Over a hundred and fifty years ago, Charles Babbage designed his Difference Engine (actually, he designed it twice, the second design requiring many fewer moving parts). Less than a decade ago, it was finally built. One can be seen at the Computing History Museum in Mountain View, CA, at least until its owner finishes reinforcing his living room floor to accommodate it (hey, you can never have too many five-ton Victorian computing machines in the house).
I remember reading about the difference engine many years ago. The wisdom at the time was that it could not have been built to acceptable tolerances with the technology of the day, or if it could have been, it would have required too much force to operate.
Both of these notions have been disproved, and in fact two working examples have been built, using modern numerically-controlled milling machines, but with carefully-introduced errors to mimic the imprecision of the time (not as much as you might think -- the Victorians knew a bit about machining). With some training and practice, and with the aid of a 4:1 reduction not in the original plans, a person of average physique can literally crank the thing up and watch it produce values of a seventh-degree polynomial to thirty-one digits.
It's something to see. Forget about retro PC keyboards, this is the real deal.
Of course, the real fun would be in cranking up a Babbage Analytical Engine, but alas, the design for that one is not complete enough to implement. Not that that stopped Ada Lovelace from hacking it anyway.
