The Key-Driven Calculator
While these single-digit adding machines have been used to illustrate how the control, which was lacking in the Hill invention, had been recognized by other inventors as a necessary requisite to the key-drive, it should not be construed that such carrying control as had been applied to their inventions was of a type that could be used in the Hill machine or in any multiple-order key-driven machine. It was thirty years after the first attempt to control a key-driven machine was made before an operative multiple-order key-driven machine, with a control that would prevent over-rotation, was finally invented.
Theory versus the concrete
Theoretically, it would seem that the only feature or element lacking in the Art prior to 1886, to produce a real key-driven calculator was means that would control the carrying and also leave the carried wheel free for key actuation. It was, however, quite a different problem. Theoretical functions may be patched together to make a theoretical machine; but that is only theory and not the concrete.
All but one of the generic elements solved
To take fragmental parts of such machines as were disclosed in the Art and patch them together into anything practical was impossible, even if one had been familiar with the Art and could devise mechanism to supply the new element. That is, leaving aside the broad or generic theoretical elements, which today, from knowledge gained by later inventions, serve the make-up of a key-driven calculator, there was still lacking any concrete example or specific design of a whole machine, as there was no such machine disclosed in the drawings of patents, or any known mechanism which, if arranged in multiples, would be operative as a practical machine even if mechanism to supply the new element were to be added.
In other words, while it is conceded from our present knowledge that all but one of the generic theoretical elements had been solved as disclosed in the various before-named machines, it required the application of these elements in a different way from anything before disclosed; which in itself required a different concrete form of the generic principles for the whole machine as well as a generic form of invention covering the new theoretical element.
It may be easy to analyze that which exists, but quite a different story to conceive that which did not exist. With reference to the Art, however, the production of the new element is a feature that may be credited without question. The concrete does not enter into it other than as proof that a new feature has been created.
Originality of inventions