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George Huxtable wrote:

> Is Mike claiming, then, that because they can't be found online, it puts
> that record somehow in doubt?

Please don't treat me like an idiot.  I'm only mentioning that the
document is not online.  I've never seen a facsimile of the original,
only the occasional extraction, precis or interpretation by other authors.

>   "A new instrument" and "the old
> | instrument corrected of some faults" suggest it might not be the
> | original design, but an improved version or even an entirely new
> | instrument.
>
> Well, yes, that might well be the case. So what?

If folks are saying "Halley used Newton's original instrument", then
that means the statement is false - Halley used a different instrument
with unknown or poorly documented specs (again - I haven't seen the
entire original Journal Book entry, so I have no idea what other info is
available on this).  Outside of the Journal Book entry, I have found
nothing - but then I don't have access to unlimited historical sources.

> A plausible scenario might be this- Halley had taken the instrument to sea
> for his first Atlantic voyage, and had found some problems, which needed
> fixing, so he had returned the instrument to Newton at the Mint on his
> return, only the previous month. Newton (or his craftsmen at the Mint) had
> promptly put those right, and reported to the Royal Society, acknowledging
> those problems and improvements.

Plausible scenarios are nice, but they are not good scholarship.

> My reading of it does. How else does he interpret the words "with which
> notwithstanding Mr Hally had found the Longitude better than the Seamen by
> other means"?

Standing on land and comparing different instruments?  On a ship of the
line in harbour at Portsmouth?  On a small yacht on the Thames?
Crossing the Atlantic?  Assumptions do not make good scholarship either.

> That's exactly why, in my original posting, I used Halley's
> precise latitudes, rather than his longitudes, as evidence that he was
> indeed using a two-mirror instrument. Newton's instrument would have been
> used for his latitudes, and also for his star altitudes to get local time,
> part of the process for finding longitude.

Accurate readings do not guarantee the identity of the instrument.  Did
he use an accurate instrument?  Apparently.  Did he use Newton's
original instrument?  Unknown.

> But Mike was disputing that Halley had taken Newton's instrument to sea, (or
> disputing that Newton had said he had- I'm unsure which), not whether or not
> he had used it to find the longitude.

I'm disputing whether we _know_ which instrument was used.  There is no
direct evidence and only one oblique reference.

> Note that all Halley had conceded was that the two instruments were "much
> different"; which they were, although they were clearly based on exactly the
> same principle. Had he been "leant on", by Hadley and his supporters, in
> order for Hadley to be able to claim a patent, which would otherwise have
> been denied? I wonder, and so does Ted Gerrard.

Halley says they are different, yet they only differ in the orientation
of the arc and index mirror.  Hadley's zeros at the end of the arc away
from the scope and Newton's zeros adjacent.  Halley was a smart enough
man to recognize how trivial that difference is.  Only in Hadley's
second version do we see any significant differences.

Why had he changed his opinion?  I don't know.  Some suggest it was
because Halley wasn't that familiar with Newton's instrument.

> WHO puts the matter in doubt, exactly? On what basis? What evidence can Mike
> offer that Halley used some other instrument? What instrument would that be,
> then? And how can he square the use of the known instruments of his day with
> the precision of his latitude observations, made at sea, on the English
> Channel coasts? To me, that's decisive.

In exactly which books did I read that - offhand I can't remember.  I
was looking for confirmation, not contradiction.  What instrument did he
use?  That's the question I'd like to see answered.

> Mike suggests that Halley's own proposed instrument of 7 years earlier, as
> referred to by Cotter, may have been what he took to sea on those voyages.
> But that was a single-reflecting device, with considerable angular motion
> between horizon and celestial body as it's rocked, which addition of a
> telescope would only worsen. It shared the same problems, used at sea, as
> the cross-staff and backstaff, which a two-mirror intrument would eliminate
> in a stroke.


But Halley's instrument can be converted to double reflection with a few
minor mods and one more mirror.  Since we know nothing about what he
used, everything is speculation but for a single ambiguous source.

Mike


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