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"The Roman Emperor Trajan, being of an impetuous and active temperament, seemed to be filled with resentment that his realm was not unlimited, but was bounded by the Ister River [Danube]. So he was eager to span it with a bridge that he might be able to cross it and that there might be no obstacle to his going against the barbarians beyond it. How he built this bridge I shall not be at pains to relate, but shall let Apollodorus of Damascus, who was the master-builder of the whole work, describe the operation."
Procopius, Buildings (IV.6.12-13)
The treatise referred to by Procopius has not survived, but Cassius Dio does provide a description of the bridge, which was constructed about AD 104, sometime before the Second Dacian War (AD 105-106).
"Trajan constructed over the Ister a stone bridge for which I cannot sufficiently admire him. Brilliant, indeed, as are his other achievements, yet this surpasses them. For it has twenty piers of squared stone one hundred and fifty feet in height above the foundations and sixty in width, and these, standing at a distance of one hundred and seventy feet from one another, are connected by arches. How, then, could one fail to be astonished at the expenditure made upon them, or at the way in which each of them was placed in a river so deep, in water so full of eddies, and on a bottom so muddy? For it was impossible, of course, to divert the stream anywhere. I have spoken of the width of the river; but the stream is not uniformly so narrow, since it covers in some places twice, and in others thrice as much ground, but the narrowest point and the one in that region best suited to building a bridge has the width named. Yet the very fact that river in its descent is here contracted from a great flood to such a narrow channel, after which it again expands into a greater flood, makes it all the more violent and deep, and this feature must be considered in estimating the difficulty of constructing the bridge. This, too, then, is one of the achievements that show the magnitude of Trajan's designs, though the bridge is of no use to us; for merely the piers are standing, affording no means of crossing, as if they had been erected for the sole purpose of demonstrating that there is nothing which human ingenuity cannot accomplish. Trajan built the bridge because he feared that some time when the Ister was frozen over war might be made upon the Romans on the further side, and he wished to facilitate access to them by this means. Hadrian, on the contrary, was afraid that it might also make it easy for the barbarians, once they had overpowered the guard at the bridge, to cross into Moesia, and so he removed the superstructure."
Roman History (LXVIII.13)
More than a hundred years later, Dio remarks that the stone piers still were standing in his own day.
The sestertius illustrated here dates from about AD 105 and likely commemorates the bridge, which is represented as having a single span. One can see the fortifications or castra that guarded access to the bridge about which Hadrian was so concerned. A series of wooden arches spanned the twenty masonry piers, each a hundred and seventy feet apart. Including the width of the piers themselves and the abutments on either side of the river, the bridge extended some 3724 feet (1135 meters), almost three-quarters of a mile.
In AD 271, the emperor Aurelian was forced to withdraw from Dacia north of the Danube and resettled its provincials on the other side of the river in Moesia, renaming that province for the one abandoned to the Goths (Eutropius, Breviarium, IX,15; Festus, Breviarium, VIII; Historia Augusta, XXXIX.7). The bridge that Apollodorus had designed more than a century and a half earlier then was destroyed by the retreating Romans.
Trajan's chief military engineer in the Dacian Wars, Apollodorus later turned to civil engineering and architecture and was responsible for Trajan's Forum and Market. Indeed, the placement of the Basilica Ulpia suggests that it may derive from the plan of a legionary encampment, which had its headquarters (principia) placed in the center of camp, behind which was the sacellum, the regimental shrine. Apollodorus also designed or supervised the Column and Baths of Trajan, and possibly the Pantheon, which was rebuilt by Hadrian.
Another work by Apollodorus does survive, the Poliorcetica (c.AD 100), a treatise on siege machines that has been preserved in the corpus of Byzantine poliorcetics (the military art of siege warfare). It begins with an epistolary preface addressed to an unnamed ruler, who has solicited advice in preparation for a campaign against an enemy occupying a fortified height in a place with which the author is unfamiliar. Asked to supply designs for siege works, Apollodorus enumerates those that could be used against an elevated position, including how to deflect objects, such as wagons and barrels, which might be rolled down a hill; screens to protect against missiles when the top is reached; and, once there, advice on undermining a wall, ramming a gate, or using assault ladders. They are "effective, protective, and safe, and that as far as possible all shall be made of easily provided materials, light in weight, well engineered and quick to produce with unskilled labor (CXXXVII)."
But then the treatise continues with a list of increasing elaborate devices that do not follow these assurances: hand-held drills; a flame thrower to crack stone; an elevated scouting ladder to look over a wall, a tower that includes a draw-bridge, a double ram, a pivoted device to sweep defenders from the wall, a platform to level the ground, and hoses and pumps to fight fires. There also are interlocking ladders that can be assembled to form a scaffold and used with other attached devices, such as a falling knife; a means for dispensing hot oil; and rams, including one with a flail propelled by a torsion spring. There even is an armored floating bridge or raft. All seem designed to display the ingenuity of the author rather than the practicality of the device.
It is this discontinuity between the pragmatic and exotic that has convinced Blyth that the latter sections of the Poliorcetica are interpolations to the original material. Indeed, he contends that two-thirds of the treatise are later additions. Even the original material is not by Apollodorus but by an older contemporary; otherwise, Blyth assumes that he would have joined the campaign, himself, instead of sending an assistant and a team of skilled workmen (CXXXVIII). That the triangular deflectors and ladders mentioned in the Poliorcetica are also depicted on Trajan's Column argues that the manual was addressed to Trajan, probably at the time of the first Dacian War, which the Column commemorates.
Poliorcetica are Byzantine compendia of earlier works on siege warfare in which the classic poliorcetic manuals of such authors as Athenaeus Mechanicus, Biton, Heron of Alexandria, Philo of Byzantium, and Apollodorus of Damascus were updated and supplemented in the context of the tenth and early eleventh centuries. This was the age of Byzantine conquest, especially the years between AD 960 and 1025, in which walled towns and fortresses were systematically besieged. Practical handbooks to this end were therefore of particular interest.
About 950 AD, an anonymous Byzantine author, traditionally referred to as "Heron of Byzantium" (after Heron of Alexander, who is cited in the work), compiled the Parangelmata Poliorcetica ("Instructions for Siege Warfare"), a manual on siege craft and the fabrication of siege engines. The work concludes with the admonition that
"If army commanders carefully complete with logic and continuous diligence these siege machines, which have been selectively compiled for description and illustration, and always contemplate divine justice...they will easily capture cities, especially those of Afar and themselves suffer nothing fatal from the God-damned enemy" (LVIII).
The author retains the original text of Apollodorus but offers additional information and explanations, seeking to make that work understood by the nonspecialist. "Having clarified only the works of Apollodorus as it were in toto with additional elaboration and secondary arguements, we have drawn our conclusions, finding and adding ourselves numerous concordant <items>" (I). The original illustrations also are redrawn and presented in three-dimensional perspective to make the siege engines more comprehensible as well. "After weaving this <material> also into the works of Apollodorus we have arranged it with the drawings, giving these precise definition, knowing that even an illustration alone, when well defined, is able to render quite clear aspects of construction that are obscure and difficult to express" (I).
It is a curious work (which understands the Poliorcetica of Apollodorus to have been addressed to Hadrian) that describes the battering ram of Hegetor, the largest one in antiquity (XXV; also Vitruvius X.15.2-7), and mentions how the pickled intestines of cattle, attached to wineskins full of water could be used as hoses to spray water on the parts of burning towers that otherwise could not be reached (XXXIX).
During the sixteenth century, there was renewed interest in this genre of literature, as can be seen in these illustrations from an Italian manuscript, written in Greek.
"To bring down brick walls quickly," the anonymous Byzantine recommends the use of a borer (XVII-XVIII), which, during a siege, was to be sheltered under the protective cover of a tortoise. The pointed iron rod was secured by a wooden cylinder that was turned by a bow and cord, the entire shaft revolving freely in a socket supported on the ground.) It was used to drill a series of holes at an acute angle into the wall about a foot and a quarter apart and three feet high. These bore holes then were to be filled with rounded wooden stakes smeared with pitch, or pitch and pulverized sulfur. (They also were intended to prevent the wall from collapsing prematurely.) After the first row of holes was drilled, a second row was bored and stuffed with chips and splinters of dry wood.
These combustible materials then were ignited, kindled and fanned by a bellows if there was insufficient wind, the props in the wall burning and causing the wall to collapse (XIX). For Blyth, both devices are hopelessly impractical. The drill, which was about five feet long, likely would not have enough torque and, even though the bricks, once bored, were to "fall outward, with a sudden, quite massive collapse of the wall," the could do so unpredictably. It was an altogether dangerous exercise, one that either was ineffectual or, if effective, then potentially suicidal. The bellows, which was to safely ignite the wooden props from a distance, also was too small to have been useful.
Apollodorus later was imprudent enough to criticize Hadrian's design for the Temple of Venus and Rome:
"But he first banished and later put to death Apollodorus, the architect, who had built the various creations of Trajan in Rome—the forum, the odeum and the gymnasium. The reason assigned was that he had been guilty of some misdemeanour; but the true reason was that once when Trajan was consulting him on some point about the buildings he had said to Hadrian, who had interrupted with some remark: 'Be off, and draw your gourds [the concave segments of which, in fact, do recall a type of dome that appears at this time]. You don't understand any of these matters.' (It chanced that Hadrian at the time was pluming himself upon some such drawing.) When he became emperor, therefore, he remembered this slight and would not endure the man's freedom of speech. He sent him the plan of the temple of Venus and Roma by way of showing him that a great work could be accomplished without his aid, and asked Apollodorus whether the proposed structure was satisfactory. The architect in his reply stated, first, in regard to the temple, that it ought to have been built on high ground and that the earth should have been excavated beneath it, so that it might have stood out more conspicuously on the Sacred Way from its higher position, and might also have accommodated the machines in its basement, so that they could be put together unobserved and brought into the theatre [presumably the Colosseum] without anyone's being aware of them beforehand. Secondly, in regard to the statues, he said that they had been made too tall for the height of the cella. 'For now,' he said, 'if the goddesses wish to get up and go out, they will be unable to do so' [the same criticism directed at Phidias' statue of Zeus at Olympia]. When he wrote this so bluntly to Hadrian, the emperor was both vexed and exceedingly grieved because he had fallen into a mistake that could not be righted, and he restrained neither his anger nor his grief, but slew the man."
Dio, Roman History (LXIX.4)
The criticism may have been made in AD 104, before Trajan departed to fight the Second Dacian War and the fire that year on the Esquiline had made land available for Trajan's baths (Thermae Trajani). Or it may have been two years later, when Trajan had returned and construction on his Forum began.
Ridley contends that Hadrian was not responsible for the death of Apollodorus, arguing that, if he had been, the Historia Augusta would have mentioned the fact when it relates that the architect was to have assisted Hadrian in the construction of a colossal statue to the Moon (XIX.13). Too, if the Poliorcetica were addressed to Hadrian, it would have to date either to the Dacian campaign or the Jewish revolt of Bar Kokhba in AD 132-135. Since Apollodorus states that he was not familiar with the local terrain, Ridley contends that the uprising in Judea must be intended, which was in response to Hadrian's plan to replace the Jewish Temple with one dedicated to Jupiter (Dio, LXIX.12-14). If so, then Apollodorus still would have been alive after the Temple of Venus and Mars had been completed.
But the Temple was not dedicated until AD 135 and actually may have been finished by Antoninus Pius. As to the Historia Augusta, it recounts that Hadrian, "although he was very deft at prose and at verse and very accomplished in all the arts, yet he used to subject the teachers of these arts, as though more learned than they, to ridicule, scorn, and humiliation" (XV.10), and possibly, to death as well. The emperor who requested that Apollodorus write his Poliorcetica is not mentioned by name, but references to his talent and kindness suggest to Ridley, too, that the treatise was addressed to Trajan.
In the relief above from Trajan's Column, Apollodorus' bridge over the Danube can be seen behind the emperor, who holds a patera, a shallow bowl used in sacrifice. (Apollodorus may be the bearded figure behind Trajan's left shoulder.) In the passage from Procopius quoted above, he is described as architekton, the only time in the book that the word occurs. Otherwise, mechanikos ("master-builder) is used of the builders who served or assisted Justinian in his undertakings, the implication being that the emperor had a greater share of responsibility for such work than his predecessors. (Curiously, having made this distinction, the translator still renders architekton as "master-builder"). Procopius, too, is the only source who indicates Apollodorus to be from Syria, where Trajan had served as a military tribune while his father was governor of that province.
The odeum (a hall for musical performance) that was restored by Apollodorus had been built by Domitian (Suetonius, Life, V; Eutropius, VII.2) and, even by the fourth century, still excited wonder (Ammianus Marcellinus, XVI.10.4).
The pen and watercolor drawings illustrating the Poliorcetica of Apollodorus have been taken from "Treatises on Engines and Weapons," an early sixteenth-century Italian manuscript of works in Greek on siege warfare. It now is in the Special Collections Department of the University of Glasgow Library. The Poliorcetica, itself, has not been translated into English. The coin is from the Freeman & Sear Catalog 12 (2005), item 561.
References: Procopius: Buildings (1940) translated by H. B. Dewing (Loeb Classical Library); Dio Cassius: Roman History (1914-) translated by Earnest Cary and Herbert B. Foster (Loeb Classical Library); The Historia Augusta (1921) translated by David Magie (Loeb Classical Library); The Architecture of the Roman Empire (1982) by William L. MacDonald; "Apollodorus of Damascus and the 'Poliorcetica,'" (1992) by P. H. Blyth, Greek, Roman and Byzantine Studies, 33(2), 127-155; Siegecraft: Two Tenth-Century Instructional Manuals by "Heron of Byzantium" (2000) by Denis F. Sullivan; "The Fate of an Architect: Apollodoros of Damascus" (1989) by R. T. Ridley, Athenaeum, 67, 551-565; "Apollodorus' Poliorketika: Author, Date, Dedicatee" (2008) by David Whitehead, in A Roman Miscellany: Essays in Honour of Anthony R. Birley on His Seventieth Birthday edited by A. Krieckhaus, H. M. Schellenberg, and V. E. Hirschmann.
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