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    Re: Testing the dip meter
    From: Brad Morris
    Date: 2012 Jul 26, 16:06 -0700

    Hi Alex

    There are two predominant points within driving distance of my house on Long Island, NY.

    #1 Montauk Point.  G.Washington commissioned a lighthouse there.
    #2 Orient Point.  more lighthouses, etc.

    Big unobstructed views of water.  Elevated!


    On Jul 26, 2012 2:55 PM, "Alexandre E Eremenko" <eremenko@math.purdue.edu> wrote:

    It is really hard to find a place on land where you can test
    a dipmeter:-(
    Inspection of maps and one field test nearby on the Baltic
    showed that there is probably only one such point in Germany:
    the island of Helgoland.
    So I made a day trip to Helgoland.
    Most of the trials I did from the ship on my way to and from.

    This is indeed a tricky business, and I understand why Russian sailors
    were reluctant to use it:-)

    First, there are some design faults. The main of them is that there is
    no handle. And too little space for your fingers such that they do not
    interfere with two obsectives and with the moving parts. And absolutely
    no way to attach a strap. So you always worry that it will slip out of
    your hand.
    Second, there is no way to remove the yellow eyeglass filter without
    a special tool:-) It does not alow you to grab it with your fingers
    because the rubber cap prevents this.
    Now, many people on tis list said that they do not like the astronomical
    telescope because it is inverting:-) (Though in all other respects it is
    superior, and it takes just a little training to get accustomed to it).
    This dip-meter has not only an inverting scope, but the light also passes through two prizms, and then two differently rotated pictures are superimposed.
    The result that the image in your field of view moves absolutely
    counter-intuitively when you move the device.
    The two horizon images look vertical, but also inverted with respect to each other, even to get them both in the field of view requires some work.
    The very important thing is the diaphragm which equalizes the brightness
    of the images. Unless the brightness is equal, you almost do not see
    the one which is less bright. And this diaphragm is too sensitive...

    Anyway, after many trials from two decks I obtained the results which
    look reasonable. The scattering in a typical series is about 0'5,
    same as with a sextant measurement. (The manual says that the
    accuracy is 0'3). The averaged results look reasonable
    (I did my best to estimate the height of these two decks using my own height for comparison).

    Of course, if you practice a lot, the device will detect the cases
    of large anomalous refraction at the horizon. I don't know how frequent these cases are.
    Based on my averages, I suspect that dip changed a little during the
    day, and could be as much as 1' different from the tables.
    But these conjectures are not reliable because this was my first trial,
    and I suppose that my own accuracy changed during the day.
    Besides that I could only approximately extimate the height of my eye
    for the ship observations, and even worse for the observations from
    the island. (It is impossible to approach the water edge in the only
    place in the island siutable for such observation).

    3 or 4 times curious passengers asked me to explain what is this
    and what is it for. This was not easy to explain in one setnence,
    and when I tried a longer explanation, I felt that people became bored,
    and regretted that they asked:-)

    Finally I decided to pretend that I can only explain in Russian:-)


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