# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Dennert & Pape MHR-1 Dennert & Pape MHR-1 HR-2 Cylindrical Slide LineRuler
From: Wolfgang Hasper
Date: 2013 Jan 30, 20:54 +0100

```Gary,
thaks for poining me once more to your paper.
I did not realise how completely you have described
ways around all conceivable special cases.
Great!
I will prepare an additional label for the outer tube
in the MHR1 style to have your rules always at hand.

Best
Wolfgang

Am Montag, 28. Januar 2013 00:38 schrieb Gary LaPook:
> 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 1�. 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 0.9� 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 85� by
> interchanging the latitude and declination and then
> computing Hc which will produce an exact value of
> Hc."
>
> Also see my prior post at:
>
>
> http://fer3.com/arc/m2.aspx/Computing-azimuth-with-B
>ygrave-special-cases-LaPook-feb-2010-g11826
>
> gl
>
>
>
> --- On Sun, 1/20/13, Marc Giordan
>  wrote:
>
> From: Marc Giordan
> Subject: [NavList 22013] Re: Dennert & Pape MHR-1
> Dennert & Pape MHR-1 HR-2 Cylindrical Slide
> LineRuler To: garylapook---net
> Date: Sunday, January 20, 2013, 11:08 AM
>
> Sir,
> 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
> Yours
> Marc Giordan
> 26 rue de Paris
> 78560 le Port Marly
> (0033)675210703
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> View and reply to this message:
> http://fer3.com/arc/m2.aspx?i=122013
>
>
> Attached File: http://fer3.com/arc/img/122138.calcul
>
> Attached File:
> http://fer3.com/arc/img/122138.bygrave manual.pdf
>
>
> View and reply to this message:
> http://fer3.com/arc/m2.aspx?i=122138

```
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