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    Re: The repeating reflecting circle.
    From: Alexandre Eremenko
    Date: 2005 Jan 12, 19:17 -0500

    Dear George,
    today I found in Purdue library a really rare thing:
    the book of Simms "Mathematical Instruments", second
    edition 1844 which Chauvenet refers to.
    Among other things, it contains an excellent picture
    of Throughton's Dip-sector (dipmeter) with a brief history
    of its invention, and details of its use.
    This is the first clear picture of a dipmeter which I encounter.
    There is also a description of Troughton's reflecting circle.
    It is different from the repeating reflecting circles we discussed.
    And it also has a prohibited range of angles.
    (at about 30 degrees), and Simms says this explicitly and
    explains that for these angles it can be used only as a simple
    sextant (the advantages specific to reflecting circles are
    thus lost for this prohibited range).
    I may try to scan and post the pages of this book concerning
    the dipmeter and/or the reflecting circle.
    Or can mail you photocopies of these pages.
    The book is very interesting, describing a wide range of
    astronomical and surveying and plotting instruments.
    On Thu, 13 Jan 2005, George Huxtable wrote:
    > I haven't yet read the Mendoza paper (but will soon). In that geometry,
    > Alex reports that the angle at which light is blocked is from 5 to 10
    > degrees.
    This is a very rough guess I could make from the pictures.
    It can as well be around 20 degrees.

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