From GibMap

>Next to the top cable-car station, traces are left of the old Spanish, and later on British, Signal Station. By means of different coloured balls on the yard arm, all shipping movements through the straits were announced, although these were invisible in low levanter. The daily sunrise and sunset guns were fired from here, which marked the opening and closing of the City Gates. In the mid-19th Century, the public eagerly looked out for the signal that the P&O packet from U.K. had been spotted off Punta Carnero, entering the Bay of Gibraltar.

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