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    Re: Dennert & Pape MHR-1 Dennert & Pape MHR-1 HR-2 Cylindrical Slide LineRuler
    From: Gary LaPook
    Date: 2013 Jan 27, 15:33 -0800
    Do you have any actual instructions for using your MHR-1 for dealing with the special case when the declination is less than the lowest  declination marked on the tangent (or co-tangent) scale, 20' for the Bygrave and 30' for the MHR-1 and 50' on my flat Bygrave? I have never been able to find instructions for this situation. I am attaching a description for the use of this instrument (in French) which says it is "inutilisable" for this situation and the original Bygrave instruction manual is silent as to this situation, also attached.

    I came up with a method of dealing with this situation and it is posted on my website at:

    (In my notation "W" is the same as "y" in Bygrave's notation and "x" on the MHR-1.)

    "When declination is less than one degree you can't begin the computation the normal way to find "W" because you have to start the process with declination on the cotangent scale and this scale doesn't extend below 1º. So in this case you just skip the computation of "W" and simply set "W" equal to declination. Using this method you arrive at an azimuth that is not exact but is a close approximation and in the worst case I have found the azimuth is still within 0.9º of the true azimuth but most are much closer. If the declination is less than one degree and the latitude is also less than one degree, follow this procedure and also assume a latitude equal to one degree. After you have computed the Az you then follow the same procedure discussed above for azimuths exceeding 85º by interchanging the latitude and declination and then computing Hc which will produce an exact value of Hc."

    Also see my prior post at:



    --- On Sun, 1/20/13, Marc Giordan <marc.giordan@gmail.com> wrote:

    From: Marc Giordan <marc.giordan@gmail.com>
    Subject: [NavList 22013] Re: Dennert & Pape MHR-1 Dennert & Pape MHR-1 HR-2 Cylindrical Slide LineRuler
    To: garylapook@pacbell.net
    Date: Sunday, January 20, 2013, 11:08 AM

    I am French and also quite familiar with the use of the MHR 1 at sea (I used it for 2 atlantic crossings by sail)
    Would any one wish to be trained in using it, I am ready to help !
    The MHR 1 was bought by my father some 30 years ago at the flee market in Paris
    Marc Giordan
    26 rue de Paris
    78560 le Port Marly
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