Fig. 305, was of this kind, with a span of 108 feet. | mortise a icb and dcio, which is cut into cach half

Another of his designs, Fig. 306, is a system of what put in and driven hard, thus holding the two logs

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

that in this case it is necessary to support the tic beam, or it would fail in large spans. The celebrate bridges of Schaffhausen, Zurich, Landsberg, and Wettingen, are constructed on this principle. In the bridge of Schaffhausen the timbers are disposed nearly as in Fig. 311; the continued tie AB retaining

and being an abutment for the compressed beams: the frame requires only to be supported, and has no other thrust on the abutments of the bridge than 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 sustaining the thrust. Hence we get the kind of construction shown in Fig. 312. But as long pieces of

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 sided 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 advantage is gained, because the angles of junction into shorter lengths than is here shown, little or no 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

it will be seen that the upper part amdnb is wholly compressed in the direction of its length, and that the lower part aresh is wholly extended in the direction of its length; and as timber offers the greatest resistance when strained in this direction, provided the joints are made secure, we are thus led to the kind of construction shown in Fig. 311, where the same pressures are evidently obtained as in the principle have, however, often been executed, as in perforated beam, Fig. 310, the only difference being | Fig. 314, which shows the method adopted for the

Fig. 31

B

arches of the bridge over the Thames at Kingston: | part alike into action, and also of the difficulty of the span is 49 feet. Combinations of this kind preventing decay at the joinings. naturally lead to the continued curved rib, which possesses advantages not found in a series of beams merely abutting end to end; for when the rib is built of short lengths with the joints crossed and the different thicknesses firmly bolted together, it becomes as one solid beam. If the straining force be applied at D, Fig. 315, it must be sufficient to fracture the

Bridges of rope or cordage are described in works on Military Engineering as early as the year 1631, and perhaps earlier, but there does not seem to be any account of the existence of iron suspension bridges in Europe before the middle of the last century. About the year 1741, the first European chain-bridge was built in England across the Tees, two miles above Middleton, chiefly for the use of the miners of that district. The length was 70 feet, the breadth rather more than 2 feet, and the height above the water 60 fect. It appears to have been a very rude work, not superior to the Chinese chain-bridges. It attracted no attention at the time, nor does the construction of suspension bridges appear to have occupied the attention of English engineers until 1814. In America however iron suspension bridges were erected as early as 1796, by Mr. Finlay, who, in 1801, took out a patent for their construction. In 1811, eight bridges had been built on Finlay's plan.

could be brought under that of a stone or cast-iron | suspension bridge, with a horizontal platform sus. bridge. Add to which, a suspension bridge would pended from the main chains. never equal in stability a common arch bridge, because it is subject to vibrations, the law of which is not sufficiently known to calculate their precise results in practice, but which certainly are more dangerous in a heavy bridge than in a light one. The object therefore in building a suspension bridge, is either to make it so light that its own vibration shall not hurt it; or if, as in nine cases out of ten, that cannot be done, then to make it so heavy and stiff, in proportion to the load it will have to carry, that the load shall not cause it to vibrate much. This, for a bridge liable to be constantly loaded with as much as it could contain, would be impracticable." For large openings where the traffic is small, suspension bridges are well adapted, because they can be carried to almost any span and any height, for a comparatively moderate expense. They are also well fitted for military bridges; the chains, or cables, the platform and even timbers being ready prepared to frame suspension piers, might be carried more conveniently than a pontoon bridge, and could be rigged up for use in a short time. They are also well adapted for crossing chasms in mountainous countries; and on a great military pass, a bridge of this kind would give the inhabitants greater command over it for by knocking out a few connecting bolts, a whole bridge might be dismantled very rapidly without being destroyed. For piers or jetties on the sea-coast they are well adapted, from the openness of their construction. "If the suspension towers are founded on piles, and themselves made of strong but open framework, and if the chains and platform are properly combined to get as much stiffness with as little weight as possible, so that they may resist vibration, without being so heavy as to be endangered by the vibration they cannot resist, a suspension picr may be buried in the waves without being hurt."

The bridges of ropes, &c. mentioned at the beginning of this article, are true suspension bridges in their rudest form. They are very numerous in various parts of the world. In China, where the germ of nearly every thing connected with the Useful Arts is found, suspension bridges are formed of five parallel chains with links one foot in diameter, on which a loose bamboo flooring is laid. Another form is described as consisting of two parallel chains 4 feet apart, suspended over stone piers about 8 feet high on cach bank. The ends of the chains pass back from thence, turn obliquely, and are bedded in the rock, each being fastened round a large stone, which is kept down by a mass of smaller stones laid upon it. A plank about 8 inches wide, extending across the river, is suspended from the chains by bands made of roots, of such length that the path is 4 feet below the chains in the middle of the length of the bridge. The suspending bands are renewed every year, and the planks are loose, so that any part can be repaired separately. The length of one of these bridges is described as being 59 feet. It is only used for foot-passengers; but it is a proper

(1) "A Memoir on Suspension Bridges," &c. by Charles Stewart Drewry. London, 1832.

In 1814, a plan was advanced by Mr. Dumbell of Warrington, for making a direct road from Runcorn in Cheshire, across the Mersey to Liverpool. The scheme included a bridge instead of a ferry across Runcorn-gap, and it was suggested to stretch across the river a web of metallic rings. From the nature of the navigation, it was necessary that any bridge to be constructed should consist of not more than three openings, the centre one of 1,000 feet, and the other two of 500 feet cach, with a height of at least 70 feet under the bridge above high water. These conditions being submitted to Telford, he proposed an iron suspension bridge to consist of 16 iron cables, each formed of 36 square half-inch iron bars, and of the segments of cylinders proper for forming them into one immense cylindrical iron cable, which was to be nearly half a mile long, including the fixings on shore, and about 44 inches diameter, The half-inch bars as well as the four segments were to be welded together into so many lines of bars, and laid in a bundle together, secured by bucklings every 5 feet, and wrapped in flannel saturated with a composition of rosin and beeswax, to preserve them from the weather. They were further to be bound together with wire of about

th inch diameter. The roadways were to be suspended from 16 of these cables, and to consist of two carriage ways and one central footpath. The main suspension piers of the middle opening were to be about 140 feet high, and the deflection of the cables in the middle th of the opening or 50 feet.

At the time this bridge was proposed, there was but little experience on the construction of suspension bridges; and in order to obtain data for proportioning the strength of the parts, Telford undertook a course of experiments in 1814 upon the tenacity of malleable iron. "The inquiry was commenced by proving what force would tear asunder lengthways pieces of iron from 14 inch to inch in diameter. The experiments upon the first or larger diameters were performed with great accuracy, with an excellent hydrostatic machine, constructed by Mr. Fuller, at Mr. Brunton's

patent chain-cable manufactory, Commercial Road, ↑ The first was built in 1816, by Mr. Lees at Galashiel, London; those upon the smaller diameters were made across Gala Water, to establish a communication by weights attached perpendicularly, and sundry times between different parts of his manufactory. It was repeated. Having ascertained their powers when made of slender wires: the span was 111 feet, and suspended perpendicularly, I next made experiments the cost about 401. Another bridge of iron wires was upon different diameters from to of an inch, built in 1817 across the Tweed at King's Meadows. drawn horizontally and with different degrees of It is 110 feet long, and 4 feet wide. It was concurvature all these were done between points structed by Messrs. Redpath and Brown, of Edinburgh, 900, 225, 140, 131 feet apart, and repeated sundry and cost about 1607. It is supported by two hollow times upon the different distances and diameters. cast-iron columns 9 feet high, 8 inches diameter, and The number of experiments in all amounts to above 200.

"In the experiments made upon inch and under, the wire was drawn over pulleys; sometimes both ends were fixed, and sometimes one end only, the other having weights attached perpendicularly, to show their effects when compared with those loaded upon the curved part of the wire: these last were disposed at the,, and divisions of the distance 중 over which it was stretched. Having completed these experiments, I next proved what force by a blow would break the wire when stretched nearly horizontal, and at different curvatures, which was done by dropping weights from a given height. The several wires upon which these experiments were made were weighed, and the weight of 100 feet in length of each noted.

The results of these experiments were, that a bar of good malleable charcoal iron one inch square, will suspend 27 tons, and that an iron wire inch diameter, (100 feet in length, weighing 3 lbs. 3 oz.) will suspend 700 lbs.; and that the latter with a curvature or versed sine of th part of the chord line, will support th part of the weight suspended perpendicularly, when disposed equally at,, and of its length; and with a curvature of th of the chord, it will bear id of the aforesaid perpendicular weight, disposed in a similar way. Experiments upon the other diameters correspond sufficiently, and it was found that increasing the distance only varied the effect by the difference of the weight of metal contained in the wire employed.

"A wire inch in diameter, drawn very tignt between points 32 feet apart, resisted the impulse of a 20 pound weight falling from a height of 74 feet. Several other experiments were tried with lesser weights upon this and smaller wires.

"From these experiments I had reason to be satis fied that English iron made with wood charcoal, had sufficient tenacity to bear itself, and a portion to spare equal to the purposes of a bridge across an opening of 1,000 feet, and therefore considered myself justified in proceeding to form designs."

inch thick in the metal, set on the opposite banks of the river 4 feet apart: in each column is placed a wrought-iron bar 10 feet high, 24 inches square; and the suspending wires are attached each separately to these bars by screw-bolts 1 inch diameter. The lower ends of the cast-iron columns stand on a framed wood foundation. The roadway is formed of wrought-iron frames, to which deal planks 14 inch thick are bolted. The side railing is of wrought-iron rods: the suspending wires are about ths of an inch diameter, and are not arranged in a catenary, but one end of each is fastened to its supporting bar, and the other end is fastened to a part of the roadway, so that the wires are a set of inclined lines forming different angles with the vertical supporting bars, Fig. 318. When the wires are

Fig. 318.

drawn tight by the screw-bolts the roadway is said to have very little motion. The strength of this bridge was tried shortly after its erection, by filling it entirely with a crowd of people, and it received no injury.

The system of inclined wires was adopted in a bridge built in 1817, at Dryburgh Abbey, across the Tweed, by Messrs. Smith. It was 260 feet span, and 4 feet wide. It was observed that the bridge had a very sensible vibration when crossed by any person, and the motion of the chains appeared to be easily accelerated. In 1818, six months after the completion of the bridge, a violent gale of wind caused so great a vibration of the chains that the longest chains broke, the platform was carried away, and the whole bridge destroyed. The vertical motion of the roadway was said to be, just before breaking, as great as the horizontal motion, and sufficiently violent to have thrown a person off the bridge.

"The defect of this plan of supporting a suspended The project for the Runcorn bridge was not carried roadway by inclined chains, fastened at the ends to the out on account of the great expense of the under- platform, is, that the chains, being of different lengths taking; but it served to direct the attention of and inclinations, form different curves, and are exengineers to the subject, and may thus be regarded posed to very different degrees of tension; the long as the origin of the great extension that has since ones being the most strained, and the shortest the been given to the suspension mode of bridge-building. | least. In a bridge of large span, and made with chains Between the time here referred to, and the erection | of any considerable weight, it would be almost imof the Menai Suspension Bridge, several small sus- practicable to strain the inclined chains (especially pension bridges had been erected in Great Britain. the long ones) quite, or even nearly, tight; for if

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