Instructions to operate the Pierre Fardoil Adding & Subtracting Machine:

- A stationary base plate in the shape of an astrolabe with a diameter of 4-1/2 inches (108mm) is divided on its outer circumference into 100 units and marked (0) 100, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 (0). Astrolabes were one of the few scientific instruments available in the 17th century and it is obvious that this form or shape Fardoil would chose to build his machine. The overall height of the machine is 5-1/2 inches (140mm).
- A second rotating disc (main disc) with 100 teeth on its 3-1/2 inch (88.7mm) measuring circumference does show two dials; the outer dial to add numbers is arranged clockwise, the inner dial to subtract numbers is arranged counter clockwise.
- The dial to add numbers starts at 0 (100) and shows marks numbered 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 (0).
- The dial utilized to subtract numbers starts at 100 (0) and shows marks numbered 90, 80, 70, 60, 50, 40, 30, 20, 10, 0 (100).
- The two dials are offset to each other by 50, presumably to simplify the needed presetting of the machine to the number from which to subtract numbers. Having the machine preset at the number 50 in its starting position to add numbers is the best position from which to preset the machine to the average number from which to subtract.
- This rotating disk can only be turned counterclockwise; a pawl protruding through the base-plate from the back at the 12:00 o’clock position prevents the disc from being turned clockwise.
- Mounted on top or in front of the rotating disc is a round casing with a diameter of 2-13/32 inches (61.5mm), and a height of 13/64 inches (5.2mm).
- On top of, or in front of, the round casing are two little discs rotating underneath two fixed hands.
- The two little discs measure 1-1/32 inch (26.2mm) in diameter and are connected to a planetary gear with a ratio of 1 to 10; turning the disc with the diameter of 3-1/2 inches from 0 to 100 will turn the two little discs 10 times.
- The actual handle to operate the machine is attached to a frame that is connected with two arms to the round casing containing the planetary gears.
- Another spring operated pawl is attached to this frame and catches the disc when the handle is turned counterclockwise to add or subtract the set or chosen number to be processed.
- The picture #13 shows the spring operated pawl engaged with the number 64; turning the operating handle counterclockwise to the stop (stationary pawl at 12:00 o’clock position) would constitute the addition of the number 64 to the number 47.
- In order to set the machine to zero, the operator has to press down the lever on the top to the right behind the base-plate, visible in the picture #2 at the 1:00 o’clock position. Pressing this lever will disengage the stationary pawl protruding through the baseplate from the back at the 12:00 o’clock position, hence free the rotating disc so it can be turned both ways to the zero position.
- To operate the machine, the operator holds the base plate with his left hand and grabs onto the operating handle with his right hand.
- The machine then will have to be set to zero to add numbers or to be preset to the number from which the operator intends to subtract numbers.
- With the operating handle, the operator then moves the operating handle clockwise to engage the moving pawl with the number to be added or subtracted.
- The operating handle then is turned counterclockwise till it is stopped by the stationary pawl.
- The previous two steps need to be repeated until all the numbers are added or subtracted.
- The numbers to be added or subtracted need to have values between 0 and 100.
- Opposite the moving pawl is a little hand attached to the round casing. This hand points to the result; outer or upper scale shows the resulting tens for addition, the inner or lower scale shows the resulting tens for subtraction. The two little rotating discs show the result for the ones; upper little disc for addition (hand points in the same direction as hand pointing to resulting tens), lower little disc for subtraction (hand points in opposite direction of hand pointing to the resulting tens).
- Both little discs turn clockwise while numbers are added or subtracted; the little disc showing the result for addition has numbers which increase, the little disc for subtraction has numbers which decrease in the direction of the rotation. Both little discs rotate 10 times 360° for a full rotation of the rotating 3-1/2 inch main disc.