During the demo of Charle’s Babbage’s Difference Engine #2 at the Computer History Museum, a docent turns a large hand crank to compute values of a 7th order polynomial by divided differences and print the results. After the demo I noticed another crank on the opposite end of the machine, awkwardly positioned near the ground:
jjb: “What’s this other crank for?”
docent: “We use that to debug the machine.”
A clutch on the drive line connecting the operator crank to the printer allows the printer and calculator to be decoupled. The secondary crank can then operate the printer independent of the calculator. Classic divide and conquer debugging.