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Paul put up a nice posting on slide rules.

Anyone with a serious interest in these devices should take a look at the
website of the Oughtred Society. Also, in Britain, there's the UK Slide Rule
Circle (not just circular slide rules!).

I still have a couple of ten-inch rules, that once got a lot of use. The
wood-based Thornton version is still in working order, the plastic Aristo is
falling apart.

William Oughtred produced the first slide rules, in hand-engraved solid
brass. I've been allowed to handle his navigator's version, all of 18 inches
diameter, of 1630, at the History of Science Museum in Oxford. With gloved
hands, I should add. Unfortunately, it's lost its two radial arms, which
were intended to move together, around the centre,  like a pair of dividers.
I'll be taking a closer look next week. It has two log sin circles, four log
tan circles, each circuit covering a decade of change in sin or tan. So four
decades of the tan scale are spread over about 20 feet of scale length,
which must rival the Bygrave.

It doesn't aim to solve great-circle triangles, as the Bygrave does, though
I suspect it could be used in a similar three-step sequence to do that job.
It's intended for a navigator to solve rhumb-line sailing problems, such as
you would do by traverse tables, and including Mercator sailing, for which
it provides a 2-turn scale of meridional parts. I have a scanned copy of
part of Oughtred's book, translated from Latin to English in 1660, available
to anyone that asks. It's just the "navigational addition", nor the complete
text, occupying  70 of his pages. For its date, it is very advanced, for
example advocating decimal rather than sexagesimal subdivision of degrees,
though rather impenetrable.

George


contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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