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    Re: New resource re ships' logs
    From: Frank Reed
    Date: 2010 Apr 5, 17:24 -0700

    George H, you wrote:
    "This is the first time, that I know of, that such a collection of digitised data has become available online."

    This collection is terrific, and thanks again for pointing it out. I spent several hours today exploring a few early 19th century logbooks.

    Just a reminder that there are dozens and dozens of old logbooks, journals, and other primary source documents from various types of American vessels (none USN) digitized years ago and available on the web site of Mystic Seaport at http://library.mysticseaport.org/initiative/MsList.cfm. It's great to see some similar British documents finally available online for comparison.

    George H, you wrote:
    "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?"

    Wales explains this in the description of the observations for the earlier voyage. The observations with z.d. are made with the (fixed) Astronomical Quadrant or in some cases they may be calculated. If they're calculated, it's usually explained in a footnote. For the observations at Point venus on Tahiti in April, 1774, he writes, "the star's true zenith distance is put down as it was computed; and the Moon's zenith distance was observed with the Astronomical Quadrant" and "where the objects are the Moon and Sun, the Sun's zenith distance was observed with the Astronomical Quadrant, and the Moon's altitude with a Hadley's Sextant. In every instance, the true time [meaning local apparent time] was got from that shown by the Clock." The clock in question was a pendulum clock that they routinely set up for these observations on land.

    On the earlier voyage, by my count Wales shot 21 back obs lunars and about 540 standard lunars. He was a busy astronomer! There is one other exotic back obs sight. It's a latitude taken in April, 1774 on an island in the Marquesas and recorded as "supplement to the the double altitude of the Sun's LL" and in the footnote it says that this was done by "Back Obs" using a quicksilver horizon.

    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.


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