There's a great early science fiction novel about the possible effect of semaphore communication technology on the Roman empire — L. Sprague de Camp's "Lest Dar

There's a great early science fiction novel about the possible effect of semaphore communication technology on the Roman empire — L. Sprague de Camp's "Lest Darkness Fall", http://en.wikipedia.org/wiki/Lest_Darkness_Fall http://en.wikipedia.org/wiki/Lest_Darkness_Fall. In the novel, de Camp argues that a time traveler with this technology (and foreknowledge of history) could have prevented the Dark Ages.