Signal Fires

Fire has the capacity for switching between two belief states, where one is indicated by the presence of fire and the other by the absence.

Greek military history provides evidence of advancements in communication deriving from signal torches.

Polybius (200 BC), a Greek historian, wrote in great detail about the communication technologies of the time.

I think that as regards the system of signalling by fire, which is now of the greatest possible service in war but was formerly undeveloped, it will be of use not to pass it over but to give it a proper discussion. It is evident to all that in every matter, and especially in warfare, the power of acting at the right time contributes very much to the success of enterprises, and fire-signals are the most efficient of all the devices which aid us to do this. For they show what has recently occurred and what is still in the course of being done, and by means of them anyone who cares to do so even if he is at a distance of three, four, or even more days’ journey can be informed. So it is always surprising how help can be brought by means of fire messages when the situation requires it.

Limitations of signal fires, according to him:

[it was possible for those who had agreed on this to convey information that a fleet had arrived […] but when it came to some of the citizens having been guilty of treachery or a massacre having taken place in the town, or anything of the kind, things that often happen, but cannot all be foreseen […] all such matters defied communication by fire-signal. For it was quite impossible to have a preconcerted code for things which there was no means of foretelling.](https://erenow.net/ancient/the-complete-histories-of-polybius/10.php)

It is one difference, and two states, which means it is effective for communicating small messages across a distance, but also has limitations.

For example, fire off means “no attack” and fire on means “attack”

But for messages such as “Coming in from the North”, “Coming in from the South”, “Move West”, “Move East”, “Send backup”, etc., the message space (i.e., the total number of possible messages) is too small.

Polybius describes a technology developed by Aeneas Tacticus — one of the earliest writers of 400 BC on the art of war. He made an improvement to the method devised by Democritus, called the Polybius square.

The method involves two locations each with 10 torches, separated into two groups of five.

First, the sender raises a torch to signal to the receiver that a message is incoming. The receiver raises a torch to signal that they are ready.

The sender then lights a number of each of the two groups. The first group of torches determines the row position in an alphabetic grid, shared by both the sender and receiver. The second group of torches determines the column position in the grid.

Where the row and the column intersect, determines the letter sent.

Each group of five torches is a symbol, which was limited to five differences, from one to five torches.

This means there are 25 differences.