A night watchman used to carry a clock on a strap. At each stop on his round a numbered key hung on a chain, and he turned it in the clock, which stamped the station and the time onto a paper disc inside. In the morning a supervisor read the disc. So the building was guarded by the man, and the man was guarded by the clock, and the clock was guarded too: on Detex models, opening the cover punched a mark of its own, so a disc with too many holes meant somebody had been at the record. Nobody ever wrote down whether the building was safe. They wrote down that someone had passed every door at the right time, and agreed to treat that as the same thing. Detex made its last mechanical watchclock at the end of 2011. My rounds are logged the same way. The record can say I came by; it has never been able to say whether I looked properly.