On 18 February 1867, François Marie Auguste Barre, a Parisian merchant, got a 15-years patent for Machine à compter (calculating machine, see the French patent N° 75026). In the same year, the device was presented during the Exposition Universelle (International Exposition, the second world’s fair was held in Paris, from 1 April to 3 November 1867).
The calculating machine of Barre was a simple, keyboard-operated counter, which can be used to add a single digit number by pressing on keys. In fact, due to its limited action, the appliance has only educational value, and cannot be used as a real calculating machine. The device was later described in the book Enciclopedia delle Arti e Industrie, volume 5, Macchine da Calcolare from Giuseppe Pastore (Torino, Italy, 1885).
Let’s examine the operation of the machine, using a drawing from the abovementioned book (see the image below).
This device consists of a nine-key keyboard A, carried by two levers C, which can rotate up of the same rotation axis. The nine keys are inscribed respectively with the numbers 1, 2, 3, … 9. They extend below with small parallelepiped rods of different lengths, arranged so that when the operator presses one of the keys with his finger, the keyboard A goes down, and the key the operator touch takes a position such as to come to meet the lower floor D; once the key is released, keyboard A returns to its initial position by virtue of a counterweight.
The rotation axis of the two levers C carries a drum, on which the next ones are marked at equal distances whole numbers 0, 1, 2, 3,… up to a certain number (depending on the size of the appliance). This drum is fitted with a crown toothed on which a pawl acts, by means of which the descent of the keyboard A produces the rotation of the drum, while during the ascent there is no communication of movement between the levers C and the drum itself.
The length of the various keys is such that the corresponding rotation of the keyboard, and therefore of the drum, is proportional to the number marked on the key itself; so that, if the drum has the digit 0 at the index, when the operator presses for example key 7, the number 7 of the drum is brought to the index. If later the operator pushes a second key, e.g. 9, the drum advances another 9 digits and the index marks the sum 7 + 9 = 16; so you can keep adding more numbers, until the maximum straight number is exceeded on the drum.
There is a panel N, arranged in the base M of the appliance, consisting of 10 numbered slots, which can be slid by hand into special grooves. Using this panel, the operator can keep track of a total or any other number that needs to be remembered.