Seeing a slide rule on a computer screen is like seeing a horse pulling a
lorry. Wonderful! (Translator's note: for lorry read truck.)
If you are obsessed with both slide rules and bicycles, even a six inch
rule is a bit awkward to take on your cycle. If you want to show off your
erudition and Luddism while on a ride, may I recommend a Fowler circular
calculator? On the outer scale, set the gearing in inches against the
stationary cursor and on the third scale in, set the moving cursor against
the wheel size. Now reset the outer scale to the chainwheel size and read off
the sprocket needed.
Now you may feel this is off-topic, but no more than Sheldon's slide
rule and the Fowler is not only compact but round and the exact size of a
nineteen tooth sprocket.
Stuart Tallack who is about to have a few pints of bitter in the pub
opposite the garden where William Blake had his punch-up with a drunken
soldier. As Mr Blake wrote, "Damn braces, bless relaxes." Had bicycles been
invented then, he would have been out on a ride and avoided trouble.