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Jaap Scherphuis
@jaapsch.net

I just added to my site this manual for the Thales GEO double-register mechanical calculator, and uploaded a scan to archive .org. archive.org/details/thal... This was a special model with two input registers and two accumulator registers, attached to a single crank. I don't have this model, but...

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Jaap Scherphuis
@jaapsch.net

I own a similar Brunsviga D 13Z/1. These were used for land surveying calculations. You can easily input the x,y components of a unit vector in the input (e.g. the sine/cosine of an angle from a lookup table), and multiply that by the length, adding it to the vector in the accumulator registers.

Jaap Scherphuis
@jaapsch.net

They were also useful for the military for other kinds of coordinate calculations. I have a letter from a US army colonel to Kurt Herzstark about a double-Curta calulator, including a copy of the design. Herzstark patented this variant of his Curta, but afaik it was never produced.