Illustration of communication by mechanical semaphore in 1800s France. Lines of towers supporting semaphore masts were built within visual distance of each other. The arms of the semaphore were moved to different positions, to spell out text messages. The operators in the next tower would read the message and pass it on. Invented by Claude Chappee in 1792, semaphore was a popular communication technology in the early 19th century until the telegraph replaced it.