Roman Semaphore Lines and Horse Drawn Cars

[SlyDessertFox]

So, on the Roman steam engine thread, there was a lot of discussion on the Romans using semaphore lines and horse drawn railcarts. Someone suggested we make a new thread for this so we could delve more fully into the topic, and I concur.

Now I thought I'd do a little research on semaphore lines, and I was interested to find that the Greeks during the Hellenistic Age in the 4th century BC , toyed with hydraulic telegraphs. What I found even more interesting, was that, according to Polybius, this was used during the First Punic War for sending messages from Sicily to Carthage.

If I may quote Polybius here:

Aeneas, the author of the work on strategy, [writing] to find a remedy for the difficulty, advanced matters a little, but his device still fell far short of our requirements, as can be seen from his description of it.
He says that those who are about to [communicate] urgent news to each other by fire signal should procure two earthenware vessels of exactly the same width and depth, the depth being some three cubits and the width one. Then they should have corks made a little narrower than the mouths of the vessels [so that the cork slides through the neck and drops easily into the vessel] and through the middle of each cork should pass a rod graduated in equal section of three finger-breadths, each clearly marked off from the next. In each section should be written the most evident and ordinary events that occur in war, e.g., on the first, "Cavalry arrived in the country," on the second "Heavy infantry," on the third "Light-armed infantry," next "Infantry and cavalry," next "Ships," next "Corn," and so on until we have entered in all the sections the chief contingencies of which, at the present time, there is a reasonable probability in wartime. Next, he tells us to bore holes in both vessels of exactly the same size, so that they allow exactly the same escape.
Then we are to fill the vessels with water and put on the corks with the rods in them and allow the water to flow through the two apertures. When this is done it is evident that, the conditions being precisely similar, in proportion as the water escapes the two corks will sink and the rods will disappear into the vessels. When by experiment it is seen that the rapidity of escape is in both cases the same, the vessels are to be conveyed to the places in which both parties are to look after the signals and deposited there. Now whenever any of the contingencies written on the rods occurs he tells us to raise a torch and to wait until the corresponding party raises another. When both the torches are clearly visible the signaler is to lower his torch and at once allow the water to escape through the aperture. Whenever, as the corks sink, the contingency you wish to communicate reaches the mouth of the vessel he tells the signaler to raise his torch and the receivers of the signal are to stop the aperture at once and to note which of the messages written on the rods is at the mouth of the vessel. This will be the message delivered, if the apparatus works at the same pace in both cases.

We hear almost nothing about this after Polybius. Why did it fall out of use, not to be reinvented for 2,000 years, in 19th century Britain? Especially since, unlike a lot of other Greek scientific achievements during the Hellenistic Era, this one would be very useful to the Roman military for them to adopt, as they were wont to do.


Now on to horse drawn rails. I am not all too familiar with this, but here's what I can guess: The ruts could certainly be made out of wood, since they would not be as worn out as they would be with a steam powered train. Also, transport by land was expensive during the imperial period, and of course traversing the Mediterranean by sea during the winter was perilous in many spots. How much cheaper could horse drawn rails make transport by land?


Discuss!

 

[mrmandias]

On the semaphore, it really is startling that the Romans didn't use it. The only thing I can think of is that the Romans didn't do the sort of long distance command-and-control that would have made it useful, but honestly I don't really believe that. It also would have been seriously useful for Roman merchants if organized over any sort of scale. You'd think that the early warning of revolts would have been much prized too.

Wikipedia says there was a railway of sorts over the Corinthian Isthmnus during much of the Classical period. It was done by grooves in the rock though, which sounds much more capital intensive than regular rails. I also don't know if those give you the same mechanical advantage that regular rails do. If they do, though, groove rails might have been a natural for Roman roads. The rockwork is already happening and the rails would complement the original function of quick military transport by also allowing for quick transport of supplies and siege equipment.

 

[pieman97405]

Wikipedia says there was a railway of sorts over the Corinthian Isthmnus during much of the Classical period. It was done by grooves in the rock though, which sounds much more capital intensive than regular rails. I also don't know if those give you the same mechanical advantage that regular rails do. If they do, though, groove rails might have been a natural for Roman roads. The rockwork is already happening and the rails would complement the original function of quick military transport by also allowing for quick transport of supplies and siege equipment.

Unless I am mistaken, the purpose of the "railroad" over the Corinthian Isthmus was to keep the ships moving in a straight line, not to make it easier to move them while they were on land. To get the benefits of being able to move more with less power, you would need something like an iron trimed wheel and a piece of strap rail, or even just a wooden rail, as the stone grooves we'd end up seeing would do little (or nothing) to decreas friction.

 

[SlyDessertFox]

So I looked more into this, and found this site: http://www.hellenicaworld.com/Greece/Technology/en/Communication.html

I also found another passage from Polybius here, that describes what is essentially a version of an optical telegraph:

The most recent method, devised by Cleoxenus and Democleitus and perfected by myself, is quite definite and capable of dispatching with accuracy every kind of urgent message, but in practice it requires care and exact attention. It is as follows: We take the alphabet and divide it into five parts, each consisting of five letters. There is one letter less in the last division, but it makes no practical difference. Each of the two parties who are about to signal to each other must now get ready five tablets and write one division of the alphabet on each tablet, and then come to an agreement that the man who is going to signal is in the first place to raise two torches and wait until the other replies by doing the same. This is for the purpose of conveying to each other that they are both at attention. These torches having been lowered, the dispatcher of the message will now raise the first set of torches on the left side indicating which tablet is to be consulted, i.e., one torch if it is the first, two if it is the second, and so on. Next he will raise the second set on the right on the same principle to indicate what letter of the tablet the receiver should write down ... Upon their separating after coming to this understanding each of them must first have on the spot a ``viewing tube'' with two cylinders, so that with the one he can observe the space on the right of the man who is going to signal back and with the other that on the left. The tablets must be set straight up in order next the tubes, and there must be a screen before both spaces, as well the right as the left, ten feet in length and of the height of a man so that by this means the torches may be seen distinctly when raised and disappear when lowered. When all has been thus got ready on both sides, if the signaler wants to convey, for instance, that a hundred of the soldiers have deserted to the enemy, he must first of all choose words which will convey what he means in the smallest number of letters, e.g., instead of the above ``Cretans a hundred deserted us,'' for thus the letters are less than one half in number, but the same sense is conveyed. Having jotted this down on a writing-tablet he will communicate it by the torches as follows: The first letter is kappa. This being in the second division is on tablet number two, and, therefore, he must raise two torches on the left, so that the receiver may know that he has to consult the second tablet. He will now raise five torches on the right, to indicate that it is kappa, this being the fifth letter in the second division, and the receiver of the signal will note this down on his writing tablet. The dispatcher will then raise four torches on the left as rho belongs to the fourth division, and then two on the right, rho being the second letter in this division. The receiver writes down rho and so forth. This device enables any news to be definitely conveyed ... Many torches, of course, are required as the signal for each letter is a double one. But if all is properly prepared for the purpose, what is required can be done whichever system we follow. Those engaged in the work must have had proper practice, so that when it comes to putting it in action they may communicate with each other without the possibility of a mistake ... In offering these observations I am acting up to the promise I originally made at the outset of this work. For I stated that in our time all arts and sciences have so much advanced that knowledge of most of them may be said to have been reduced to a system. This is, then, one of the most useful parts of a history properly written.

That site also has more on Aeneas' hydraulic telegraph, and, crucially, the whole excerpt from Polybius on it:

"I think that as regards the system of signaling by fire, which is now of the greatest possible service in war but was formerly underdeveloped, 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 that it is always surprising how help can be brought be means of fire messages when the situation requires it. Now in former times, as fire signals were simple beacons, they were for the most part of little use to those who used them. For the service should have been performed by signals previously determined upon, and as facts are indefinite, most of them defied communication by fire signals. To take the case I just mentioned, it was possible for those who had agreed on this to convey information that a fleet had arrived at Oreus, Peparethus, or Chalcis, but when it came to some of the citizens having changed sides or 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--and it is chiefly unexpected occurrences which require instant consideration and help-- all such matters defied communication by fire signal. For it was quite impossible to have a pre-concerted code for things which there was no means of foretelling....

Aeneas, the author of the work on strategy, withing to find a remedy for the difficulty, advanced matters a little, but his device still fell far short of our requirements, as can be seen from his description of it. He says that those who are about to communication urgent news to each other by fire signal should procure two earthenware vessels of exactly the same width and depth, the depth being some three cubits and the width one. Then they should have corks made a little narrower than the mouths of the vessels [so that the cork slides through the neck and drops easily into the vessel] and through the middle of each cork should pass a rod graduated in equal section of three finger-breadths, each clearly marked off from the next. In each section should be written the most evident and ordinary events that occur in war, e.g., on the first, "Cavalry arrived in the country," on the second "Heavy infantry," on the third "Light-armed infantry," next "Infantry and cavalry," next "Ships," next "Corn," and so on until we have entered in all the sections the chief contingencies of which, at the present time, there is a reasonable probability in wartime. Next, he tells us to bore holes in both vessels of exactly the same size, so that they allow exactly the same escape. Then we are to fill the vessels with water and put on the corks with the rods in them and allow the water to flow through the two apertures. When this is done it is evident that, the conditions being precisely similar, in proportion as the water escapes the two corks will sink and the rods will disappear into the vessels. When by experiment it is seen that the rapidity of escape is in both cases the same, the vessels are to be conveyed to the places in which both parties are to look after the signals and deposited there. Now whenever any of the contingencies written on the rods occurs he tells us to raise a torch and to wait until the corresponding party raises another. When both the torches are clearly visible the signaler is to lower his torch and at once allow the water to escape through the aperture. Whenever, as the corks sink, the contingency you wish to communicate reaches the mouth of the vessel he tells the signaler to raise his torch and the receivers of the signal are to stop the aperture at once and to note which of the messages written on the rods is at the mouth of the vessel. This will be the message delivered, if the apparatus works at the same pace in both cases. "

Here's a picture of Aeneas' hydraulic telegraph:

I was also wrong in saying there is no mention of it after Polybius-It was mentioned again by Livy, who says they were used by Phillip V in 207:

From there, in order that he [Philip of Macedonia] might meet every movement of his enemies, he sent men into Phocis, Euboea and to Peparethus, to select heights from which signal fires might be visible. For himself he placed a watch-tower on Mountain Tisäon, whose peak rises to a great height, so that by fires on distant heights he might in an instant receive a message as to where his enemies were activ.

Xenophon also mentions them in his Hellenica, where it seems signals were sent via sunlight via mirrors (which would be as simple as a shield). Using fire in the night and sunlight reflected by mirrors Greeks could send messages 40-100 or more kilometers, depending on the visibility (25-62 miles).

On the fifth day as the Athenian ships sailed up, Lysander gave special instructions to the ships that were to follow them. As soon as they saw that the Athenians had disembarked and had scattered in various directions over the Chersonese--as they were now doing more freely every day, since they had to go a long way to get their food and were now actually contemptuous of Lysander for not coming out to fight--they were to sail back and to signal with a shield when they were half-way across the straits. These orders were carried out and as soon as he got the signal, Lysander ordered the fleet to sail at full speed. Thorax went with the fleet. When Conon saw that the enemy were attacking, he signaled to the Athenians to hurry back as fast as they could come to their ships. But they were scattered in all directions Xenophon, Hellenica, 405 BC

A simple method that was used also were colored flags to symbolize specific messages that were used also for a long time in the Navy.

In Alcibiades, The Lives of The Noble Grecians and Romans, by Plutarch ( 46-120), he referres to an event that took place in 410 BC:

Upon his first appearance, both sides formed a false impression; the enemy was encouraged and the Athenians terrified. But Alcibiades suddenly raised the Athenian ensign in the admiral ship, and fell upon those galleys of the Peloponnesians which had the advantage and were in pursuit.

That all comes from the sight, which has a wealth of information on Greek communication, including acoustic signals and cryptography.


So the simple question is: Why was this not picked up by the Romans? Or was it, and we just are not aware?

 

[profxyz]

Well rails are only really useful if you need to constantly send large quantities of goods over long land distances. The big trade lanes (i.e.grain and luxuries) were the Levant-Rome, North Africa-Rome and Alexandria-Rome routes (eventually Constantinople-Rome I guess), all of which could transport more faster using ships (after all, horse-drawn rail is still only as fast as a horse).

Planning and laying down rails also needed a massive empire-wide bureaucracy that the Romans didn't really have (because of the influence of local/middle-class elites). Even after Diocletian's reforms, the Roman bureaucracy was about 30,000 people (and obviously they can't all be planning/building rails and whatnot)

Same with the semaphores: I'd expect an exorbitant cost for not much benefit - you'd need another massive bureaucracy to operating the signalling stations, often over quite poor land like Northern Britannia which couldn't actually shoulder the burden of maintaining such infrastructure. Indeed, putting so many bureaucrats in far flung regions might actually increase revolts, since it would give potential usurpers a fully-functioning government right from the get-go.

And furthermore - what benefit would an earlier knowledge of invasions have brought to Rome?

A military signalling system is important primarily because you want to know where the enemy is, and where your forces are. Why is this, in turn, important? Because you needed to make sure that your forces could intercept the enemy. But this answer assumes that your forces are marshalled in some central location and have to go meet the enemy before he penetrates your borders in depth.

Which is not what the Romans did in the early Empire, when they came closest to affording this system. The Romans did not concentrate in a central location and go out to meet the enemy - they were spread out in a cordon along the border (be it the Danube, the Rhine, the Syrian limes) in garrison forts. Any raid would be immediately met and contained on the border, perhaps even before it.

And if it was anything bigger than a raid - say, a Parthian invasion - a faster knowledge of the fact in Rome wouldn't have changed much, since it would still take the better part of one season to march troops over to the East. A delay of 2-7 days isn't going to change things (usually) in such a case, since the Parthians would have to siege the walled cities in order to gain any territory. A semaphore system, therefore, required much cost/taxes/resentment for very little practical (i.e. military) benefit.

 

[SlyDessertFox]

So I just did more research, and according to this, Aeneas' hydraulic telegraph was used to great effect across Alexander The Great's (and I will presume by extension, Seleucus') empire. That source also has some nice visuals (and a more concise and clearer description IMO than Polybius:

 

[SlyDessertFox]

Well rails are only really useful if you need to constantly send large quantities of goods over long land distances. The big trade lanes (i.e.grain and luxuries) were the Levant-Rome, North Africa-Rome and Alexandria-Rome routes (eventually Constantinople-Rome I guess), all of which could transport more faster using ships (after all, horse-drawn rail is still only as fast as a horse).

Planning and laying down rails also needed a massive empire-wide bureaucracy that the Romans didn't really have (because of the influence of local/middle-class elites). Even after Diocletian's reforms, the Roman bureaucracy was about 30,000 people (and obviously they can't all be planning/building rails and whatnot)

Same with the semaphores: I'd expect an exorbitant cost for not much benefit - you'd need another massive bureaucracy to operating the signalling stations, often over quite poor land like Northern Britannia which couldn't actually shoulder the burden of maintaining such infrastructure. Indeed, putting so many bureaucrats in far flung regions might actually increase revolts, since it would give potential usurpers a fully-functioning government right from the get-go.

And furthermore - what benefit would an earlier knowledge of invasions have brought to Rome?

A military signalling system is important primarily because you want to know where the enemy is, and where your forces are. Why is this, in turn, important? Because you needed to make sure that your forces could intercept the enemy. But this answer assumes that your forces are marshalled in some central location and have to go meet the enemy before he penetrates your borders in depth.

Which is not what the Romans did in the early Empire, when they came closest to affording this system. The Romans did not concentrate in a central location and go out to meet the enemy - they were spread out in a cordon along the border (be it the Danube, the Rhine, the Syrian limes) in garrison forts. Any raid would be immediately met and contained on the border, perhaps even before it. And if it was anything bigger than a raid - say, a Parthian invasion - a faster knowledge of the fact in Rome wouldn't have changed much, since it would still take the better part of one season to march troops over to the East, so a delay of 2-7 days isn't going to change much. A semaphore system, therefore, required much cost/taxes/resentment for very little practical (i.e. military) benefit.

Does the need to transmit multiple and various messages quickly have to only be military though? Could it not be used for transmitting official messages across the empire, military or civil, as it may have been used in Alexander's empire?

 

[profxyz]

You could have used the messages for bureaucratic purposes... but that probably wasn't how the Romans ran things. It was mostly local elites and local bureaucrats doing whatever 'seemed right' for the community at the time, and it was part of the Roman deal: you accept our rule and our taxes; we let you run things. (And furthermore, what kind of messages were so important that you had to build a whole system around it instead of taking a 2-7 day delay?)

Any breach of this deal would likely have led to big unrest, especially in the East where the majority of the troops were located. And in any case, the Emperors probably trusted Roman elites to share his worldview.

 

[Richard V]

Not impressed with this telegraph. It's clever but impractical. You can only transmit this code as far as you can observe a torch signal, which is just a few miles. It can only be done at night, in good visibility conditions, with constant monitoring and still has the problem of transmitting very limited data.

Much easier just to send a messenger on a horse.

 

[mrmandias]

OTL, horse-drawn rail was first used in mines. It would be equally beneficial to the Romans to do that.

How did the Romans get grain from Ostia to Rome proper? By the Tiber? If shipping got crowded and congested, as it may have, a rail line there may have made quite a bit of sense.

A rail line allowing transhipment from the Rhone to the Rhine and from the Danube to the Adriatic would have helped Rome a lot. I can't say it would have been economical, and maybe it wouldn't have, and terrain would also have been an issue, but at least conceptually the potential is there. Maybe in England too, where you had to have troops stationed in non-coastal areas as part of your defense line. Shoot, now that I think of it the single biggest area this could have made a difference is in mesopotamia. You have these really rich areas right on the Roman frontiers that they could never really control very well because they were too far from the Med. Animal-drawn rail may have also made sense in places for Nile-Red Sea transhipment, but that would mainly be of commercial value. The main strategic value would be a better connection to mesopotamia.

Now, I think you're mostly right about the semaphores. But the Roman defense were partly structured that way because the transportation options were poor. The long Rhine - Danube - Syria/Mesopotamia frontier were simply to inaccessible by boat to move up troops rapidly. So you had to have significant forces stationed there. But this contributed significantly to the Roman Empire's governance problem. Significant forces scattered around that were hard to reach meant commanders way out on the periphery had the wherewithal to revolt and the isolation to bring their forces along with them on the revolt and the isolation to have some time to prepare before they went out to meet the Emperor or before the Emperor came breathing down their throat. But if the Emperor starved the frontier of adequate troops, which they often did, the local commanders didn't have the strength to beat off attacks.

Having better military transportation could alleviate this problem by allowing the emperor to have more of a central force. Which in turn would make semaphore or heliograph more directly useful.

 

[SlyDessertFox]

You could have used the messages for bureaucratic purposes... but that probably wasn't how the Romans ran things. It was mostly local elites and local bureaucrats doing whatever 'seemed right' for the community at the time, and it was part of the Roman deal: you accept our rule and our taxes; we let you run things. (And furthermore, what kind of messages were so important that you had to build a whole system around it instead of taking a 2-7 day delay?)

Any breach of this deal would likely have led to big unrest, especially in the East where the majority of the troops were located. And in any case, the Emperors probably trusted Roman elites to share his worldview.

I'm not so sure. The emperor received piles and piles of petitions, requests, advice requests, etc. etc. from provincial and local officials on even the smallest matters. We hear of Trajan giving his judgement on Christians in Asia when Pliny asked him as governor, and we hear of Hadrian judging on a similar inquiry in the same area-all the way from Britain. Caligula reportedly was so swamped with work that he could hardly sleep, begging for the son to come up.

And the empire did develop the large bureaucracy you are talking about eventually, under the Severans and then finally under Diocletian. So the possibility for enlarging the bureaucracy is always there.

 

[profxyz]

I'm not so sure. The emperor received piles and piles of petitions, requests, advice requests, etc. etc. from provincial and local officials on even the smallest matters. We hear of Trajan giving his judgement on Christians in Asia when Pliny asked him as governor, and we hear of Hadrian judging on a similar inquiry in the same area-all the way from Britain. Caligula reportedly was so swamped with work that he could hardly sleep, begging for the son to come up.

And the empire did develop the large bureaucracy you are talking about eventually, under the Severans and then finally under Diocletian. So the possibility for enlarging the bureaucracy is always there.

Sure, but none of those messages seem, again, to warrant the high costs/taxation/resentment that could (especially the latter) result in the adoption of semaphore. Sure a Caligula or a Commodus might well try to establish one (and good for them for trying to do so), but I think an Augustus or a Vespasian would have stuck with the perfectly-fine horse postal system.

In the end, it's all about opportunity cost: you have X amount of money; do you want to spend it on games and soldiers' wages, which definitely make the people happier; or a communications system/bureaucracy which doesn't and might even require more taxation?

 

[profxyz]

Now, I think you're mostly right about the semaphores. But the Roman defense were partly structured that way because the transportation options were poor. The long Rhine - Danube - Syria/Mesopotamia frontier were simply to inaccessible by boat to move up troops rapidly. So you had to have significant forces stationed there. But this contributed significantly to the Roman Empire's governance problem. Significant forces scattered around that were hard to reach meant commanders way out on the periphery had the wherewithal to revolt and the isolation to bring their forces along with them on the revolt and the isolation to have some time to prepare before they went out to meet the Emperor or before the Emperor came breathing down their throat. But if the Emperor starved the frontier of adequate troops, which they often did, the local commanders didn't have the strength to beat off attacks.

Having better military transportation could alleviate this problem by allowing the emperor to have more of a central force. Which in turn would make semaphore or heliograph more directly useful.

Sure, if the Romans had adopted the strategy of central forces in the early Principate (they eventually adopted it under Constantine - the comitatenses) the semaphores would likely have been a valuable addition - when they switched to it in the fourth century it was by far too late to be building something like that.

The cordon strategy of the Romans was designed with also an overarching political idea in mind, however - the idea that, given enough peace, barbarians would become good little Romans. It's not as effective if you have barbarians pillaging the borders periodically.

 

[SlyDessertFox]

I'm at a loss for how this would make the people more resentful and more inclined to rebel.

 

[profxyz]

Because of the taxes and bureaucracy that come with undertaking this project might encourage rebellion, especially as there will be people who point out that this largesse can be used somewhere else - like for soldier's wages or games.

The operative word here is 'might'. There was a dual republican-imperial legitimacy in Rome, and after Vespasian you had the loyalty of the army to contend with as well. It's not really as if being the son of an emperor shielded you from trouble (as perhaps was more the case in Han China), so you didn't have the leeway to think too long-term.

I would expect any rational Roman Emperor with such a sword of Damocles hanging over him to be inherently conservative, and not spend money on stuff that doesn't produce immediate or tangible returns (to use investment parlance) - like better communications.

 

[Reader347]

How did the Romans get grain from Ostia to Rome proper? By the Tiber? If shipping got crowded and congested, as it may have, a rail line there may have made quite a bit of sense.

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There probably were barges on the Tiber early on, water transport is the cheapest by weight/volume, but later there was a canal between Ostia harbor and Rome. A horse/oxen/man can pull many more times weight on a canal as on land. I'm sure there's an illustration of that online somewhere.

 

[Reader347]

Unless I am mistaken, the purpose of the "railroad" over the Corinthian Isthmus was to keep the ships moving in a straight line, not to make it easier to move them while they were on land. To get the benefits of being able to move more with less power, you would need something like an iron trimed wheel and a piece of strap rail, or even just a wooden rail, as the stone grooves we'd end up seeing would do little (or nothing) to decreas friction.

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Guided wheels in a smooth hard groove would still have been easier work than wheels on dirt or gravel or cobblestones.

 

[SlyDessertFox]

What if this was put into use during the republic, with Rome's constantly expanding and fluctuating borders and wars? It would be very useful to, say, be able to relay where the Macedonian army is marching and when reinforcements are coming from one place to the next.

 

[profxyz]

What if this was put into use during the republic, with Rome's constantly expanding and fluctuating borders and wars? It would be very useful to, say, be able to relay where the Macedonian army is marching and when reinforcements are coming from one place to the next.

I think the semaphore lines were always a good idea - the question was whether you could actually afford the capital/political will to create them.

The Roman Republic would have been the best time to do it, and maybe it would have actually saved the Republic as well (since the main problem with the Senate was that it wasn't designed to manage a massive empire - the main issue being communications delays, requiring lots of power be given to local generals and leaders).

Perhaps if the Macedonians/Pontians kept on defeating Roman armies sent over to 'punish' them, the Senate might have decided to have such lines in order to exercise direct control over the general.

 

[Dathi THorfinnsson]

Optical semaphores really need telescopes to be practical.

Ya, sure balefires can send a 1 bit signal 'we are being invaded', but for anything much more complicated you need telescopes.

Note the hydraulic semaphore had a limited, preselected set of a small handful of messages that could be sent. And was useless for anything more general.

Note, too, that it required identical instruments at both ends, which the Romans werent very good at.