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    Early XVIII century nav book
    From: Alexandre Eremenko
    Date: 2012 Mar 11, 08:33 -0700

    I recently found this book, which is not in our list of navigation books:
    Compleat Modern Navigation Tutor, or
    a Tretease of the Whole Art of Navigation
    in its Theory and Practice etc. by Joshua Kelly, 1724.

    It describes among other things the details of construction, use and
    adjustment of the cross staff. For example, I learned from it, how cross
    staff was really used: one did not look on the Sun directly, but held the
    cross staff with it crossbar behind, and aligned the horizon seen in the
    slot of a vane with the shadow of the cross bar end seen on the same vane.
    Only for star observations, the cross staff was reversed, and
    the ends of the cross bar had to be aligned one with the star and another
    with the horizon, as shown in the pictures in most books.

    (So the statement one can frequently read that navigators hurt their eyes by looking directly at
    the Sun seems to be incorrect).
    Moreover, he mentions that a lens was used on the improved models of the
    cross staff so that one had to align with the horizon the bright spot created
    by the Sun on the vane.

    The Davis back staff and its use is also described in great detail.

    One can download the book from my web site http://www.math.purdue.edu/~eremenko/Navigation/Kelly/


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