"[Lord Moulton] told how Babbage, one of the most brilliant inventors of the last century, spent a long life on his calculating machines, and left nothing not because his plans were impracticable, but because before ever a machine was finished a new and better idea had displaced it in his fancy."
-- H. Fletcher Moulton, The Life of Lord Moulton, 1922.
This point made by Lord Justice Fletcher Moulton in his Presidential Address to the Junior Institution of Engineers on January 22, 1904 is worth expanding on. His words on July 24, 1914 in his Inaugural Address at the Napier Tercentenary Celebration in Edinburgh, commemorating the 300th anniversary of the publishing of Napier's description of the new science of logarithms:
"One of the sad memories of my life is a visit to the celebrated mathematician and inventor, Mr Babbage. He was far advanced in age, but his mind was still as vigorous as ever. He took me through his work-rooms. In the first room I saw parts of the original Calculating Machine, which had been shown in an incomplete state many years before and had even been put to some use. I asked him about its present form. 'I have not finished it because in working at it I came on the idea of my Analytical Machine, which would do all that it was capable of doing and much more. Indeed, the idea was so much simpler that it would have taken more work to complete the Calculating Machine than to design and construct the other in its entirety, so I turned my attention to the Analytical Machine.' After a few minutes' talk, we went into the next work-room, where he showed and explained to me the working of the elements of the Analytical Machine. I asked if I could see it. 'I have never completed it,' he said, 'because I hit upon an idea of doing the same thing by a different and far more effective method, and this rendered it useless to proceed on the old lines.' Then we went into the third room. There lay scattered bits of mechanism, but I saw no trace of any working machine. Very cautiously I approached the subject, and received the dreaded answer, 'It is not constructed yet, but I am working on it, and it will take less time to construct it altogether than it would have taken to complete the Analytical Machine from the stage in which I left it.' I took leave of the old man with a heavy heart. When he died a few years later, not only had he constructed no machine, but the verdict of a jury of kind and sympathetic scientific men who were deputed to pronounce upon what he had left behind him, either in papers or in mechanism, was that everything was too incomplete to be capable of being put to any useful purpose." (Napier Tercentenary Memorial Volume, Cargill Gilston Knott, ed., 1915, p. 20)
I firmly believe that by basing the Babbage-Boole Rodulator on binary operations, and by designing the mechanisms of the machine in a modular style, more concentration can be put on the actual mathematical operations of the machine rather than on the mechanics of the machine itself. I think this is what Moulton was referring to when he accused Babbage of never settling on a design.
Babbage was so full of mechanical ideas that he never stopped improving the mechanics. But much of this intellectual energy was spent on the physical implementation of logical operations, rather than on the logical operations themselves. I believe that had Babbage settled on a general, modular mechanism, he would have been able to complete his Analytical Engine and more.