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--- 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@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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