Reader Kevin Anetsberger is a fan of nomography (basically, the use of printed charts as calculating aids) and he wrote to say that the sorts of “slide charts” I mentioned in my April 5, 2008 entry are far from extinct—and interestingly enough, the world center of manufacturing for slide charts is back in my home town, Chicago. Kevin mentions three companies, and their Web sites are worth a quick look: Perrygraf, Datalizer, IWA, and American Slide Chart.
And although this was posted in the comments on my LiveJournal mirror, it's worth reposting for everyone else: A monograph on nomographs, courtesy Bill Leininger.
I spent a little time looking for instructions on my father's circular slide rule, and by now I'm pretty sure that the device is a Dietzgen/Gilson Midget Circular Slide Rule. I found what may be a manual for it, but it consists of bad TIF scans of the pages, and it is not easy to read—and the rule itself is so worn that making out the scales in some cases is impossible. It was evidently made by Gilson but private labeled and sold by Dietzgen to fill out their product line. The operation is something I would not have guessed: You position the two sliders separately to appropriate scales, and then slide the two as a unit to a third point to read out the answer. The friction clutch is made such that sliding the short pointer moves only the short pointer, but sliding the long pointer moves them both. (I had not noticed this while fooling around with it.) So you set the long pointer first, then the short pointer (which does not disturb the position of the long pointer) and then move the short pointer by moving the long pointer, at which time the long pointer reads out the answer. Whew.
Interestingly, about 120° of the front face is much more worn than the rest of it, suggesting that my father did a lot of calculating within a relatively narrow range of values. What he used it for is only one of a multitude of things I would ask him, if only I had the chance. What he probably would have said (over a grin) is, “I made things not blow up,” which when your stock in trade is bulk methane would be a very good thing.
If circular slide rules interest you, this page presents a number of different models, none of which precisely match the specimen that I have. And if you want to make your own circular slide rule, here's a page with a full how-to. And here's another site that explains how to make both circular and linear slide rules.
