in computer science semaphores are constructs which are used to protect access to shared resources. Few centuries back Semaphores had a very different meaning. They were knows as optical telegraphs and were used to trasfer information by means of visual signals. Think of it as a series of relay towers each within visible range and operators conveying information using visual signals such as flags , mechanical arms with different position representing different things.Early messages would pass through 15 stations in 9 minutes, and speed varied with weather, and obviously it all would happen only from dawn to dusk. It was all before electric telegraphs which when introduces had to face resistance from it on the ground that any one can break the system by cutting the wire. Optical method regained its ground with the introduction of optical fibres which are backbone of todays communication infrastructure.
last year adobe has installed a semaphore atop its Almaden Tower which transmits a coded message.From SanJoseSemaphore – “When the Semaphore was launched on August 7, 2006, the challenge of decoding of its message was put to the public. On August 14, 2007, the solution was announced, and the winning code-breaking team of Mark Snesrud and Bob Mayo was honored for being the first (and only) team to break the code. The solution: For the past year, the San Jose semaphore has been broadcasting the full text of Thomas Pynchon’s 1966 novel, The Crying of Lot 49.”