Editing Tower Bridge (section)

=== Signalling and control ===
[[File:Tower Bridge London uk.jpg|thumb|Tower Bridge in 2018]]
Originally, river traffic passing beneath the bridge was required to follow several rules and signals. Daytime control was provided by red [[Railway semaphore signal|semaphore signal]]s, mounted on small control cabins on either end of both of the bridge piers. At night, coloured lights were used, in either direction, on both of the piers: two red lights to show that the bridge was closed, and two green to show that it was open. In foggy weather, a [[gong]] was sounded as well.{{sfn|Tower Bridge|1994|p=47}}

Vessels passing through the bridge were required to display signals. By day, a black ball at least {{convert|2|ft|m|2}} in diameter was mounted high up where it could be seen. Night passage called for two red lights in the same position. Foggy weather required repeated blasts from the ship's [[steam whistle]]. If a black ball was suspended from the middle of each walkway (or a red light at night) this indicated that the bridge could not be opened. These signals were repeated about {{convert|1000|yd|m}} downstream, at [[Cherry Garden Pier]], where boats needing to pass through the bridge had to hoist their signals/lights and sound their horn, as appropriate, to alert the bridge master.{{sfn|Tower Bridge|1994|p=47}}

Some of the control mechanism for the signalling equipment has been preserved and is housed in the Tower Bridge's museum.<ref>{{cite web |url=https://www.towerbridge.org.uk/Visit-Engine-Rooms/ |title=Engine Rooms |website=Tower Bridge |publisher=[[City of London Corporation]] |access-date=30 January 2020 |archive-date=9 April 2020 |archive-url=https://web.archive.org/web/20200409193415/https://www.towerbridge.org.uk/Visit-Engine-Rooms/ |url-status=live }}</ref>