"Babbage... had early conceived the notion he picturesquely called "the Engine eating its own tail" by which the results of the calculation appearing in the table column might be made to affect the other columns, and thus change the instructions set into the machine. ...[A]fter a striking mathematical digression into difference functions new to mathematics, and suggested only by the operation of the engine, he built ...a machine capable of carrying out any mathematical operation instead of only the simple routine of differences ...Such a machine would need instructions both by setting in initial numbers, as in the , and also far more generally by literally telling it what operations to carry out, and in what order. [The arithmetic unit was] capable of repeated additions, of multiplication which is hardly more than that, and of reversing the procedure for subraction and division... It would work on previously obtained intermediate results, stored in the memory section... or upon freshly found numbers. It could use auxiliary functions, logarithms, or similar tabular numbers, of which it would possess its own library. It could make judgements by comparing numbers... proceeding upon lines not uniquely specified in advance... carried out wholly mechanically. ...The operation depended upon punched cards... modeled on the already well-worked-out scheme of the . ...[T]he process was elaborately safeguarded against the perils of friction, jamming, and even errors of human attendants..."