Fuller calculator - Francis WEB

This is a weird one: a spiral slide rule.

I have been following this calculator for a long time through sales, but I never really got into the details on how it works, and what we can do with it.
First of all, the first patent for this calculator dates from 1879 !! You can find a copy of it below
U.S. Patent number 219246

Almost a hundred years later, these calculators were still sold, and did not change, except for small fabrication details. In fact, you can find the serial number on one of the rule index. This number matches closely the year, and the highest one I found so far is 13473 for the year 1970. This confirms what I read about  a maximum number of 14000 units total produced of this calculator.
The earliest serial found is 890, which is probably from the year 1880. Here is a chart of the serial numbers vs the year:



I was under the impression that it was a simple addition, maybe multiplication (using logs) slide rule. However by reading the user manual, I see that we can do complex calculations.
Here is an example on how to find a side of a triangle knowing 2 other parameters:



The user manual is full of such examples.

Overall, a decent tool to perform many calculations, and certainly a commercial success if you think that it was sold for almost 100 years, without big change in the design. Certainly very classy when displayed on a desk.