It is evident that if the Parmelee machine was ever used to add with, the operator would have to use a pussyfoot key-stroke or the numeral bar would over-shoot and give an erroneous answer, as no provision was made to overcome the momentum that could be given the numeral bar in an adding action.

Foreign digit adders
Single digit adders lack capacity

The foreign machines of the key-driven type were made by V. Schilt, 1851; F. Arzberger, 1866; Stetner, 1882; Bagge, 1882; d’Azevedo, 1884; Petetin, 1885; Maq Meyer, 1886. These foreign machines, like that of Parmelee, according to M. le Colonel d’Ocagne, were limited to the capacity of adding a single column of digits at a time. That is, either a column of units or tens or hundreds, etc., at a time. Such machines, of course, required the adding first of all the units, and a note made of the total; then the machine must be cleared and the tens figure of the total, and hundreds, if there be one, must then be added or carried over to the tens column the same as adding single columns mentally.

On account of these machines having only a capacity for adding one order or column of digits, the unit value 9 was the greatest item that could be added at a time. Thus, if the overflow in adding the units column or any other column amounted to more than one place, it required a multiple of key-depressions to put it on the register. For example, suppose the sum of adding the units columns should be 982, it would require the depression of the 9-key ten times and then the 8-key to be struck, to put the 98 on the machine. This order of manipulation had to be repeated for each denominational column of figures.

Another method that could be used in the manipulation of these single-order or digit-adding machines was to set down the sum of each order as added with its units figure arranged relative to the order it represents the sum of, and then mentally add such sums (see example below) the same as you would set down the sums in multiplication and add them together.

Example of method that may be used with single column adder.

Such machines, of course, never became popular because of their limited capacity, which required many extra movements and caused mental strain without offering an increase in speed of calculation as compared with expert mental calculation. There were a number of patents issued in the United States on machines of this class which may well be named single digit adders.

Some early U.S. patents on single-digit adding machines

The machines of this type which were patented in the United States, preceding the first practical multiple order modern machine, were patented by D. D. Parmelee, 1850; W. Robjohn, 1872; D. Carroll, 1876; Borland & Hoffman, 1878; M. Bouchet, 1883; A. Stetner, 1883; Spalding, 1884; L. M. Swem, 1885 and 1886; P. T. Lindholm, 1886; and B. F. Smith, 1887. All of these machines varied in construction but not in principle. Some were really operative and others inoperative, but all lacked what may be termed useful capacity.