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    Re: Transatlantic cables and measuring dip.
    From: Richard M Pisko
    Date: 2003 Mar 21, 20:50 -0700

    Back before the dawn of time (on Fri, 21 Mar 2003 23:53:04
    +0000, to be exact), George Huxtable
    >I know of another type of dipmeter which was made by Carl Zeiss and was
    >used by the Carnegie oceanographic expeditions in the 1920s. I havent seen
    >one, but I think it's rather as Richard indicates. The observer looks
    >through a telescope at a pair of prisms (could be mirrors) with reflecting
    >surfaces at 45 deg to his sight line. These provide a split view of the
    >horizon to his left and the horizon to his right. One of the prisms or
    >mirrors can be tilted sideways slightly by a calibrated fine-adjustment
    >until the two views of the horizon are aligned. Having taken a reading, the
    >instrument is inverted and the measurement repeated. The difference between
    >the readings is 4 times the dip.
    Many thanks.  I had not imagined the first (Blish prism)
    attachment to a sextant; I think that would be more useful
    than a purpose built instument ... although at �130 for the
    glasswork, probably more expensive than a couple of porro
    prisms and a micrometer at the tip of a lever.
    I guess I could try measuring the "dip" differences day to
    day from a specific open area in the prairies, horizon to
    horizon, with a wood mockup.  Although it would be easier to
    take shots from a theodolite on the mountains about seventy
    miles away to measure the amount they seem to float higher
    on certain days.  When the conditions are right, you quite
    clearly can see the foothills that are normally below the
    Richard ...

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