It may be said, if such advantages are gained by forming bridges in this simple manner, why not have adhered to their principle all along. instead of adopting difficult and intricate plans of construction, which give rise to great additional ex pense, besides making the subject so much more involved and uncertain ? but on the other hand, it must be remarked, that independently of the short duration of timber in comparison of stone, iron. &c., where large spans have to be crossed, it cannot always be obtained of sufficient dimensions, as respects length and depth, and besides, in some localities, a supply of such timbers is not always to be obtained ; and even in those where a great quantity was at one period to he found, the very fact of a large consumpti.m leads to a deficiency in the course of time, and trees of a later growth and smaller scant ling would necessarily conic to be used ; and further, in con nexion is ith the application of beams to large spans, it must be borne in mind, that where the distance between the sup ports becomes greater, the dimensions require to be aug mented, and that when the size is increased in an arithmetical proportion the weight is added in a much greater ratio. Under these circumstances, it may easily be conceived, that beam or girder bridges could only be employed within certain circumscribed limits ; now, however, that the Britannia Tube has originated the system of building up hollow beams, by which their strength may be made sufficient for the work required of them, without adding so enormously to their weight, we may expect to see wooden-bridges, thrilled of hol low beams, taking the place of lib and arched timber bridges, and indeed, in some instances superseding both stone and iron, as being calculated for much larger spans than have hitherto been attempted, and ea} of being constructed at a comparatively small cost.
But to go back to the history of wooden bridges, we will find that is here risers were broad. and their channels deep, it would be impossible to cross them by single beams of tim ber. In such cases, a timber franlin; o sc dfulding W01111:1 be firmed in the lied of the river. by driving piles, or a pier night be thrilled of stones, or other materials. On these, beams of timber would be placed, with one extremity resting on the pier, and the other un the bank of the i iver, or on an abutment raised at the water's edge, and upon several piers in the is ater, as the case might be. Where the dis tance between the supports were too great for the dimensions of the timber forming the roadway, the main beams were propped up by struts projecting from the sides of the piers or piles, is hi•h were sometimes made to meet in the centre, or if that was not practicable on account of the distance between the supports, they could each be made to sustain the beam, and a cross-piece on which their ends should abut be placed between them, and fastened to the underside of the beam, these struts, or stays, were then multiplied and disposed in various ways, until at length a rib or arch of timber was firmed to support the roadway, while the spandrels were filled up with struts and ties to resist the compression.
The ribs of bridges constructed in this manner were com posed of frames, the lower portion of which form segments of circles, frequently made up of several pieces of wood placed immediately one over the other, and joggled together, so arranged, however, that their ends should break joint. To these circular arcs, or polygonal frames, upright pieces were attached either by bolts, mortises, or iron straps, by which the weight of beams supporting the roadway was sustained at intervals, and so disposed as that each part might, as far as possible, conduce to the strength of the is bile, The spandrils, or spaces between the lower rib and the road way, were differently arranged. according to circumstances; in some there were perpendicular braces, and in others pen dant pieces made to radiate towards the centre of the circle, or polygon, of which the ribs were composed. Again ; instead of thrilling the lower bar of the polygon, &c., by layers of timber bound closely together, it was, in some cases, constructed of two rails placed at some distance front each other, and fastened to radiating pendants, the intermediate spaces being filled up by abutting pieces placed regularly across, so as to keep all the pacts of the ribs in their proper places. Besides this, latt,ce-bridges may be mentioned, which consist of two or more frames isanpt)sed of top and bottom rails, either or slightly cambered. and the spaces between them filled in with diagonal pieces, enissing one another at an angle of -15°, or other suitable angle; these pieces are bolted to one another at every crossing, as also to longitudinal walings, and the top and bottom rail ; bridges of this description are easily constructed, exceedingly cheap, and have been found to answer extremely well tuff large spans. In addition to those enutnerated, there are a large class of timber-bridges formed on the tension principle, by framing king and queen-posts to the main beam on which the roadway is supported, and bolting to them an upper rail, at the same time filling in the intermediate spaces is ith ties and truss-hams.
The Pons Sublicius was the first bridge ever built across the Tiber. It was at first constructed of timber in the reign of Ancus Martins. It was put together is ithout either bolts or ties, so that it could readily be taken asunder ; and was built for the purpose of connecting together the Aven tine and Janiculum The bridge built over the Danube by Trajan is almost one of the oldest timber-bridges of is !deb we have a detailed account. It was supported on twenty stone piers, which were 150 feet high and ti feet broad ; on these were framed timber arches, each I'll) feet span, and f coned of three concentric timber-rings bound together by radiating pendants ; these, together with the arches, supported the longitudinal beams on which the flouring joists v. ere placed across the bridge.