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I had wondered about the following problem, which related to Cook's third
voyage, to be found in
http://www.canadiana.org/view/17414/0003
And if you go to page 43, that's a page showing lunar distance observations
taken at Ulietea, in 1777. I wanted to compare those with the ones we have
been discussing from the earlier voyage, in 1773, taken at the same spot.

 "Note how the Sun column is shown as an altitude, but for some reason the
Moon is shown as a ZD; presumably Zenith Distance. Why should this be?
Well, it strikes me that this may be a use of the sextant fitted with
backsight. If the Moon can only be seen over the island, so would call for
an estimation of dip-short, can it be that instead, the Moon altitude is
being measured up from the opposite horizon? Does that explain the use of
ZD? Can anyone suggest another reason for logging ZD for one body, and
altitude for another?"

On second thoughts, I later managed to convince myself that the sextant
backsight explanation must be wrong, because that would provide the
supplement (180 - alt), rather than the complement (90 - alt, = zenith
distance).

But I expect that Frank has put his finger on the right answer; that the
Bird 1-foot radius astronomical quadrant, which had been supplied to both
vessels, and doesn't need a view of the horizon, was being used for
observations on both voyages, for bodies under which the true horizon was
blocked. It's very likely that this instrument was divided in degrees down
from the zenith, rather than up from the horizon. (Alternatively, Frank
suggests that the altitudes may have been calculated rather than observed)

It was explained elsewhere that the quadrant had been used, in other
observations, placed on a well-bedded-in cask, which had been filled with
wet sand. It was set by means of a plumbline, which would presumably have
been in an enclosed tube viewed by a microscope, for alignment. If it had
been left set-up on land, as had been the case on Tahiti, it would then
require a
24-hour guard to prevent it being stolen. So, not nearly as simple in use
as a sextant, but its big advantage was its ability to measure precise star
altitudes from on land at night, needing no horizon.

If Frank has ready access to that Wales volume of the second voyage, or to
its ECCO digitisation, I wonder if he can tell us whether Bayly has
provided any explanatory notes about his lunar observations, for which we
have seen only page 174. We would be interested in an explanation of those
symbols in his lunar distance column.

Frank ends-

"But the lunar observations that really interested Wales haven't been
mentioned. In July, 1774, Wales notes with obvious exasperation that he
cannot figure out why the two sextants sometimes disagree substantially
when used for the same lunar observations. He sets up a deliberate and
careful experiment using the two instruments at nearly the same time. The
longitudes after identical preparation and testing of both instruments and
identical computation on July 31 differ by 40' corresponding to a
discrepancy in the observed distance of something like 1.3 minutes of arc,
which for his expectations at the time was quite large, and Wales describes
it as "extraordinary indeed!" (his exclamation mark). He records this
dutifully and says he leaves it to someone more mechanical to figure out
why this happens."

I wrote, on 2 April,

"Bayly, in a letter to Banks about the third voyage, quoted in David vol 3,
writes about the poor accuracy he found in the 15-inch Ramsden aboard
Discovery, (which may well have been the same instrument used on Adventure
in the second voyage) finding that it was producing longitude errors of
sometimes more than a degree; "in consequence of which I made no other use
of it during the remaining part of the voyage but to observe altitudes at
the time of observing distances, as in that case a small error on altitude
would seldom cause any effect in computing of the correction for parallax
and refraction"

So those are two similar comments about discrepancies between instruments.
Ramsden's dividing engine was due to come into use soon after that time,
but these instruments would have been made using hand-division, and I think
we can see the effect of its imperfections.

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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