IV
TREE-BRIDGES WITH STONE PIERS
Themost famous bridge in this kind is the one built by Trajan over the Danube, just below the rapids of the Iron Gate. Trajan required it for his wars against Dacia, which inA.D.106 he brought to a successful end, the Dacian leader Decebalus being slain and his people subdued. The bridge had played its part, yet Hadrian, the next Emperor, who began his reign ten years afterwards, looked upon it as a dangerous highway, open to incursions from Dacian revolts, and for this reason he destroyed some piers and the footway. Perhaps Hadrian was jealous of Trajan’s work, for two fortified gates and a handful of Roman troops could have defended the bridge against barbarians.
There has been much controversy over this great structure. Its architect was Apollodorus of Damascus, who designed also the Trajan column placed in the centre of the Forum Trajanum. A bas-relief on this column represents the bridge, but in a manner at odds with the written description given by Dion Cassius, who held important offices under Commodus, Caracalla, and Alexander Severus,A.D.180-229. Dion Cassius wrote a history of Rome, in eighty books, and a small portion of this work has comedown to us entire. His evidence then is worth having, and it states that the bridge had twenty piers of hewn stone, 150 feet high and 60 feet wide, with openings between them of 170 feet, spanned by arches. Doubt has been thrown on the accuracy of this description, because the bridge on the Trajan column is unsuited to a span of 170 feet; “nevertheless thirteen piers are still visible out of the twenty, according to Murray’s ‘Handbook.’ The writer has not been able to find any accurate measurement of the width between these piers, but as the ‘Handbook’ speaks of the length of the bridge as perhaps 3900 feet, and as the Conte Marsigli, writing from personal observation, in a letter to Montfaucon, gives the total length as probably 3010 feet, there can be no doubt that the spans were very considerable and that the representation of the design in the bas-belief is almost wholly conventional. The one point as to which it gives clear information, not supplied elsewhere, is that the superstructure was ofwood.”[43]
In other words, this colossal work was a descendant of the earliest tree-bridges, in so far as the footway was concerned. Whether arched timbering was carried from pier to pier to uphold the roadway, as in the bas-relief, is a question of no great moment; the horizontal bearing beams would need support, no doubt, since they had to span openings far wider than the longest trees; and itis useless for us to guess in what way this support was carried to them from the lofty piers, which were built with enormous blocks of stone. The main point is that one phase of bridge-building, whose first models were fallen trees lying astride rivers and chasms, seems to have culminated in the masterpiece of Apollodorus of Damascus. Much inferior work of the same kind, very varied and entertaining, has been common everywhere; some of it belongs to Kurdistan, for example (p.73); and in the Lledr Valley there is a good Welsh specimen called the Pont-y-Pant, whose wooden footway is primitively rustic, and whose piers are fragments of rock gathered from the river-bed and piled together. I have found at Thirlmere a quaint thing which is partly a dam and partly a bridge. The dam, an undulating wall of unmortared stones, has at equal intervals a few angular openings over which wooden hand-bridges are thrown. It would be easy in a shallow river to make a fish-pool by heaping boulders into a dam of this rude sort, and the completed work would rank no higher than the beavers contests against running water. So I tell myself that many a tribe in the great period of prehistoric art, about 50,000 yearsB.C., ought to have built for itself a bridge as elementary as the Pont-y-Pant and a perforated dam as uncouth as the one at Thirlmere.
From this untutored handicraft we look back again at the great art of Apollodorus, whose vast bridge over the Danube was near the ancient town of Nicopolis.What a long travail in the gestation and birth of infrequent ideas! Even half a million years ago a man of the eagle-beaked tools may have put a boulder under a tree-bridge because the tree was thin and swayed too much on a windy day; half a million years ago, and yet we do not feel ashamed of the Pont-y-Pant!