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Henry Halboth pointed out-

>George - the discrepancy you describe is the compass error to which
>deviation must be applied to obtain variation. Perhaps Brother Cook was
>not too much troubled with deviation, but it's there nevertheless; even
>the metal ring in an officer's hat while taking an azimuth can deviate a
>standard magnetic compass as much as 3-degrees, as I have proven by
>experiment. Please don't construe that I am suggesting Cook's hat,
>whatever that may have been, as the start of another thread, but I am
>saying that, without a knowledge of the deviation the variation recorded
>must be suspect.

===================

Henry is quite correct, as usual. I fell into the mistake of assuming that
for Cook's wooden vessel, there would be no deviation, but should have
known better. I've recently been studying a voyage to the "Greenland
Whaling" (West of Spitzbergen) in which the log notes that on certain
courses, the two compass positions in the binnacle differed by a whole
point (11 and-a-quarter degrees).

Two compasses were placed a few feet apart in the wide binnacle (somewhat
like a domestic sideboard), presumably so that the helmsman could read a
compass straight-on, wherever he needed to stand. Checks had been made to
ensure that it wasn't the effect of interaction between them. It must have
been deviation resulting from some local ironwork, to which the compass
would be very susceptible in such high latitudes. Such problems were only
poorly understood in the early 1800s.

I'm interested in Henry's account of the effect of the "metal ring in an
officer's hat". Unfortunately, most such portraits in my books of Cook's
period are posed hatless, but none of the others show such a ring. What was
it FOR, and how big was it, I wonder? Some sort of buckle, perhaps, for a
chinstrap to hold down such awkward headgear, with its immense top-hamper,
in a blow? But an IRON ring (which it would have to be to affect the
compass)? I can imagine it being of ivory, perhaps, or brass, or even
silver; all non-magnetic. But an iron ring doesn't sound right to me, even
for an officer's working headgear. Perhaps Henry will explain further.

Of course, on Lisa's steel vessel, compass deviation and its correction is
a serious matter, and in her case it's likely that compass bearings of low
celestial bodies would be useful, not for checking variation, but instead,
taking compass variation from the chart, for checking deviation, and how
well the compass has been corrected..

George.

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