324. The oldest wooden bridge that we have any account of is the Bridge of Sublicius, which existed at Rome in the reign of Ancus Martius, about 500 years before the Christian era, and which it is said was put together without nails or iron of any kind. It owes its celebrity to the combat of Horatius Cocles, a renowned Roman knight, who saved the city by a noble defence of this bridge.

325. The next in point of antiquity was that erected by Julius Caesar, for the passage of his army across the Rhine. It is described at some length in his ' Commentaries;' and Alberti, Palladio, Scamozzi, and others, have attempted, from the description, to restore the design; but their representations differ considerably. Caesar's army passed over this bridge ten days after they began to carry the timber to erect it.

326. The bridge built by Trajan over the Danube appears also to have been of timber, except the piers, which were of stone; at least so it is represented in basso-relievo upon Trajan's Column. The roadway of this bridge appears to have been supported by three concentric curved ribs of timber, connected by radial pieces; and it is certainly a good specimen of the art of building timber bridges at that early period. Trajan's Bridge consisted of twenty or twenty-two stone piers, with wooden arches; each arch above 100 feet span.*

* Gibbon's ' Rome.'

327. In the Middle Ages, when bridges were first established on the passages over the principal rivers, they were almost always constructed with piers, at from 15 to 20 feet apart, consisting of one or more rows of piles. These piers were generally defended by a kind of jetty to break the ice, which also protected them from the shock of bodies borne down by the current; nevertheless, in process of time, and from the frequent repairs that were necessary to protect the piers, the water-way generally became almost wholly blocked up; and consequently the bridge soon became incapable of sustaining the pressure of water which accumulated in high floods.

328. Palladio, in his 'Treatise on Architecture,' has given several designs for bridges, which display a considerable knowledge of the subject; indeed, many of the designs of the present time are merely improvements on those exhibited in his valuable work. Palladio appears to have been the first among the moderns who attempted a mode of construction that would admit of greater spans for the openings, and by reducing the number of piers avoids exposing the timber-work to the shock of bodies carried down by the current. The bridge he erected over the torrent of Cismone, near Bassano (Fig. 102), was of this kind, the span being 108 feet.

Fig. 102.

Among the designs for wooden bridges given by Palladio, the most remarkable is that exhibited by Fig. 103; as it appears to have been the first where the idea was adopted of constructing a system of what may be termed framed voussoirs similar to those of a stone bridge.

Fig. 103.

329. In Switzerland several excellent wooden bridges have been erected, one of the most celebrated was that at Schaff-hausen, constructed in 1757 by John Ulrich Grubenmann, a village carpenter of Tuffen, in the canton of Appenzel, but certainly one of no ordinary capacity. It was composed of two arches, the one 172 feet, the other 193 feet span, supported by abutments at the ends, and by a stone pier in the middle, which remained when the stone bridge was swept away in 1754. In this bridge the oak beams which rested upon the masonry of the abutments and pier not having been sufficiently seasoned, nor raised from the stone-work so as to admit of a circulation of air round them, became rotten, and the frames began to settle. Grubenmann being dead, a carpenter of Schaffhausen, named George Spengler, undertook in 1783 to remedy the defect. He raised the whole bridge, by means of screw-jacks resting upon scaffolding, supported by piles; and replaced the decayed timbers by others of a better quality. This was the only repair that it underwent during the forty-two years of its existence. In 1799 it was burnt by the French army.

The construction is ingenious, the principle of which is shown in Fig. 4, Plate XLVII. It has been remarked as a most serious defect in this bridge, that all the principal supports were so dependent upon one another, that a single part could not be removed without the whole being first supported. It had also a defect common to others constructed on the same principle, that it bent considerably sideways.

Schaffhausen Bridge was completed in less than three years: and Mr. Cox says that "a man of the slightest weight felt it almost tremble under him; yet waggons heavily laden passed over it without danger." It is often stated that the middle pier was not necessary as a support; this however is a mistake, as it certainly would not have borne its own weight without the assistance of the middle pier.*

330. The Bridge of Bamberg, on the Regnitz, in Germany (Plate XL.), is an example of a method of construction introduced by Wiebeking. It was built in 1809, and is the widest span that has been executed according to his principle.

Plate XL.

BRIDGE AT BAMBERG.

Fig.1

It consisted of one arch of 208 feet span, with a rise of 16.9 feet, and the width of the roadway was 32 feet. A stone bridge had formerly been erected on the same site, but its heavy piers contracted the water-way so much, that the water, in a flood, accumulated to such a height as to overturn the bridge by its pressure. In consequence of this accident the wooden bridge was made to span the whole width of the river.

In the middle of the width of the bridge, three ribs were placed side by side, the centre one being five beams in depth at the abutments, and only three in depth at the crown; but those on each side of it were three beams in depth throughout. On each side of the bridge there were two ribs placed side by side, and bolted together; each of these consisted of

* Cox's 'Travels in Switzerland;' Rondelet, ' L'Art de Batir Gauthey, ' Construction des Ponts.' five beams in depth towards the abutment, and three beams in depth at the crown. The depth of the beams was from 13. to 15. inches. The three compound ribs were united together by cross ties, with diagonal stays or braces between.

In the elevation (Fig. 1) the boarding is supposed to be removed from one-half of the bridge, and the abutment cut through, to show the manner of framing the timbers. Fig. 2 is a section to a larger scale across the bridge at A A, on the elevation.

The joints of all the parts built into the abutments were well soaked in hot oil, and they were also covered with sheet lead. The ribs and joists were of fir, the cross ties and plates of oak.