Call it an exercise in elegance.

This pocket watch-like device is a circular slide rule calculator with five scales. In expert hands, it can be used to quickly compute powers, logarithms, and trigonometric functions to 3 digits of precision. Its small size and compact shape probably made it a desirable item to carry by many engineers, even though its limited precision was not sufficient for more elaborate computations.

The device has five scales, that can only be moved together by rotating the large knob. The scales cannot be moved independently of each other (they're on a single sheet of cardboard paper that serves as the watch's "face".) This shortcoming is alleviated by the fact that the device has two pointers; one is fixed as the 12 o'clock position, while the other can be rotated using the second knob.

The innermost scale is marked in degrees from 0º to 90º. The outermost scale, with values from 0 to 1, is the sine of the values on the innermost scale. I.e., labeling the scales with the letters $A$ through $E$ starting from the inside, $E=\sin A$. Scale $D$ is the power-of-ten of the value of scale $E$: $D=10^E$. Scale $B$ is the square root of scale $D$. Lastly, the values on scale $C$ are the values on scale $B$ multiplied by the square root of 10.

I am no slide rule expert, but I do believe that these scales are somewhat unorthodox. Nevertheless, it appears that they are quite sufficient for many typical engineering computational tasks. Together with a mechanical digital calculator like a Curta, an expert user could accomplish many of the same computational tasks as with today's scientific calculators, almost as easily, almost at the same speed, although with a significantly lesser precision.