This logarithmic instrument is built in the form of a pocket watch, and uses the so-called “BOUCHER” method. It consists of a fixed dial and a movable dial plate which can be moved by the knob of the device. A button can disengage the rotation of the needle.

The mobile dial contains several scales:

• Numbers on the third scale
• Two scales of square numbers
• The sines of the angles on the periphery

The fixed dial contains:

• 3 scales of cubic numbers
• A scale of logarithms on the outskirts

To do the product of 3×2: you have to rotate the dial clockwise to place the number 2 on the vertical index which is underneath the knob. One must then place the needle on the origin of the scale and finally position 3 on the needle. With this operation we have added two lengths of a circular arc corresponding to figures 2 and 3. The result 6 can be read on the index of the dial.