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    Re: Dip measurement.
    From: Alexandre Eremenko
    Date: 2004 Nov 23, 01:02 -0500

    I think I have to think more to understand what they say
    (I only downloaded these papers yesterday).
    Someone on this list mentioned that this journal,
    Transactions of the Royal Soc. is available for free
    on the web, but if you wish I can e-mail or post .ps files.
    (I am using a JSTOR archive of old science journals
    which is subscribed by Purdue University.)
    I am talking mostly about two papers:
    1. Hadley, "The description of a new instrument..." May 13 1731
    2. Maskelyne, "Remarks on the Hadley's quadrants..." May 28 1772.
    Hadley recommends three wires: two parallel to the plane
    of the arc, dividing the field of view into 3 parts.
    and one perpendicular to the arc, passing through the optical
    axis. He seems to insist that touching of the bodies has
    to occure on the middle line between the parallel wires,
    and as close as possible to the perpendicular wire.
    He also insists, for the reasons that totally escape me
    "the distance from the parallel wires to the middle line
    has to be 4 and 100/146 of the focal length of the object glass,
    so as to comprehend between them the image of an object
    whose breadth to the naked eye is a little more than 2 and 3/4 deg".
    (In general, I find Hadley's paper very obscure).
    Maskelyne recommends only two wires, parallel to the arc.
    He says:
    "The two parallel wires will be very useful on many occasions,
    as well in the fore and back observation."
    On taking the altitudes he says:
    "The nearer the object is brought to the imaginary line
    in the middle between the wires and the truer the wires are
    kept perpendicular to the horizon, the more exact will the
    observation be".
    On taking the distance between the Sun and the Moon, he says
    "...having brought the Moon to appear anywhere on or near
    the diameter of the field of view of the telescope which
    bisects the interval between the wires, give a sweep to the quadrant
    and the Sun and Moon will pass by one another;
    if in this motion the nearest limbs, at their nearest approach,
    just tome to touch one another, without lapping over,
    on or near any part of the diameter of the telescope which
    bisects the interval between the wires, the index is rightly set..."
    "This method of observing will be found much more easy
    and expedious than without the wires, since in that case
    it would be necessary to make the limbs touch very near the centre
    of the telescope, but here it is only necessary to make them do
    so anywhere on or near the diameter of the field of the telescope
    which bisects the interval between the wires."
    I only add at this point that
    a) Maskelyne assumes that a telescope is used for back sights
    as well as for the fore sights.
    b) The following collimation test was proposed in one of these
    XVIII papers (and also in Cotter, for example): measure
    a LARGE distance between Sun and Moon, say more than 120.
    Bring them to touch near one wire. Then (if collimation is OK)
    they will also touch near other wire.
    c) Soviet sextant has 4 wires in its inverting scope.
    The only mentioning of them in the manual is for a collimation
    test. (Unlike all other modern sextants, SNO-T has collimation
    adjustment screws).
    On Mon, 22 Nov 2004, Fred Hebard wrote:
    > What do they have to say about the wires?

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