A classic of fantasy. A line of towers light a fire to announce an expected event (usually an invasion). I understand that this was actually used throughout history and later led to the semaphore.
How fast could a signal travel in this way? (wiki says 300 miles in a few hours but that is vague)
Who had this system well implemented in history? How far did it go? Was it ever used? Did it ever fail? Could towers see the tower one over as redundancy for a tower that failed?
Actually, any trivia or source about this topic would be very welcome.
Since the signals travel between the posts at the speed of light, the main slowdown is due to decoding and encoding the signal at the post.
Semaphore lines could apparently send a single symbol over 15 stations in 9 minutes, or a complete message of 36 symbols in 32 minutes. So a symbol would take about half a minute to get from one post to the next.
In general this is dependent on how efficient you can encode messages (taking less symbols), how many different symbols you can use (like long and short smoke bursts, maybe using different colors of smoke, if you’re using smoke signals) and how quickly you can send a single symbol (again with smoke, it will take longer to send more complex symbols).
Assuming that the signal you want to send is just the announcement of a single expected event (like, “We’ll light the beacons if we’re ever invaded and want you to send help”), then the speed would be approximately given by the maximum distance at which one could see a beacon, divided by the time that it takes to set a fire that could be seen at that distance.
This was exactly the answer I reached myself. If the next tower is 1 mile away and it takes you one minute to light that fire, then you are covering 1 mile per minute (assuming the guy in the next tower is not using the restroom or what have you).
Was this ever really used? Is there any historical data on actual speeds achieved?
Hard to find much quantifiable information about signal fires, although every authority agrees that they were used by every earlier civilization. Early semaphore info is easier to find. France had the largest network and used it to great advantage. The Wiki article sort of understates how big it was compared to the others.
That seems about right. Many places cite this quote on semaphores from the 1862 Papers on subjects connected with the duties of the Corps of Royal Engineers By Great Britain. Army. Royal Engineers:
Because it’s logarithmic. Just like, to indicate one out of eight possibilities, you don’t use 8/2 = 4 bits; you use log(8)/log(2) = 3 bits. Similarly, to indicate one out of three possibilities, you use log(3)/log(2) bits.
Put another way: With n bits, you can encode one out of 2^n possibilities. So to encode one out of 3 possibilities, the number of bits you need (in an amortized sense) is the n such that 2^n = 3. What value of n is that? It’s log(3)/log(2) = 1.585…
Well…assuming the Longfellow poem describes the situation completely, “no invasion” wasn’t one of the possibilities, but a pre-condition to valid data display. The Old North Church belfry was initialized to an ambiguous state.
Two separate fires might significantly complicate things, and might even require more than two fires at each station. The fires would have to be physically separated enough that they could be distinguished as separate light sources. That might require two teams of fire starters.
Also, if the message is being distributed in several directions at one, you would need additional fires. If there is a station directly to the south, you might separate the fires on an east-west axis, but to another station to the east or west, those two fires might look like a single light.
