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    Re: How Many Chronometers?
    From: Greg R_
    Date: 2009 May 6, 15:03 -0700

    
    --- On Wed, 5/6/09, George Huxtable  wrote:
    
    > By the way, there was a wonderful picture, a few weeks back, occupying a
    > double page in the Guardian. 
    
    A link would have been nice. Since George didn't provide one, here's a couple for those interested:
    
    http://3276.e-printphoto.co.uk/guardian/index.cfm?z=z&action=view&c_id=42104&p_id=11032928
    
    Thumbnails:
    http://3276.e-printphoto.co.uk/guardian/index.cfm?z=z
    
    --
    GregR
    
    
    --- On Wed, 5/6/09, George Huxtable  wrote:
    
    > From: George Huxtable 
    > Subject: [NavList 8167] Re: How Many Chronometers?
    > To: NavList@fer3.com
    > Date: Wednesday, May 6, 2009, 2:55 PM
    > Jim Wilson wrote-
    > 
    > | How about a log which keeps error and rate. John
    > Harrison's chronometer
    > | had to be accurate for three years at sea for him to win
    > the prize. And
    > | there were zero sources of time ticks then.
    > 
    > That's a common misunderstanding. But the test that was
    > applied to 
    > Harrison's chronometer H4 was between Portsmouth and
    > Barbados, a voyage of 7 
    > weeks, over which an error of 38.6 seconds had accumulated.
    > 
    > Cook took a different chronometer, a copy made by Kendall
    > (K1), around the 
    > World on "Resolution" on his second
    > circumnavigation. There is much 
    > misunderstanding about that too, partly the result of the
    > Sobel book 
    > "Longitude". A useful analysis was made of its
    > performance by Derek Howse, 
    > in a section, "Navigation and astronomy", of the
    > book "Background to 
    > Discovery", ed. Howse (1990). I attach a plot. Over
    > most of the period, it 
    > was gaining at over 10 seconds per day. Over a year, that
    > puts it an hour 
    > out, or 15� of longitude (= 900 miles in the tropics).
    > Over two years, twice 
    > that. It was only by checking the error, against lunars or
    > Jupiter 
    > satellites, at each stopping-point, where the rate could
    > also be checked by 
    > star transits, that Cook was able to keep its inaccuracy
    > within bounds. As 
    > long as that was done, it became a useful tool, and he was
    > delighted to have 
    > it aboard. The Arnold chronometer, carried on the consort
    > "Adventure", 
    > behaved much worse, as the plot shows.
    > 
    > Jim added, in another posting,
    > "I guess I thought that a chronometer was defined as a
    > clock with a constant 
    > rate of change."
    > 
    > Alas, there's no such thing. Every chronometer has some
    > temperature 
    > dependence, and the aim is to minimise that over the widest
    > possible working 
    > range. And every mechanical chronometer is subject to wear,
    > and to 
    > deterioration of its lubrication, which changes its rate
    > over time. It 
    > became standard practice, whenever a long stay in port
    > occurred, to return 
    > the instrument to an expert for cleaning and re-rating.
    > 
    > By the way, there was a wonderful picture, a few weeks
    > back, occupying a 
    > double page in the Guardian. Harrison's original
    > chronometer, the enormous 
    > H1, had failed, after all these working years. One of the
    > four coil springs, 
    > which contol the motion of its balance, had broken;
    > presumably, from metal 
    > fatigue. The picture showed from above Jonathan Betts,
    > curator of the 
    > horological gallery at the National Maritime Museum,
    > surrounded by the 
    > component parts of the clock like an enormous exploded
    > diagram. I do hope he 
    > manages to get it together, to run for a further 250 years.
    > 
    > 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.
    > 
    > 
    
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