Caesar’s Rhine Bridge – Rome Showcases its Technical Prowess

Obviously, there are gaps in our knowledge of Roman construction techniques and tools because we lack either written or physical evidence of these. Non-permanent materials have eroded away over two millennia and their characteristics elude us. Great structures like Hadrian’s Wall survive because of the use of stone or concrete.

There is, however,  one clear example of a non-permanent structure which we know a lot about – Caesar’s first bridge over the Rhine. We know the story because Caesar tells us in volume four of The Conquest of Gaul. But construction techniques are only part of the story. Caesar built this bridge to show the Germans they were never safe from him because he had the skills to cross the river and attack them.

In 55 B.C, the German tribes Usipetes and Tenctheri crossed the Rhine to the Roman occupied west bank and plundered corn they found there. Crossing back over they joined forces with the Sugambri tribe. When Caesar sent an embassy to the Sugambri and asked that the culprits be returned to him, the response was that the Rhine was the limit of Roman authority. Simultaneously, the Ubii tribe, an ally of Rome, asked for help against the Suebi tribe who were harassing them.

Caesar, deciding he had enough reasons to cross the Rhine, planned an expedition to attack the Sugambri.

Downstream from Koblenz, there are two small towns named Andernach and Neuwied. Historical scholarship suggests Caesar planned to cross the Rhine in between these two locations. The distance between the shorelines in this area is 800 to 1000 feet. The average depth of the river is 16 feet.

Before we talk about the construction of the bridge, it will be useful to show a photograph of the model that sits in The Museo Della Civilta Romana in Rome.

 

Before starting construction, the Romans built a crane on a raft and floated it out into the river. You can see the framework of this apparatus on the left of the picture. The piers supporting the bridge were constructed of one and a half foot thick logs. These tigna bina were tied together in pairs with a two foot gap between them. They were then driven into the river bed at an oblique angle using a pile driver. This was apparently a large stone, attached to block and tackle on poles, that could be swung at the posts once they were tied into position. Once two sets of these posts were secure a large log (fibulae) was placed between them in the slot formed by the two foot gap. Angled supporting posts were tied to the tigna bina to provide additional support.

Caesar says the piles were positioned forty feet apart before they were driven into the riverbed.

Once the support structure was in place, logs were laid across the piers and then boards were used to form the roadway.

I estimate that the piers were about twenty feet apart so a thousand foot river span would have required 50 piers. The bridge was built in ten days which meant workers would have placed five piers per day assuming the roadway was built as a parallel process over piers already completed.

The Romans had a variety of familiar tools at their disposal, including adzes, hammers, saws for cutting boards, and block and tackle. They used 9 inch nails to fasten boards together.

Once the bridge was complete, the army crossed over the Rhine. They spent 18 days in the land of the Sugambri, burning abandoned villages because the enemy had fled their homeland in fear of the Roman juggernaut. After treating with the Ubii, Caesar crossed back over the Rhine and destroyed the bridge behind him. Ten days in the making, Caesar’s Rhine bridge lived for eighteen days.

A footnote to the story. There have been endless arguments about this event among scholars. Some reinterpret Caesar’s Latin and challenge the construction methods described. Others question the engineering saying that the bridge would not have held. As an engineer, I am comfortable with the construction methods described and think these debates are a waste of time. Scholars will be scholars, however, and they’ll debate any issue until each has had his say. How many engineer historians are there, anyway?

Roman Field Camp Construction and Engineering

As we have discussed in the past, the Romans were an extremely methodical race. While the Greeks were happy to contemplate the universe, the Romans were always in left brain overdrive --  breaking everything down into a structure that could be duplicated -- over and over again. So it was with field camp engineering.

I define field camp here as a short-term stopover for a field army traveling between two points. The destination could be permanent summer camp, permanent winter camp, or battle. Field camp construction is most impressive when you compare the effort involved with the short duration. You might imagine that at the end of a hard day of marching the commanders would pick out a clearing, pitch some tents, and call it a day. Not even close.

The marching army included an engineer unit which was dispatched to locate and prepare the camp site for the troops. They selected a site which would meet as many of the tactical requirements of the army as possible. The first and most important criteria was location. The engineers looked for a patch of even ground elevated from the surrounding area near a water supply. Terrain sloping away from the campsite would made attacks more difficult.

Once the site had been selected, surveyors precisely measured out the walls, roads, and sites for the structures to be placed in the camp.

 

This diagram shows the principal landmarks. The camp was formed by intersecting roads, with gates at the end of each. The main building of the camp was the Principia, located at the intersection of the two main roads.

The walls of the camp were designed to neutralize an attack. The first step in building them was to dig a trench six feet deep. When the trench was complete its sides were graded into a “V” to prevent an attacker from jumping across. The soil from the trench hole was then piled into a rampart frame of logs, making the total thickness of the rampart 3-5 meters. The top of the rampart was covered with palisades to produce a fence. Each legionnaire carried two 6 foot palisades in his kit which were tied together when placed.

Obstacles were never placed more than 40 meters from the defensive wall because javelins could not be hurled with accuracy beyond that distance.

 

Camp gates were designed to prevent the rush of an attack force. Techniques to prevent this usually involved a ditch in front of the gate or an angling the wall at the gate formed into a curve. Roads in the camp were 18 meters wide which was also the distance from the rampart to the first tent line.

Camp structures were sized according to importance of the occupant. The commander occupied the Praetorium (part of the principia), while the tribunes had their own barracks. The legionnaires slept in leather (calf or goat) tents designed to be waterproof. Each tent was ten feet square and housed eight men. Each century had eight tents for eighty men since sixteen were always on guard duty. Centurions had their own larger tent.

The camp was essentially a small city with wagon and horse parks, a hospital, washing and latrine facilities, and a cook tent. The granary was constructed to keep stored grain ventilated and raised off the ground.

Roman Siege of the Fortress Masada

Masada is a barren and desolate place forsaken by God. Two hours south of Jerusalem, it stands as nature’s fortress against those who would try to conquer it. Ironically it was also the place God’s chosen people had to defend against the Roman military machine of 72-73 A.D.

The photos below give a sense of this barren landscape:

 

The upper photo is a view from 16 miles up, showing the Dead Sea; below it is the fortress from 10,000 feet. At Masada one finds no water or vegetation, only heat. It sits in the Desert of Judah stretching from 21 miles west at Hebron to the Dead Sea, where the Jordan River ends. Masada is a lozenge-shaped table mountain 730 yards long and 215 yards wide, sitting 440 feet above the Dead Sea. Its main access point is a steep old road from the east called the Snake Path.

Masada was one of the three treasuries of King Herod, who also had a small palace there. Fortified by Jonathan, brother of Judas Maccabeus circa 140 B.C, protected by a Jewish garrison until the establishment of Judea as a Roman province in 6 A.D, and retaken by the Jewish soldiers under Manaemus during the confusion of the Jewish revolt of 66 A.D, it was overrun by the Sicarii in 70 A.D. Under their leader, Eleazar, the Sicarii set about terrorizing the surrounding territory.

Formed in the 50s A.D, Sicarii was a fringe sect of Zionist fanatics known for carrying concealed daggers (Sicae) and using them against their enemies and Jews they considered enemy sympathizers. Their lives were dedicated to the expulsion of the Romans from the promised land by terror or whatever means they could employ. When the Roman general Titus captured Jerusalem in 70 A.D, the stage was set to retake Masada, destroy the Sicarii and return the fortress to Roman control. Flavius Silva was given command of the assault in 72 A.D. and immediately set to his task.

It is estimated that the fortress held approximately 1,000 people including women and children. Of these there were about 500 warriors.

Silva’s approach, like all Roman generals, was methodical in the extreme. He knew Rome could not afford to have any of the Sicarii survive the attack so he put together a plan to deploy a series of forts and built a circumvallation around the rock.

The photograph above provides a recent view of the fortress. At the lower right you can see the rectangular outline of Camp B, positioned to allow line of site observations of Masada. 

Above is another aerial view of Masada showing the circumvallation. Other key points are also labeled. According to Josephus, the Romano-Jewish historian, the 8,580 yard circumvallation and thirteen attached forts were built to completion in three days by four legions. Its walls were ten foot high piles of rubble, the only raw material available at the site. At eighty to one hundred yard intervals in the wall, towers were constructed for the purpose of observation and archery attacks.  

Eight field camps were also constructed with unique features designed to protect troops from the heat. Stone barracks were substituted  for the traditional field tents with tent material used for roofing. This provided more headroom and better air circulation in a climate where daily high temperatures range from the high eighties to low one hundreds from May through October.

To the west of the fortress sits a naturally occurring chalk ridge nicknamed the “white spur.” 

 

This ridge was the only logical means of moving a siege tower up to the fortress, but it could not be utilized until a proper ramp had been constructed. So the Romans built a twenty foot wide causeway 675 feet long rising 225 feet at a one to three incline. It was 700 feet wide at the bottom and was capped by a 75 foot wide stone platform at the top.

The focus of the assault was a 90 foot siege tower, built with a battering ram, ballastas and scorpiones. As the Roman soldiers pushed it along, it was protected by ballistas located on the sides of the spur. Once the tower reached the fortress wall the ram was put to work attacking the gate and blazing torches were sent over the wall. The next morning the Romans threw gangways from the tower to the top of the walls and entered the fortress. All inside were dead. The Jews had arranged a mass murder to avoid the taboo of suicide. Each Jew was assigned someone to kill in progression until only one was left and he committed suicide. Two women were found alive hiding in a basement and revealed what happened.

We know that the final assault took place in May of 73 A.D, but the rest of the timetable is obscure. If Silva took command at the end of 72, he would have put his plans together before beginning construction, perhaps in early 73 A.D. This was not a classic siege where the goal is starving out the enemy. The Sicarii were small in number and had an adequate food supply. The timetable for victory was determined by the availability of the siege tower and its ramp. Once the tower had been pushed to the walls of Masada, Roman forces were able to enter the fortress and the assault came to an end.

Today Masada is a monument preserved by desert heat -- the structures and raw materials of 73 A.D. left for us to view. Monument to a cult who chose death rather than surrender and to those who took the rock from them.

The Mystery of the Ancient Oarsmen

Any study of ancient sea battles generates related discussions that go beyond numbers of ships, men, and battles. For this post I have selected one of those -- the oarsmen who acted as the means of propulsion for the ancient warship.

This subject is interesting because no one knows how the warships of antiquity were propelled. We know the ships had oarsmen, but where were the oarsmen placed and how were they organized to maximize the ship’s power and speed? The number of oars had to be chosen to avoid collisions between them, which when they occurred, would destroy the rhythm of the stroke and cause a loss of the ship’s propulsive force.

The trireme, original warship of the ancient world, was developed from the pentekontor, an open hulled craft which commonly used 25 rowers on each side. This is Homer’s craft and the craft of the Vikings. Extant examples of the pentekontor can be seen in the Oslo History Museum.

The trireme was developed during the early fifth century B.C. in Greece probably from designs the Phoenicians were using. The “tri” in trireme refers to the three types of oarsmen in the ship - thalamites, zygites, and thranites. Why these types existed and how they functioned is a mystery.

There are four principle ways to configure oarsmen in a trireme: in a single line on each side of the boat; vertically, one group above another; assigning multiple rowers per oar; or placing rowers next to each other on a common bench. The following discussion will attempt to determine which of these options was in use.

Greek triremes had a length of approximately 120 feet, as verified from available dry-dock ruins. One hundred twenty feet is also the engineering maximum because, even with internal support cabling, it can be shown that wave action will break a longer wooden ship in half. Because of the limit on the number of oarsmen who could be placed on each side of the ship, designers had to come up with other ways to arrange them.

The absence of historical writing on the configuration of the rowers leaves us in the dark – but we have other sources. The poets, philosophers, and playwrights of Athens have come to the rescue with tidbits of information.

From the standpoint of pictorial evidence (pottery and the like), we come up empty. The most widely analyzed artifact showing a trireme is called the Lenomant of the Acropolis believed to have been carved in 400 B.C.

The arguments over what this image tells us about the rower positioning are on-going.

Many historians assert the three types were placed along each side of the ship at three levels bottom to top and superimposed over each other, but this configuration is problematic. Collisions between the oars were likely and the requirement for longer oars for oarsmen farther from the water sacrificed mechanical advantage because of the inefficient angle of attack.

Regarding multiple rowers per oar we take Thucydides word (Book 2, 93.2) that each rower during the Peloponnesian War used one oar.

That leaves us with the rowers seated next to each other as the remaining option. Let us look at some quotes from those who have made a contribution to solving the mystery.

*In Aeschylus’ Agamemmon the statement is made that the thalamites sit below the zygites.

*Polybius describing a collision during the battle if Chios in 201 B.C. states that one ship penetrated the middle of the hull of the other under the thranites thole (fulcrum pin).

*Polyaenus notes that in a storm additional thranite oars were put out to aid in steering indicating that the thranites sat in the rear of the ship. Some historians believe that the Thranites were in the aft position, the zygites in the center, and the thalamites in the bow. This suggests a bow to stern division of rowers as opposed to a vertical division which doesn’t make sense given the need to layer the rowers and equip them with different length oars.

*A trireme oar inventory has been uncovered which lists 62 thranite oars, 54 zygite oars, and 54 thalamite oars.

*There is an Athenian dockyard record that indicates that thranite oars no longer serviceable could be made into zygite oars, i.e. be shortened by cutting off the worn parts.

*Aristotle in his Mechanics says that the mesoneoi (young rowers) sat in the “middle” and had the longest oars. One assumes he uses middle to mean the center line or farthest from the side of the ship.

In the crude drawings below I show how the rowers could have been positioned to satisfy all the historical statements. The rowers sat on benches angled to the back of the ship, with the thranites, zygites, and thalamites seated next to each other. The bench was stepped to allow the rowers to swing their oar without hitting the men sitting next to them.

A demonstration of this configuration was made using oar lengths of 10,12 and 13.5 feet and showed good results.

During the Golden Age of Athens, the trireme was the warship of choice. A dockyard record from 325 B.C, however, lists 360 triremes, 50 quadriremes, and 7 quinqueremes indicating an evolution in craft types.

By the First Punic War the Romans were using mainly quinqueremes as discussed in the previous article. With incomplete knowledge of how triremes were manned, we must throw up our hands as we contemplate how a quinquereme could be rowed using five groups of rowers. Most likely there was a group who moved their oars from a standing position, but this is a guess.

A few more relevant points are interesting. The trireme was said to be capable of 8 knots, which is good compared to sailing ships of the time. Pliny cites some Roman sailing records the fastest of which is a run from Ostia to Africa of 270 nautical miles at an average speed of 6 knots. How long the rowers could maintain their speed is unknown. One would imagine exhaustion setting in after 10-20 minutes, but we don’t know whether all rowers were pulling in order to make an 8 knot speed.

And one more thing. Rowers were not slaves as portrayed in popular culture – at least the Greek oarsmen weren’t. There are pay slips extant showing that the thalamite rowers were paid less because their oars were shorter.

Ecnomus – One of the Greatest Naval Battles of Antiquity

The naval battle at Ecnomus between the Romans and Carthaginians sits at the top of the list of great sea battles of antiquity along with Salamis. Ecnomus was pivotal to Rome’s success in the first Punic War because it established Rome as a legitimate sea power, equal to the Carthaginians, and set the stage for the Roman invasion of Africa. Our best source of information about the battle comes from Polybius writing one hundred years after the event.

In 256 B.C. Rome assembled a large naval force for the purpose of defeating the Punic navy and opening the door to an invasion of Africa. The fleet supposedly included some 330 ships including troop transports, although there has been much debate about the accuracy of these numbers. The Carthaginian fleet was approximately equal to that of the Romans.

The Roman plan was to sail south from Messana round Cape Pachyrus and head west to Cape Ecnomus where they would rendezvous with Roman land forces encamped there. Then the fleet would proceed on to Africa. If the Punic fleet was encountered, the Romans planned to drive it out of their way.

The Carthaginians through their spy network were able to follow the Roman advance and arrange the battle on their schedule. Moreover the Carthaginians had spied on Roman maneuvers where the attack formation was rehearsed.

The diagram above shows the Roman attack “V” made up of force 1 and 2. Force 3 overlapped the "V" and completed the triangle formation. It was made up mainly of troop transports. Force 4 was an auxiliary battle fleet nicknamed “Triarii”. 

The Punic fleet consisted of a long line stretched across the Roman front. The left flank of this line was pulled in at an oblique angle because of the nearby coastline.

The diagram shown above depicts the Punic battle plan. Hamilcar planned to collapse his center so his wings could attack the Roman fleet from the flanks. This is in fact what happened during the battle.

This diagram shows the progression of Hamilcar’s attack. He made the mistake of focusing his forces on the transports, Triarii, and the rear of force 1 and 2 as shown above.  That allowed large segments of force 1 and 2 to wheel around and attack the Punic force from behind after they had disposed of the Punic center.

 

The above diagram depicts the way the Romans were able to achieve victory by a circular attack. Hamilcar lost 30 ships and had 64 taken, while Rome lost 24 ships.

One reason why this battle is interesting is that it anticipates the later military battles with Hannibal. The Romans always used massive forces to punch through the center of the enemy line, unlike the Carthaginians who preferred to let their center sag and set up an attack from the wings. Regarding the interesting use of the word "Triarii" which was the familiar designation for the third (and most experienced) line in the Roman army formation behind the Hastati and Principes, the term as used at Ecnomus likely refers to "third" line and has no relationship to the sailor's experience.

Roman Naval Battles of the First Punic War – Introduction

The Punic Wars fought between Rome and the Carthaginian Empire comprised three separate encounters over 118 years, starting in 264 B.C. and ending in 146 B.C. The first was a battle for control of Sicily, the second the famous war with Hannibal, and the third resulted in the destruction of the Carthaginians. The first war, which we will concentrate on here, saw the initial efforts by Rome to deploy naval forces and use them in battle. The fact that Sicily is an island made this war as much about navies as armies.

The Roman people were not seafaring by nature. The city was 16 miles from the coast and their focus had always been on agriculture rather than trade. Before the advent of the Punic Wars Rome did not possess navy or merchant marine because she did not need them. Her wars were fought on land and she relied on the Greek traders of Magna Graecia to carry her cargo. Carthage was the opposite -- a great seafaring nation of the western Mediterranean whose ships traveled the waters from England to Egypt. Not belligerent by nature, the Carthaginians maintained a substantial navy which was necessary to protect their trade interests in the Mediterranean and beyond.

The First Punic War began over a silly dispute. The Mamertines, Italian mercenaries from Campania, attacked and took control of Messana (Messina) Sicily. They were soon defeated by Hiero, tyrant of Syracuse, and afterward appealed to Rome and Carthage for aid. The Carthaginian’s “helped” by placing a garrison there, which was not what the Mamertines were expecting. Angered by this offense, they requested a treaty with Rome. The Senate knew a treaty would mean war with Carthage so they debated long and hard before deciding to move forward.

After the first land skirmishes of the war, Rome realized that Carthage would not be a pushover and defeating them would depend on their ability to fight at sea. With that in mind, the Romans proceeded to build 100 fivers (quinqueremes) and 20 triremes during the winter of 261/0 B.C. 

The fivers where probably adapted from a beached Carthaginian craft the Romans had captured. Construction was completed in 60 days and the ships were prepared to support the anticipated attack on Messana.

Although the Romans were inexperienced at sea, this lack of skill was partly offset when they fitted their ships with a corvus. This thirteen foot bridge was rigged to fall on the deck of an enemy ship, hold fast during battle, and allow soldiers to board and defeat the enemy.

Following construction, the consul C. Cornelius Scipio took 20 ships on to Messana while his consular colleague C. Duilius followed behind.

Scipio immediately received a proposal to be handed the Carthaginian naval station at Lipara, but the information was also leaked to the enemy. While ashore there, he was surprised by a Punic attack force and captured with all of his ships, earning him the sobriquet Asina (ass) for his stupidity.

After the fall of Scipio, Duilius was put in overall command. He set out for the north coast of Sicily to intercept the armada of Hannibal (not the general) which had attacked the port of Mylae. As the Romans approached, the Punic navy put to sea. Overconfident, the Carthaginian commander allowed his battle formation to fall apart, making himself vulnerable to the Roman corvus. The thirty leading Punic ships were boarded and taken, while Hannibal escaped via longboat. In all 50 Carthaginian ships were captured in Rome’s first great naval victory.

Duilius did not pursue Hannibal because he had to rescue Segesta from a Carthaginian siege by deploying his marines from the Gulf of Termini. He returned to Rome for a triumph in 259 B.C. carrying with him the beaks of the captured Punic ships which went on display in the Forum. Oddly, he was never given another commission.

Fall of the American Republic?

In a previous post (June 6^th, 2011) I wrote about the fall of the Roman Republic. The key milestones of the collapse were described with commentary describing the impact of each of them. The fall of the Republic can be seen as a connected string of events that snowballed into an unstable political system with no center of power. Once anarchy became the rule, the strongest man with the will to defy tradition was destined to take power.

The fall of the Republic began with the plight of the ex-soldier landowner whose loss of property produced a permanent underclass of poor in the city.

This is similar to the period of the turn of the twentieth century in the United States when poor immigrants flooded our cities and could not obtain work. And today we have a thinning out of the middle class as some move up the social-economic ladder and a greater number move down. America is currently experiencing a chronic state of high unemployment which may have broad social implications – dependency, crime, etc.

In the time of the Republic the brothers Gracchi (Tiberius and Gaius), tied to pass laws which would solve the problem of poverty and at the same time increase the number of men eligible for the army. This Agrarian Law would take land from the agar publicus (public land) and give it to the those who would agree to raise crops. Tiberius got his law through the assembly by very dubious means but then was assassinated by agents of the Senate who were not pleased with the loss of land they controlled and were not willing to share their wealth with the poor. Tiberius’ brother Gaius continued the distribution of land and pushed for increased power of the equestrian class at the expense of the Senate. He too was assassinated during a riot.

The deaths of the Gracchi drove a permanent wedge between the Senate and the people causing two political factions to appear: Optimates, who were the champions of the patrician class and Populares, who were the champions of the plebs. These factions were more divided than the political parties in America today because in Rome it was a case of human survival -- wealth versus poverty rather than a battle over ideology.

The other fallout of the deaths of the Gracchi was the public perception of the destruction of the rule of law. For any political system to be legitimate, it must be willing to stand behind a legal system that will protect those without power. By stooping to assassination, the Senate had proved themselves illegitimate – no better than the barbarians they felt superior to.

When the Jurguthine War broke out in 110 B.C. the Senate’s appointed commanders were repeatedly defeated to the embarrassment of the Roman people. Finally the people, through the assembly, picked their own commander, Marius, to win the war. Marius solved Rome’s recruitment problem in an eyelash by removing the property qualification for military service and at the same time shifted the soldier’s loyalty from Senate to commander. From that point on the supreme commander was the “emperor maker” of Rome.

How is this similar to what we see in America today? There is no question that the power dynamics in the United States are different than they were in the time of the late Roman Republic. Fortunately for us the military has always been more loyal to the president than its commanders. Loyalty is so ingrained in our soldiers DNA that it would be hard to image any deviation from that course. Commanders have occasionally defied presidents, most notably in the case of Douglas MacArthur during the Korean War, but these were the rarest of events in our country’s history.

The problem in the United States is the growing power of the central government as it increases in size. This trend is driven by the Progressive Movement’s belief that big government is necessary to achieve what it perceives is society’s social contract with its people. But progressive programs cost money and at some point, maybe now, we’ll run out. Conservatives resist this growth as wasteful and against the founding principles of the republic, because it robs us of fundamental freedoms.

Expanding the bureaucracy, extends the power of the executive branch by creating programs that the legislature can’t touch and won’t eliminate. When is the last time a failed program was defunded? Never, because the bureaucracy takes on a life of its own, driven by self-preservation. Funding an expanding executive branch requires that property (net worth) be taken from the people – not just from the top earners, but from all who pay any kind of tax.

This growth in the power of the federal government resembles the welfare experiments of Europe in the twentieth century, which failed. What is there in the progressive ideology that drives it to duplicate a failed system? Why don’t they understand the communism analogy?

Communism had many friends until the Soviet Union failed. Now, those friends are hard to find because they don’t want to embarrass themselves by supporting a philosophical system that is unworkable and idealistic. The problem was that Marx fitted the data to his system instead of the reverse. If he had done his analysis correctly, he would have seen the fundamental flaws in his point of view.

The danger to the American political system rests in the accumulation of power in the central government and the consequent loss of freedom that accompanies it. Rather than leveling the classes, this trend will divide them further. It’s up to the people to push for a balance between government and liberty. If we take the middle road, our republic can survive.