# NavList:

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

**The Bygrave Slide Rule**

**From:**UNK

**Date:**2009 May 28, 11:32 -0700

My thanks to Ron and Gary for responding to my posting. I should say that I had already read both their papers on this topic. Indeed it was the reference to 'cotangent' in both papers that caused me so much difficulty that I had to dig deeper. I decided to post here after I had sorted out in my own mind how the Bygrave Slide Rule worked. It seemed to me there was a case for questioning the cotangent idea itself. Both Bygrave's prototype and the production models employ scales on the inner and outer cylinders which have low-numbered degrees at the left end, and high-numbered degrees at the right end of the unwrapped linear equivalent. I think we all understand why it's a good idea to have both degree scales the same way round, and we would probably also all agree that left-to-right increasing is intuitively 'right'. If we go with Ron and Gary and think of the inner scale as a log cotangent, then the log value of the left end is log(cotan(0)) = +infinity and the log value of the right end is -infinity. That is, decreasing to the right. The same applies to the outer scale if we go with Ron and Gary and consider it as a cosine scale. I am not suggesting that Ron and Gary are wrong, but that there is an alternative view and that is not wrong either. Indeed it has some good points. If I think of the inner scale as a log tangent, then the logarithm values it represents increase to the right. If I also think of the outer scale as a log secant, the log values also increase to the right. This is how a conventional slide rule is organised. The numbers and the logarithms both increase left-to-right. This feels intuitively 'nice' to me. In Ron's paper, he details an experiment in which he aligns the zero degree mark of the outer scale to the 45 degree mark on the inner scale. By noting that the 60 degree mark on the outer scale then aligns exactly with the 63 degree mark on the inner scale, and noting that cosine(60) = cotangent(63), he cites this as confirming his assertion that the two scales are indeed log cosine and log cotangent respectively. However, I would point out that secant(60) = tangent(63), so this same experimental result is consistent with my secant/tangent hypothesis too! Maybe it boils down to an arbitrary choice of interpretation. We can't even go back to Bygrave and ask his help in the argument, since he firmly labels the inner scale 'log tangent' and the outer scale 'log cosine' - a hybrid of the two alternatives. My guess is that Bygrave would say that he just wanted to keep the degree markings on both scales so that zero was on the left, which is what the user would expect. Sticking with tangents and cosines (the only functions named in any of the historical documentation), this meant that the log tangents increased to the right and the log cosines increased to the left. This wasn't a problem: it just meant careful wording of the instructions to produce the required result. He would have seen no need to rename one of the scales, since the scale names don't appear in the instructions anyway. As I said in my first posting, these instructions, printed clearly on both the prototype and production devices, transpose the sequence of operations normally used on a conventional slide rule for multiply and divide. They appear to multiply the 'cosines' when the equations require division, and vice versa. It is this that led me to the realisation that it would be more constructive to discard the cotangent/cosine hypothesis and think of the cosine scale as a 1/cosine, or secant scale. Peter Martinez -------------------------------------------------------- [Sent from archive by: peter.martinez-AT-btinternet.com] --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---