The first attempt at a positive control for a key-driven adding device is found in a patent issued to W. Robjohn in 1872 (see illustration). As will be noted, this machine was referred to in the foregoing discussion as merely a single-digit adding machine, having the capacity for adding but one column of digits at a time. ROBJOHN MACHINE Referring to the illustration of the patent drawings of the Robjohn machine, it will be noted that there are three sight openings in the casing through which the registration of the numeral wheels may be read. The numeral wheels, like those of all machines of this character, are connected by devices of a similar nature to those in the Hill machine for carrying the tens, one operating between the units and tens wheel and another between the tens and hundredths wheel. [Sidenote: _Description of Robjohn machine_] The units wheel shown in Fig. 3 is connected by gearing to a long pin-wheel rotor, marked E, so that any rotation of the rotor E, will give a like rotation to the units numeral wheel to which it is entrained by gearing. To each of the nine digital keys, marked B, is attached an engaging and disengaging sector gear device, which, as shown in Fig. 3, although normally not in engagement with the rotor E, will upon depression of its attached key, engage the rotor and turn it. A stop device is supplied for the key action, which in turn was supposed to stop the gear action; that seems rather doubtful. However, an alternative device is shown in Figs. 4 and 5, which provides what may without question be called a stop device to prevent over-rotation of the units wheel under direct key action. [Illustration: From Drawings of Bouchet Patent 314,561] It will be noted that the engaging and disengaging gear device is here shown in the form of a gear-toothed rack and that the key stem is provided with a projecting arm ending in a downwardly projecting tooth or detent which may engage the rotor E, and stop it at the end of the downward key action. While the stopping of the rotor shows a control in the Robjohn machine which takes place under direct action from the keys to prevent overthrow of the units numeral wheel, it did not prevent the overflow of the higher or tens wheels, if a carry should take place. There was no provision for a control of the numeral wheels under the action received from the carry of the tens by the transfer mechanism. [Sidenote: _First control for a carried numeral wheel_]