thrust up to the abutments. The flattest brick arches | of large size are those of the bridge that carries the Great Western Railway over the Thames, at Maidenhead. These are semi-elliptical in form, and of 128 feet span, and 24 feet rise. The abutments are stepped on raking benches on the chalk stratum upon which they stand; and the resistance thus obtained appears to be sufficient. Mr. Law remarks, that, taking into consideration the materials of which it is composed, this bridge is certainly the boldest which has ever been constructed, the actual pressure at the crown of the arch being about one-third of that which would begin to injure the cohesive strength of the material of which it is composed. "And although the construction of this bridge has shown that it is practicable to approach much closer to the load which would cause failure than had before been considered safe, it is questionable how far prudence would warrant such an approach in ordinary cases; especially when we consider how many accidental circumstances may deteriorate the stability of the arch, to guard against which, it seems desirable that a much wider margin should usually be given, and that the greatest load upon the keystone should not be greater thanth of that which would begin to crush its material, in bridges exposed to only ordinary traffic; and in those which are continually exposed to the tremour and vibration occasioned by a continuous and very heavy traffic, not more thatth." |

SECTION IV.-ON TIMBER BRIDGES.

The oldest wooden bridge on record is the bridge of Sublicius, which existed at Rome 500 years B.C. It is celebrated for the combat of Horatius Cocles, a Roman knight, who saved the city by his noble defence of this bridge. It is stated to have been

put together without iron or nails. A wooden bridge was erected by Julius Cæsar for the passage of his army across the Rhine. The passage was effected ten days after they began to carry the timber for its erection. The bridge built by Trajan over the Danube appears to have been of timber, except the piers, which were of stone. The roadway seems to have been supported by three concentric curved ris of timber, connected by radial pieces. There were 20 or 22 stone piers, and each wooden arch was above 100 feet span.

In the middle ages, when men began to establish bridges on the passages over the principal rivers, it was customary to erect piers 15 to 20 feet apart, consisting of one or more rows of piles. They were defended by a kind of jetty to break the ice, which also served to protect them from the shock of bodies borne down by the current; frequent repairs were, however, required, and the accumulation of matters against the piles blocked up the water-way, and the bridge became incapable of resisting the pressure of water in the time of high floods. In these early times of modern bridge-building, abundance of material was used, without much skill in its arrangement; but in places not subject to floods, and in situations where the piers could be kept light, more elegant structures were erected. A bridge built by Palladio over the Brenta, near Bassano, is a good example of this kind of bridge. This great architect has given several designs for bridges, which have been adopted in later times. He appears to have been the first among the moderns who attempted that species of construction which renders numerous piers unnecessary, thereby avoiding the shock to the timberwork of bodies carried down by the current. His bridge over the torrent of Cismone, near Bassano,

Fig. 3v5.

beam, is considerably larger on the outside than on the in, where the two mortises meet. Two keys, B B cc, are formed, each with a notch, bed and aio, on its side, which notch fits one end of the mortise. The inner side of the key is straight, but so formed that when both keys are in their places they have a space between them wider at one end than at the other. A wedge, A A, having the same taper as this space, is

Fig. 306.

may be termed framed voussoirs, similar to the archstones of a stone bridge,—a principle that has since been successfully adopted, both in timber and in iron bridges.

One of the best modern methods of construction is that of forming curved ribs for the support of the roadway. This method, introduced by Mr. Price, is thus described by Tredgold. "He proposes the curved rib to rise about one-sixth of the opening, and to divide it into a convenient number of equal parts, according to the span, or to suit the lengths of the timber. For a bridge of 36 feet span, he proposes to make the ribs of oak, in five lengths, and 3 inches in thickness; each rib to consist of two thicknesses, one 12 inches deep, and the other 9 inches deep; the joints crossed, and the thicknesses keyed together with wooden keys. Two of these ribs, with joists framed between, he says, will be sufficient for the roadway." This method of construction, which has been brought to considerable perfection in Germany, America, and other places, will be understood from Fig. 307, in which D E F are three beams of the arch, consisting of logs of timber of small lengths, such as

E

B

B

A

с

Fig. 307.

is double, consisting of two logs, applied to each other side by side, breaking joint, as the workten term it. They are kept together by wedges and keys, driven through them at short intervals, as at K, L, &c. The manner of joining and strongly binding the two side-pieces of each beam is as follows:-the

firmly together.

Colonel Douglass, in his work on Military Bridges," notices the wooden arch, of 250 feet span, across the Portsmouth River, in North America. It was put together with wooden keys, on Price's method of construction, applied to a larger span, except that there is some difference in the form of the keys. The Colonel observes, that the arch is extremely flexible, and that diagonal braces would be an improvement in it. Also that if the three ribs had been placed close above one another, and firmly connected together, the bridge would have been much better adapted to resist any unequal load, because, in such case, they would have formed a solid beam, equal in depth to the sum of their depths. Tredgold remarks that it would have been still better to have made the same quantity of timber into two ribs, with cross ties and diagonal braces between them; that the method of connecting the parts by means of dovetail keys is objectionable, as the timber must be greatly weakened by such large mortises, and a very slight degree of shrinkage renders them useless; that it is still more objectionable as applied to the radial pieces, which would have been much better notched on in pairs, and bolted through.

There are many excellent wooden bridges in Switzerland. One of the most celebrated was that at Schaffhausen, constructed in 1757 by John Ulrich Grubenmann, a village carpenter of Tuffen in Appenzel. It was composed of two arches, 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 properly seasoned, nor raised from the stone-work so as to allow the air to circulate around them, they rotted, and the frames began to settle. Grubenmann being dead, a carpenter of Schaffhausen named Spengler, undertook in 1783 to supply a remedy. He raised the whole bridge by means of screw-jacks upon scaffolding supported by piles, and replaced the decayed timbers by others of better quality. This was the only repair required by the bridge during the 42 years of its existence. It was burnt by the French army in 1799. The principle of its construction is shown in Fig. 311. Its chief defect was that all the principal supports were so dependent upon one another that a single part could not be removed without first supporting the whole bridge. This bridge in common with others constructed on the same principle bent considerably sideways.

Mr. Coxe says, that a man of the slightest weight felt this bridge almost tremble under him; yet waggons heavily laden passed over it without danger. It is frequently stated that the middle pier was not necessary as a support: the bridge however could not have borne its own weight without such assistance.

The general principles upon which timber bridges are constructed are very simple, and belong rather to CARPENTRY than to bridges. It will, however, be desirable to introduce a short notice of them here. If AB, Fig. 308, be a solid beam resting upon the supports A and B so as to form a roadway, it will

and being an abutment for the compressed beams: the frame requires only to be supported, and has e other thrust on the abutments of the bridge tha a solid beam would have. Framed bridges such as that designed by Palladio, Fig. 306, may be referred to the same principle.

Advancing a step further it is easy to construct a bridge in which the tie is entirely removed, but in such cases the abutments must be capable of sus taining the thrust. Hence we get the kind of construction shown in Fig. 312. But as long pieces of

3

Fig. 309.

less effect in bending the beam than a similar strain in the middle. But if the weight be sufficient it will cause the beam to bend however it may be distributed, in which case the fibres at the upper side d will be compressed, and those on the lower side e extended. A line may however be drawn at the middle of the depth acb, where the fibres remain in their natural state, being neither extended nor compressed. But all the fibres between c and d are compressed, and all those between c and e are stretched; but not equally so, because the nearer a fibre is to the points d or e the more it is strained. Now as the middle part of the beam is very little strained compared with the upper and lower side, it is obvious that the same quantity of timber can be employed more effectually by using a deeper beam and cutting out the middle, as in Fig. 310. On examining the forces exerted by the parts of the beam

very common form. It was adopted by Palladio in a bridge across the Brenta. By dividing the span into shorter lengths than is here shown, little or no advantage is gained, because the angles of junction become more obtuse or open, and the strain in the direction of the pieces is much increased. Although such a bridge might bear a constant load, a load moving over it would soon derange it, because the strength of such a system to resist a variable load must depend wholly on the strength of the joinings, which cannot be made very strong. Bridges on this

Fig. 31

principle have, however, often been executed, as in Fig. 314, which shows the method adopted fo. the

In cases where it is difficult to form abutments and it is desirable to keep the roadway as low as possible, Tredgold recommends such a construction

P

Fig. 317.

B

Fig. 315.

rib at C,D and E; therefore when the strength of the rib is capable of sustaining the strains at C, D and E, and the curve is a proper curve of equilibrium to the constant load, this combination is both secure and simple. The use of curved ribs of this kind was known at a very early period, and it has been further improved by bending the pieces that form the ribs. Many considerable structures on this principle have been erected, among which may be mentioned the bridge near Bamberg over the Regnitz, designed by Wiebeking, and built in 1809. Cast-iron bridges are constructed on nearly the same principle, as in Southwark Bridge, designed by Mr. Rennie, consisting of three arches, the span of the centre arch being 236 feet.

As a bridge with a curved rib and of considerable span yields at D, C and E, Fig. 315, when the load is applied at the middle, the strength must be increased by increasing the depth of the rib. This leads to the construction of a framed rib, Fig. 316. But in

such case the two curved ribs must be continuous, and put together so as to resist either extension or compression: for when a load is placed at D, the lower rib will be extended to d and compressed at c and E; while the upper one will be compressed at D and extended at c and e; a weight applied at any other point would produce a similar effect. In bridges of large span framed ribs greatly add to stability. The framed voussoirs by Palladio, Fig. 306, resemble this kind of construction, but they are better adapted for iron bridges, where the parts cannot be very firmly united, in consequence of the expansion and contraction of the material according to variations in temperature; but in timber, where nothing is to be feared from this cause, it is, as Tredgold well remarks, losing one of the greatest advantages of the material to interrupt the connexion of the parts: besides, many joints should be avoided, on account of the difficulty of making them fit so as to bring every

as is shown in Fig. 317. Where the width of the bridge is considerable a rib may rise in the middle of the width so as to divide the roadway into two parts, or a double rib with a footway between. As cross ties will be required at the top, the middle parts may be covered with a roof to protect them. A continued coping a a a' a' might be put over each truss, which would improve the appearance and protect the framing.

By the application of some of the simple combinations thus noticed, the carpenter will be able to span any opening of moderate dimensions.

Wiebeking states that a rise of 1 in 24 is a convenient ascent for a bridge; that in timber bridges the settlement is generally about 1 part in 72; that is, if a timber bridge of 144 feet span rise one foot in the middle when first framed, it will settle so as to become nearly horizontal; so that when it is intended for the bridge to have an ascent of 1 in 24 when finished, it must be framed so as to have a rise of 1 in 18; for

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SECTION V.-SUSPENSION BRidges.

A suspension bridge derives its chief value from the fact that it is independent of the bed of the river which it crosses. Hence it can be thrown across an opening where it is impossible, either from the rapidity of the current or from the altitude of the banks, to erect centering for a stone bridge. It can also be built with ease and expedition; and as it requires only a small amount of materials, it is economical. There is also an elegant lightness about a suspension bridge; it is however on this account much slighter than stone or iron bridges, and there is probably no suspension bridge that would bear permanently the load that is passing every hour over London Bridge. "A bridge destined to be a great and perpetual thoroughfare, exposed not only to be frequently quite filled with people, and to the passage of troops, but also to the rapid motion of great numbers of heavy vehicles; in short, a bridge in a busy part of a great city, ought not to be on the suspension principle. For if it were made no stronger than our strongest suspension bridges, it would not possess sufficient stability. If on the other hand the strength were increased to a sufficient extent to enable it to bear safely its constant work, the weight, the difficulty of getting up the chains, and the increase in the masonry part, would so raise the expense, that it is doubtful how far it