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    Index corr., Octant as dipmeter
    From: Alexandre Eremenko
    Date: 2004 Nov 20, 20:10 -0500

    On Fri Nov 19 2004 - 19:53:20 EST Alex wrote:
    >I made an unexpected discovery:
    >Hadley's quadrant can be used as a dipmeter!
    And on Sat Nov 20 2004 - 11:33:41 EST he wrote:
    >I propose the following procedure for Index correction
    >for back sights on land. It seems simpler, and independent
    A little research shows that both discoveries are
    not new (what else could one expect?).
    They are both contained in the paper by
    W. H. Wollaston, M.D., F. R. S.
    "Observations on the Quantity of horizontal Refraction;
    with a Method od measuring the Dip at Sea,
    Phil. Trans., November 11, 1802.
    (This is an extremelly interesting paper, in many respects).
    Wollaston also says:
    "This method of correcting the index error for the
    back observation at sea, was many years since recommended
    by Mr. Ludlam ["Directions for the use of Hadley's Quadrant,
    1771, S 82, p. 56"]; yet I do not
    find that it has been noticed
    by the subsequent writers on that subject, or suggested
    by any one for determining the dip..."
    So Wollaston thought that the method of determining dip
    was new in 1802. So did I yesterday:-)
    P.S. Phil. Transactions, which I combed today looking
    whether they knew how to determine the dip with a quadrant,
    contain great wealth of info on all CelNav subjects.
    For example, the following paper addresses most issues
    that are permanent topics on this list:
    A Letter from the Rev. Nevil Maskelyne, MA, FRS to the
    Rev. Thomas Birch, DD. etc., containing the Results of
    Observations of the distance of the Moon from the Sun
    and fixed Stars, made in a voyage from England to the Island
    of St Helena, in order to determine the Longitude of the Ship,
    from Time to Time; together with the whole Process of
    Computation used on this Occasion,
    read on June 24 and July 1, 1762.

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