Locations "appointed as fixed points where a PoliceConstable? is to be permanently stationed from 9 p.m. to 1 a.m.
"In the event of any person springing a rattle, or persistently ringing a bell in the street or in an Area, the police will at once proceed to the spot and render assistance."
Charles Dickens, Jr., Dickens's Dictionary of London 1888 (see LondonGuidesAndMaps). Dickens provides a complete list of all fixed points, but it takes many pages of tiny script.