Pascaline

Pascal built it for his father Étienne, a tax commissioner in Rouen who spent his nights on sums in livres, sols and deniers. Digits were entered with a stylus into spoked wheels; subtraction was done as addition of the nines' complement, which meant reading the answer off a second row of windows.

The hard part was the carry. Pascal's sautoir raised a weight as a wheel passed nine and dropped it onto the next wheel, so a carry rippling through the whole register was driven by gravity rather than by torque from the operator's hand. It worked, which is more than several later attempts managed.

Commercially it went nowhere much. Around twenty were built and nine survive. The price ran to a couple of years' wages, and since French currency was not decimal, every machine had to be built for its particular set of units.