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be sent 700 mi in 24 hours. The Ming dynasty (1368-1644) used artillery as another possible signalling method. While the signalling was complex (for instance, flags of different colours could be used to indicate enemy strength), only predetermined messages could be sent. The Chinese signalling system extended well beyond the Great Wall. Signal towers away from the wall were used to give early warning of an attack. Others were built even further out as part of the protection

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