punch card reader. Two hooks assigned to each counter can be used to either increment or decrement the register (if you accidentally select both at the same time, the machine will jam up until something snaps!). A spring-loaded lever attached to each counter gets raised whenever that counter is reading ‘zero.’ An extra hook is used for each counter to selectively pull down this lever, so that it gets ignored. A ‘zero-detect’ shaft runs the length of the machine, and gets rotated 30 degrees or so whenever a zero-detect lever gets triggered. This in-turn rotates a pulley attached to the end of the shaft, which pulls on a string, which lifts a ‘catch’ mechanism to rotate the ‘cylinder’ of the punch card reader and advance to the next instruction! Tadaa! A computer!