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Nothing in this world is perfect. You guys have identified a problem 
that does not arise in practice with this model of the Bygrave. To the 
level of accuracy expected from this device, the scale distortion 
produced errors that you guys are concerned about just don't occur. The 
errors you are concerned about "fall into the noise" of the two minute 
expected accuracy. I have made 12 of these and worked more than a 
hundred sample problems and checked the results against the results from 
a digital calculator and all the results agreed within two minutes of 
arc. Two minute accuracy is sufficient for practical off shore 
navigation and is certainly good enough for a "backup" system. Even my 
ten inch long Keuffel & Esser 4080-3 slide rule can produce calculated 
altitudes that are accurate enough for off shore navigation but not at 
all places on the scales as they become bunched near the ends. What 
makes this flat version of the Bygrave very good for celestial 
navigation is that the cotangent scale is not ten inches long it is 
351.5 inches long, 29.3 feet, 8.9 meters! Having logarithmic scales this 
long allows for much greater accuracy than from a ten inch long slide 
rule and they consistently produce results agreeing within two minutes 
and often are in exact agreement.

Many of the buildings you stand in, many of the airplanes that you fly 
in and most of the bridges that you drive across today were designed 
with the use of slide rules so they have for many years provided 
calculations that we still rely upon for our daily safety.

So make one and give it a try, just don't expect agreement within 
one-tenth of a minute and you will see the usefulness of this for a 
backup celnav system. As it says on the side of the medicine bottle, 
"safe when used as directed."

gl


Brad Morris wrote:
>
> This problem does not exist on a �real� Bygrave because the cylinders 
> are stiff and remain concentric with each other. I do recognize the 
> difficulty in getting the scales mounted and keeping them referenced 
> to each other when going through the zig-zag pattern of solution.
>
> I think that local distortions of the one scale to the other will 
> clearly result in errors. Slide rules in general work when the one 
> logarithmic scale is referenced to another logarithmic scale. 
> Distorting one or the other cannot be permitted. NSG21 is absolutely 
> correct.
>
> I remember being the last class to take slide rule instruction in High 
> School. When you use a slide rule today, most people think of it as 
> black magic and have no idea how it works. Further, the electronic 
> calculator leads young engineers to give me as many decimal places as 
> their calculator does, without judgment as to the meaning of those 
> digits. Empty resolution without addition to accuracy.
>
> Best Regards
>
> Brad
>
> *From:* NavList@fer3.com [mailto:NavList@fer3.com] *On 
> Behalf Of *Gary LaPook
> *Sent:* Wednesday, February 25, 2009 4:39 PM
> *To:* NavList@fer3.com
> *Subject:* [NavList 7430] Re: New compact backup CELNAV system
>
> I haven't seen the problem you mentioned. I sealed the Cotangent scale 
> in normal plastic protection sheets (about one buck each at Fryes) 
> used for protecting documents which are quite rigid. I will experiment 
> with bending the scale and working a sample problem and get back to you.
>
> gl
>
> --- On *Wed, 2/25/09, nsg21@hotmail.com //* wrote:
>
> From: nsg21@hotmail.com 
> Subject: [NavList 7427] Re: New compact backup CELNAV system
> To: NavList@fer3.com
> Date: Wednesday, February 25, 2009, 9:47 AM
>
> I would like to share some experience in using this (transparency over printed)
>
> style of the slide rule. The surface it is placed for calculation MUST
>  BE
>
> ABSOLUTELY flat. Even small warping of the surface (such as normally found on
>
> small plastic tables) leads to big errors in calculations.
>
>   
>
>   
>
>   
>
> From: glapook---NET
>
> Date: Tues, Feb 24 2009 12:06 pm
>
> > There are often posts on the Navlist regarding using celestial as a backup
>
> to
>
> > GPS and finding a simple way to do this. I think I have found a method
>
> that
>
> > is simple, self contained, takes up little space, needs no almanac or
>
> sight
>
> > reduction tables 
>
> ...
>
>   
>
>   
>
>   
>   
>
>   
>
>   
>
>
> >


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