Building the World's First Computer: Babbage's Analytical Engine

In most of Babbage's designs, he calls for horizontal framing plates to separate each digit position in the stacks of digit wheels and pinions. Here's an example drawing from Plan 27. In this design each digit position is composed of a "cage" with digits from two interleaved numbers on the same stack. There are two primary motivations for the framing plates:  To provide a bearing surface for the wheels and pinions of a shaft so they don't bear on the ones below them. to constrain, using close holes through the plates, the lateral motion of the shafts, to the extent that they might have a tendency to bend or were not manufactured sufficiently straight. After all, for designs with 30 digits the shafts might be 9 or 10 feet long. But there are some significant drawbacks: The massive weight of all those thick iron plates. Obscuring the view of the mechanisms during operation. Making detection and repair of problems difficult because parts can't easily be accessed....

I've mostly adopted Babbage's Plan 27 arrangement for the Mill, including the use of independent columns of pinions for the various meshing: the rack with digit wheels, digit wheels with long pinions, and long pinions with the anticipating carriage. But is is appealing for the Store to use a simplification that he introduced in Plan 28: to engage the digit wheels by lifting, rather than with separate columns of pinions. That not only eliminates one of the lifting motions, but also two rotating motions for locking the digit wheels, since their vertical motion can lock them to fixed fingers on the frame.  The store wheels are duplicated many times, so having only two independent motions -- lift for meshing, and finger rotation for giving-off -- instead of five is a huge advantage. It is unfortunately not possible to do this in the Plan 27 Mill because both digit wheels in a "cage" (a pair of wheels representing the same decimal position of two different numbers) are som...