# NavList:

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

**Re: Dennert & Pape MHR-1 Dennert & Pape MHR-1 HR-2 Cylindrical Slide LineRuler**

**From:**Brad Morris

**Date:**2013 Jan 27, 20:38 -0500

After poking around with German to English translators, here are the special instructions:

When the hour angle or the declination are between 0 deg and 0 deg 20 min, as well as when the hour angle is 90, the slide rule is not to be used.

Hi Gary

On the inner tube of my MHR1 (noted as tube II in the German instructions & also the scale upon which we set the declination), the minimum angular dimension is 20 min (179 deg 40 min). At the other end of this same scale I find 89 deg 40 min (90 deg 20 min).

Tube I (outer tube, upon which in the first step we set zero, or in the third step, upon which we set azimuth) starts at zero. The other end of tube I is marked 89 deg 40 min (90 deg 20 min). The double markings begin at 35 deg (145 deg). Below 35 deg, there is only a single mark, obviously just due to limited room.

I thought to mention this as your email indicates that the smallest angular dimension on this scale is 30 min. Not true for mine. I simply don't know about others.

There ARE special instructions (in German) for when "stundenwinkel" (t) and "deklination" are between 0 and 0 deg 20 min. I cannot read German. I include the text following. All typo's are mine.

Bei stundenwinkel und deklination zwischen 0 und 0 20 sowie bei stundenwinkel 90 ist der Hohenrechenschieber nicht zu verwenden.

The first 'o' in Hohenrechenschieber has the two dots above it.

Regards

BradOn Jan 27, 2013 6:39 PM, "Gary LaPook" <garylapook---.net> wrote:

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:

https://sites.google.com/site/fredienoonan/other-flight-navigation-information/modern-bygrave-slide-rule

(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:

http://fer3.com/arc/m2.aspx/Computing-azimuth-with-Bygrave-special-cases-LaPook-feb-2010-g11826

gl

--- OnSun, 1/20/13, Marc Giordanwrote:<marc.giordan---com>

From: Marc Giordan <marc.giordan---com>

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 AMSir,

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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Attached File: 122138.calcul de navigation, v2 (courante).pdf

Attached File: 122138.bygrave manual.pdf

View and reply to this message: http://fer3.com/arc/m2.aspx?i=122138