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On 12 May 2008 at 1:49, frankreed@HistoricalAtlas.net wrote:

> > http://articles.adsabs.harvard.edu/full/1891Obs....14..122P
>
> In this article, the author says of H4:
> "although popularly styled a 'chronometer,' is really only a verge
> watch".
>
> He then desctribes the "tremulous" motion of the seconds hand. What is a
> "verge watch"?

A verge watch is one with a verge escapement (and I suppose size and the
use of a balance spring defines it as a "watch", though it was a pretty
big watch). It's true that Harrison used a verge escapement although he
put diamonds with a cycloidal curve on the pallets. I've restored a couple
of old verge pocket watches and one feels an immense sense of
accomplishment if they can be brought within 5 minutes/day. That Harrison
could make a verge with such accuracy is just slightly short of
miraculous.

> Is it connected to the tremulous motion (which makes it
> difficult to read H4 to the nearest second)? When he implies it's not a
> chronometer, is he speaking of some fairly specific contemporary
> definition
> of a chronometer, c.1891?

He was probably just defining "chronometer" by the escapement rather than
the helical balance spring and thermal compensation mechanisms.

I don't know the period of the balance in H4 but if it was typical of a
verge watch then the second hand would have advanced in several small
increments for each second (I think mine go 5 but I may be confusing them
with my old lever watches). Earnshaw's chronometer escapement only moved
after two full oscillations of the balance so it may have had a second
hand that ticked out whole seconds.

Ken Muldrew.

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