were always a check,  and a very valuable one, on the primary
    method, whether it was dead reckoning or  longitude by chronometer.
    In Britain, and in particular in the Royal  Navy, subtract 25 to 30 years
    from the dates above. For a specific case, one  "Lt. Ashe" writing in 1849 to the
    Royal Astronomical Society noted that he had  seen only a single instance of
    lunars being used to check chronometers in twenty  years at sea. Chronometers
    were not only widely available in the Royal Navy but  widely trusted by 1830.
    By contrast, British ocean-going merchant vessels seem  to have been closer to
    the American schedule.
    You also wrote:
    "Slocum  for example opted for an old tin clock instead of his chronometer
    and gives me  the impression that on those occasions that he did use celestial
    navigation that  it was almost as if he were checking his celestial navigation
    against his dead  reckoning as opposed to the other way round. He was always
    confident that he new  where he was and how to approach those various
    landfalls. He didn't need a fancy  expensive chronometer to get by although he was
    clearly as familiar with  celestial navigation as I would assume most navigators
    would be at that time. I  get the impression that sailors of the day were more
    tuned in to all the  elements affecting dead reckoning and relied on it
    heavily which may be a bigger  reason for the slow acceptance of the LOP."
    Yes, that's a good point.  Dead reckoning was far more popular and more
    successful than many modern  navigation histories (mostly anecdotal histories)
    would lead us to believe. And  navigators believed in their dead reckoning. Even
    in the 1850s, I have seen  examples of commercial vessels sailing to Java using
    nothing but dead reckoning  for longitude. As for Slocum, he had an
    advantage. He could choose his season  and his timing. As Slocum proved, you can sail
    around the entire world using  nothing but latitude by Noon Sun and dead
    reckoning longitude. Of course, if you  have to do that every day and you have a
    cargo to deliver on schedule, your luck  will eventually run out.
    42.0N 87.7W, or 41.4N  72.1W.

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