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    Re: New resource re ships' logs
    From: George Huxtable
    Date: 2010 Apr 6, 14:21 +0100

    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{at}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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