NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How Many Chronometers?
From: Greg R_
Date: 2009 May 6, 15:03 -0700
From: Greg R_
Date: 2009 May 6, 15:03 -0700
--- On Wed, 5/6/09, George Huxtablewrote: > 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. > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---