What Is Semaphore? Sending News Faster Than a Horse Could Carry It

The network devised by Claude Chappe and adopted by revolutionary France from 1794 used a tall mast carrying a pivoting crossbar with a hinged arm at each end, giving a large number of distinguishable configurations. Each tower had a telescope trained on the tower behind it and one on the tower ahead, and an operator copied whatever the previous station displayed, so a configuration propagated down the line without anyone in the middle needing to know what it meant. That is the essential design: the intermediate operators were relays, not readers. The messages themselves were encoded against a codebook held only at the ends, so a signal indicated a page and a line number rather than letters, which made transmission fast and kept content secret from the operators. The first line ran between Paris and Lille and reported a military victory within about half an hour of the event, which was an unprecedented demonstration and secured the funding that grew the network to thousands of kilometres.

The technology's constraints were severe and dictated everything about how it was built and used:

• •Line of sight, so towers had to stand on high ground roughly ten to fifteen kilometres apart, and the route was chosen by terrain rather than by where people lived
• •Daylight only, since the arms could not be seen at night, which halved availability and made a message's arrival time depend on the season
• •Weather, with fog, haze, rain and low cloud stopping the network entirely, and records show substantial proportions of days lost in winter
• •Staffing, since every tower needed operators present through all usable hours, which made the network extremely expensive to run
• •Error accumulation, since a misread configuration propagated onward, addressed by acknowledgement signals passed back up the line
• •Capacity, since only one message could occupy the line at a time, so traffic was rationed and reserved for the state

The French success prompted imitation across Europe. Britain built lines from London to the naval bases at Portsmouth, Plymouth, Chatham and Great Yarmouth, initially using a shutter system with six pivoting panels and later a two-armed design, and the Admiralty ran them for naval signalling with reported times from London to Portsmouth of a few minutes in good conditions. Sweden developed its own shutter system, and Spain, Denmark, Prussia and Russia all constructed networks, with a Russian line between the capital and Warsaw among the longest ever built. The United States used optical telegraph lines commercially, including a line reporting shipping arrivals into Boston harbour, which points at what the networks were actually for in peacetime. Information that arrives before anyone else has it is valuable in markets, and the French network was the subject of a documented fraud in the 1830s in which two bankers bribed operators to introduce deliberate errors carrying financial news, which is arguably the first recorded case of tampering with a telecommunications network for profit.

The electric telegraph destroyed the optical networks within a decade of becoming practical in the 1840s, because it worked at night, worked in fog, cost far less to staff and could carry many more messages. France maintained its optical lines for a surprisingly long time out of institutional attachment and security concerns before conceding, and the last of the European networks closed in the 1850s and 1860s. The towers survive in numbers across France, Sweden and Britain, frequently as ruins on hilltops, and place names referring to telegraph hills mark the route in many countries including places where no wire was ever strung. The word itself survived in the flag signalling system used at sea and by scouts, where an operator holds two flags in positions around a clock face, which is a direct descendant of the arm positions and is still taught. The deeper survival is conceptual, since the optical network established store-and-forward relaying, coded messages, network scheduling and the idea that information could usefully move faster than people.

Towers within sight of each other, each copying the arm positions of the one behind, relayed a message across France in under an hour when a rider would have taken days. Operators in the middle relayed configurations without knowing their meaning, since codebooks sat only at the ends. Daylight, fog and staffing costs limited it severely. The electric telegraph killed the networks within a decade, and a bribery scheme to front-run financial news was run on the French line in the 1830s.