How Did Messages Travel Before Wires? Fresh Horses and Line of Sight

A single messenger travels at the speed a person and animal can sustain, which is limited by fatigue rather than by top speed, so a horse ridden hard covers far less ground in a day than a series of horses each ridden hard for a short distance. A relay system exploits that by stationing fresh mounts and riders at intervals, so the message travels at the sprint speed of each leg continuously. The same logic applies to signalling systems, where the limit is how far a signal can be seen and the solution is a chain of stations each passing the message onward. In both cases the message moves faster than any individual component, and the cost is maintaining the entire chain permanently whether or not anything is being sent.

Several historical networks operated at considerable scale:

• •The Persian royal road with stations at intervals of a day's ride, described admiringly by Herodotus
• •The Roman imperial courier service, using state-maintained stations along the road network
• •The Mongol system, which covered enormous distances with stations supplied by local communities
• •The Chinese postal relay, operating for many centuries with detailed regulations
• •The Inca system of running messengers, which worked without horses or writing
• •The optical telegraph in Europe from the 1790s, using towers with movable arms

The last pre-electric system is the most instructive because it was fast and its records survive. Networks of towers built across France from 1794, and later elsewhere, carried a movable apparatus whose configuration encoded characters, read through a telescope at the next tower some ten kilometres away and repeated onward. A message could cross France in a few hours, which was transformative for a state that had previously waited days. The system required an operator at every tower during daylight, was useless in fog, darkness and heavy weather, and carried a very low volume of information. Codebooks compressed common phrases into short signals. It was displaced within a few decades by the electric telegraph, which worked at night, in fog and at far higher volume.

Before the optical telegraph, simpler signalling systems carried very little and did it well. Beacon chains on hilltops carried a single bit of information, meaning that a prearranged event had occurred, which is sufficient to warn of an invasion and useless for anything else, and networks of them existed across Britain and elsewhere with the fuel maintained permanently. Smoke, drums and horns did comparable work at shorter range. Flag systems on ships developed into elaborate codes that could spell out messages, and semaphore with handheld flags remains in use. Bells announced categories of event. What these share is encoding a small number of distinguishable states, and the step to the optical telegraph was making the number of states large enough to spell arbitrary text, which required a codebook and trained operators at every station.

The physical requirements shaped the networks in visible ways. Station spacing was set by how far a horse could sprint or a telescope could see, which meant terrain determined the route as much as the destination did, and optical networks followed high ground rather than roads. Every station needed staff, stabling, fodder and security permanently, which made these systems enormously expensive and generally restricted to state use, with private messages either prohibited or charged heavily. Volume was low, so messages were compressed and prioritised. Security was a permanent problem, since anyone along the chain could read what passed, which is why codes were used. And the networks were fragile, since one failed station broke the chain entirely.

Handing a message between fresh riders lets it travel at sprint speed continuously rather than at the pace one animal can sustain, at the cost of maintaining the whole chain permanently. Optical telegraph towers spaced by how far a telescope could see carried messages across France in hours and were useless in fog or darkness. Expense restricted these systems largely to state use.