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From Wikipedia :-
The One O'Clock Gun is a time signal, and is fired every day, except Sunday, at precisely 13:00. The gun was established in 1861 as a time signal for ships in the Firth of Forth, and complemented the time ball, which was installed on Nelson's Monument in 1852, but which was useless during foggy weather. The gun could easily be heard by ships in Leith Harbour, 2 miles (3.2 km) away. Because sound travels relatively slowly (approximately 343 metres per second (770 mph)), maps were produced in the 1860s to show the actual time when the sound of the gun was heard at various locations in Edinburgh.
