Semaphore

Semaphore is an instrument for signaling messages with disks and shutters in a framework and all methods of communicating by the code which grew out of it.  In 1975, a version was used by the British Admiralty for communications between London and Portsmouth.  The instruments were placed on church towers, high hills, etc., at distances of up to 10 miles apart.  With a telescope, they were efficient in fine weather.  In the present system, the relative position of two arms, human or mechanical, indicate letters.