# NavList:

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

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From: Bob Goethe
Date: 2016 Oct 20, 12:37 -0700

Good afternoon, Paul.

>>I wonder why the trig scales don't include a small overrun to the next
convenient graduation at each end. For instance, at the top left of the
rule, is there some reason T0 cannot extend down to 33′? The practice of
chopping off trig scales exactly in line with the C and D indices is
widespread on slide rules, and has long been a sore point with me.<<

I don't foresee any graphic-software reason why I should not be able to do this.  It would make the physical dimensions of the rule grow a bit...but it is already an enormous slide rule.  Maybe having it a bit wider would be no problem.

However, as I was working out test problems on my own during my design phase, I noticed that it was sometimes an issue to figure out just WHICH T-scale to use to read an answer on.  You choose the proper T-scale either by knowing the range of the answer you should be getting....

...which would be true if you were doing once-daily great-circle calculations on a trip from San Francisco to Tokyo.  You know that your azimuth/course-to-steer would only change a little bit from day to day.  So also with your Hc value.  Your distance-to-destination will change by 100 to 200 nm per day unless you are becalmed.

...or by converting your trig values into numbers and doing some math in your head.  I attempt to address this in sections 19 and 40 of the user manual.

I try to support the user's effort at doing mental math by making the scales self-documenting with the small notations out along the right edge.  For instance, scale T4 deals in values from 10 to 100.

I have, just this moment, noticed some errors in the user manual on page 71, and one on the rule itself.  I have corrected them.  Once I have rolled through the changes to the speed/time/distance calculations, I will repost all my materials.

If I extend the scales, it muddies the water a bit, in terms of mental math.

Scale ST, which currently deals in values from tangent values from 0.01 to 0.1, would - if extended - deal in values of 0.0096 to 0.105.

Not as clean for the hypothetical-slide-rule-novice for whom I am trying to design the rule and write the manual.

In part, it would make the small notations out to the right of the scales look ugly.  Making the self-documenting notations ugly offends some esthetic corner of my soul...in perhaps a way not the same as, but analogous to the way cutting the scales off grates on you, Paul.

I don't have any strong convictions on what is required to help people do mental math in year 2016.  I only know that my ability to do numbers in my head (which was pretty good in the 60s and early 70s) deteriorated as soon as electronic calculators became affordable.  I have this vague instinct that this is an area where I need the Mark 1 rule to be as simple/unambiguous as possible.

From that point of view, keeping the scales for values like 0.01 to 0.1, 0.1 to 1.0, etc. could be a good thing.  Now that I consider it further, for ME - and I am not a slide rule novice - it feels like it is a good thing to go with the existing design.

In practice, probably 9 out of 10 people who ever purchase a Mark 1 rule (and there may only be 10  who do this in the entire world) will already be members of NavList.  Perhaps designing the rule with a total slide rule novice in mind would be an error.

What do you all think about this?  What would you (plural) like to see in a navigator's slide rule, in terms of the length of trig scales?

Maybe - and I just thought of this at this very moment - I should look into open-sourcing my design.  Protect my copyright with GNU General Public License, and allow others to extend the design on their own.  I have lived my life in a proprietary world, and have never before used the GNU GPL, and have only a vague idea of what that entails.  I need to look into this.

If I DID open-source the design, then folks like Paul could create the Mark 2 Navigator's Slide Rule, even if Bob decides he likes the Mark 1 design better.  Hmmm....

>>You have 80 and 90 numbered on scale S, but the Deci-Lon omits 80. Most slide rules do.<<

I am aware of this.

When I was a youngster working with my grandfather in his shop, I was too small to really manage a hammer.  Yet I still remember him saying to me, "Sonny Boy, the whole hammer is paid for."  By which he meant, "Grab it closer to the end of the handle."

My general thinking was that "the whole slide rule is paid for", and I might as well make my notations explicit and clear, as long as the real-estate was available and it didn't add significantly to general clutter.

>>Is the alignment of the top and bottom of the body adjustable? If so, it
would be nice to include a couple marks on the top of the slide to match
the left and right indices of scale A. If the body is aligned properly,
these should coincide when the C and D indices coincide.<<

Great idea.  Thanks!  I have just made this so on my master drawing.

Bob

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