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    Re: Bygrave position line slide rule
    From: Wolfgang K�berer
    Date: 2004 Mar 1, 20:28 +0100

    Dear Mr. Doron,
    the Bygrave Slide Rule, a cylindrical slide rule like the Fuller or the Otis
    King, was invented in the 1920s to solve the navigational triangle in
    position line navigation. The ?Admiralty Navigation Manual?, Vol. III
    (London 1938) describes it on p. 128:
    ?The Bygrave Slide Rule consists of three concentric tubes, the innermost
    carrying a scale of logarithmic tangents and the one next to it a scale of
    logarithmic cosines, both scales being arranged in spiral form. This
    construction is made possible by dropping a perpendicular from X on the side
    PZ of the spherical triangle, as in the Ageton method, and solving the two
    right-angled triangles thus formed.?
    Another description can be found in the 1977 edition of the ?American
    Practical Navigator?, Vol. I, p. 604, in the historically interesting
    chapter on ?Comparison of various methods of Sight Reduction.There is also a
    picture of this slide rule on this page.
    A similar gadget called ?H?henrechenschieber MHR1? was produced by Dennert &
    Pape in Germany before and during WWII. Although they must have been
    produced in sizable numbers most appear to have been lost during the war
    and/or thrown away as outdated later on; so they seem to be quite scarce
    these days: One ?MHR 1? reportedly was sold on e-bay for $ 1080 a couple of
    years ago. The ?Deutsches Museum? shows a couple of ?H?henrechenschieber? in
    their new department on ?Computing? (shelf mark 78/255). I could also send
    you a picture of my ?MHR1? off-list, if you like.
    W. K?berer

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