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It had a simple parapet, formed of a top and bottom rail,supported at intervals by upright posts, with diagonal bracesfilling in the intermediate spaces. This bridge was unfortu-nately destroyed by the succeeding emperor, to prevent thepossible incursions of the barbarians, by its means, into theRoman territory.

The timber-bridge of Schaulfhausen, built over the Rhine ,by Ulrick Grubenmann, was remarkable for its ingenious con-struction ; it consisted of two openings, one 172 feet span,and the other about 190. Its abutments and centre-pierwere of stone ; on these were laid a kind of compound beam,formed of three rails or walings, each of which consistedof two longitudinal beams bolted together, and toothed intoeach other, so as to be perfectly united ; these were sup-ported by an infinity of struts, kept in their places byvertical binding-pieces, all tending to transfer the thrustto the supports of the bridge. It was roofed in forthe ostensible purpose of protecting the timber from the badeffects of the weather ; but there can be little doubt but thatthe roof added greatly to its strength ; and it is not impro-bable that its ingenious architect was aware of the importantadvantages to be derived from its introduction. This bridge,and others designed and executed by the brothers Gruben-mann, were in fact Timber Tubular Bridges.

The beautiful bridge of Schauffhausen was unfortunatelydemolished by the French , about the year 1800.

John Grubenmann, assisted by his brother Ulrick, built asplendid bridge of a similar edscription over the Limmat ,having the enormous span of 390 feet. It was formed of twoimmense circular ribs, one at each side of the roadway, formedof several beams bolted together, and made to fit one into theother, in a similar manner to the Schauffhausen Bridge.

This bridge had also angular struts, longitudinal beamsperpendicular binding-pieces, and abutments of masonry ; itwas likewise furnished with a roof, which added considerablyto the stability of the structure. It was, however, burneddown shortly after the destruction of the Schauffhausen Bridge.The bridge at Ceslingen, as also that of Zurich , the oneupwards of 200 feet, and the other about 130 feet span, aresplendid specimens of this kind of construction, as likewisethe bridges of Mellingen and Berne, by Ritter, and thoseover the Saone by Gauthey.

The timber-bridge of St. Clair, built over the Rhone atLyons, has seventeen openings, the centre one having a spanof 45 feet, and the others diminishing towards each bank.This bridge has a roadway of about 36 feet, which is sup-ported upon piers each formed of 13 piles, arranged in a singlerow, running parallel with the banks of the river; on the topof these piles a sill was framed, and longitudinal timbers weremade to bear over the head of each pile; and upon theselongitudinal timbers the flooring was laid.

The bridge of Grenelle, erected over the Seine, near Paris ,by M. Mallet, consists of two equal and symmetrical bridges,separated by an intermediate piece of dry ground; each ofthese is formed of three timber-bays of 82 feet span, sup-ported upon two abutments and two piers of masonry. Thewidth of this bridge is nearly 33 feet. The piers are about11 feet wide at their foundations, and are diminished to about9 feet at the springing. The abutments have a half-pierattached to them, on which the timber-work rests. Those atthe banks of the river have, besides, two wing-walls. Theground in the centre, measuring 85 feet, the whole bridge,reckoning the entire distance from the abutments on eitherbank of the Seine , is 632 feet long.

All the foundations were built on piles, upon which a plank-ing was laid. These foundations were formed by means ofcoffer-dams, easily constructed on account of the shallowness

of the river, which, at low water, was not more than 5 feet P’each bay is composed of seven timber frames, placed about 5 te®apart from each other. The frames consist of segmental i> ’formed of three pieces of bent timber, bolted and hound to n®ther by means of iron, which support the roadway by ra<ating pendants ; each of these compound segmental beams

9 inches square, and of such a length as to take in two

of the

spaces formed by the pendants. In cutting and placing

the

an inch thick, in order to prevent the water from injuringtimber of the bridge; however, it must be admitted, that aivery heavy rain, the mositure has been found tocolated through this pavement, either owing to its havingbadly laid, or perhaps in consequence of cracks made mcoating by the contraction and expansion of the timber road wA'It might be better, therefore, in future, to place a layer of nsand between the flooring-boards and the bitumen. Six drapipes in each bay convey the water off the surface. g

The pavement of the roadway consists of a fiagg>n£’ ^inches thick, laid upon sand in the ordinary manner. &bay is loaded to the extent of about 40 tons. .

There is another bridge over the Seine , at lory, w , i pSwas put up in the year 1828, and which greatly rese©the bridge of Grenelle in construction. e

Besides these, which are given by way of example, th ^are almost an endless number of wooden-bridges erec©

frames in position, they were put 5 inches higher than the!should have been, in order to allow for sinking, a ' tel t0flagging was put upon the bridge ; and even when openedthe public, the bridge had not settled into its place, sowould seem that 2, or at most, 3 inches, would have beamply sufficient to have allowed for. ^

Upon the roadway, a layer of bitumen was laid about

fter

have pe‘"beentlu«

throughout the world ; among which may be mentionedbridge at Trenton, in America , of 180 feet span; a bn 1 ^over the Tees , 150 feet span ; the bridge of Neucetringe©Bavaria , 102 !eet; the bridge across the Necker, 210span ; the bridge of Bamberg , with an opening of 206 * e ’erected by M. Wiebeking, an engineer, who has construean immense number of timber-bridges; the bridge of Fehn 1 ' ’with a span of 65 feet; the bridge at Zete, built by M-net, with a span of 125 feet, besides several put up bycelebrated Perronet ; and a number of others remarkabletheir ingenious construction and variety of form. ^ ut .

Timber-bridges are either supported upon piers and a ,ments of masonry built on the solid foundation of the g r ° jor on a platform constructed upon piles driven into the e*or they are supported upon piers formed of one orrows of piles driven in a line with the road or river p flS ®’ g f |under the bridge. There are an almost infinite va . rlB ] 1 QW'ways in which such props or piers may be made. It )S ’ treever, usual to drive the piles about a yard apart from c fto centre, and to bolt capping-pieces or walings to thesuch piles, and either filling up the spaces betweenlarge stones laid dry, or else grouted with mortar ; onthe masonry for the supports should be placed, or a ti ^ )0 jframing, if desired, or else the piles may be carried up 1 3 ,

height of the roadwajq being kept in their places by w .j eS jand diagonal pieces, bolted on each side of them ; ^ ie ?® j, inshould be about a foot square, and when they aresalt water, or in tidal rivers, their surfaces, up to '” gC up'mark, should be sheathed with copper, or protected t fieper-nails, from the ravages of the worm. At each aI -e

piers in the water, in cases where several rows of P 1 e ^ i#r jdriven, a sort of cutwater should be formed, in order 0 el) toff heavy bodies, such as floating trees, ice, &c., and pthem from injuring the superstructure. This is usu< / ^yby driving one pile by itself in advance of the res ,