A model of an iron suspension bridge, upon a new construction, was exhibited in the year 1829 It was constructed by Mr. Mottley as a design for a bridge across the Thames, from Scotland Yard, on the Middlesex side, to the King's Arms, on the Surrey shore. The annexed design for an ornamental bridge for parks or pleasure grounds, will serve to explain the nature of Mr. Mottley's plan, which consists in a combination of the principles of tension and compression. The arch represented by the curved line is confined between two parallel lines, and those lines are connected together by vertical bars, to which the arch is inflexibly attached, and the parallel lines or bars also inflexibly jointed, and the whole combined by strong bolts at the top and bottom of every vertical bar, rendering it a skeleton of a trussed beam; thus affording an opportunity of having two paths across a river, the lower one of which may be used as carnage way, and the upper may serve for a footway or promenade. This second floor is not an absolutely necessary part of the structure ; but if it be not adopted, it will be needful to have diagonal braces placed horizontally to stiffen the upper part. By this mode of construction, it will be seen that the stress or pressure instead of acting upon the highest point of the structure (as in the case of bridges where the main chains pass over lofty towers) exerts its force at the lowest point, viz. at the springing of the supporting arch, whist the piers have only to support the weight of the bridge, the thrust being counteracted by tie bars, which connect the two extremities of the arch.
The accompanying engraving represents a portion of a bridge, or of a jetty, supported upon floating piers, as proposed by Messrs. Delafons and Littlewort, for situations where the water is too deep for the erection of piers, and where the expanse of water is too great to be passed over by chains, without any other support than that at the abutments. a represents the river or channel to be crossed ; b the bed of the same ; c the suspension bridge ; d one of the floating piers, of which there may be any number, according to the distance between the shores; e the mooring chains, which are attached to the heads of piles driven perpendicularly into the earth. The piers are supported upon rows of properly constructed boats, whose buoyancy is sufficient to bear the weight, that is, without their being submerged at low water; and these boats are confined in this situation by the mooring chains. By this arrangement, when the tide rises, the piers cannot ascend and lift the bridge, which would be the case were the boats not tied down to the extreme point of low water, while the bridge is supported by their buoyancy in either case. The machinery by which the piles may be driven at considerable depths under water, forms the subject of a patent taken out by Messrs. Delafons and Littlewort.
The readiest, and probably the most ancient material for the construction of bridges, is timber ; and, in most countries, wooden bridges are common ; but, in their construction, Germany and America appear to take the lead of all other countries. The most celebrated of these structures was that over the Rhine at Schaffhausen, which was destroyed by the French in 1799. It was designed and executed by Ulric Grubenman, a common carpenter, who produced a model of a bridge to consist of only one arch of 364 feet span ; but the magis trates insisting that the bridge should, near its middle, be supported by a pier, the remains of a former bridge at the same place, he seemingly complied, but executed the work in such manner as to leave it doubtful whether the bridge really received any support from the pier. John, the brother of Ulric, about the same time constructed a timber bridge of the same sort at Kui chenaw, 240 feet in length; and they afterwards conjointly constructed one near Baden, of 200 feet in length, and another at Wuttenghen, 198 feet in length.
Several timber bridges were erected in Germany during the years 1807, 1808, and 1809, by Wiebeking. The widest span is that over the Regnitz, at Bam berg, which is 208 feet. There have been many capital timber bridges con structed in America. The Trenton bridge, over the Delaware, built by Burr in 1804, is a segment of a circle of 345 feet diameter, its chord measuring 200 feet, its versed sine 32 feet, and the height of the timber-framing at the vertex only 2 feet 8 inches. That called the Colossus, over the Schuylkill at Phila delphia, is of the extraordinary span of 340, and is the segment of a circle of 1405 feet diameter ; its versed sine being only 20 feet, and height of the wooden framing of the arch at the vertex only 7 feet: it was finished in 1813.
Bridges of Boats are made of boats composed of either copper, tin, or wood, fastened across the stream by means of anchors or stakes, and laid over with planks. At Beaucaire, Rouen, and Seville, are very fine stationary bridges of boats, which rise and fall with the tide ; that at Rouen. is nearly 300 yards long, and paved with stone, so that laden carriages and horses go over it in safety. Another kind of bridge of boats is that called flying bridges, the use of which is mostly confined to those rivers in which the stream is always running down. A flying bridge generally consists of one or more boats, covered over by a platform, and connected by a long cable to an anchor midway of the river, and considerably higher up the stream than the landing place on either side ; upon putting the helm over to one side, the side of the boat is presented obliquely to the stream, which impels it across the river to the opposite shore. Sometimes, instead of laying down an anchor in the sti earn, two stout shears or masts are erected, one on each bank, and firmly secured by guys. A stout hanger is then stretched tight from the top of one mast to the top of the other. Upon the heuser is a large iron ring or grommet, to which is attached one end of the boat rope, the other end of which is made fast to the boat, or boats, of which the bridge is composed. By the action of the tide upon the rudder the boat is sheered across the stream, dragging the grommet along the hauler. The annexed cut represents another species of flying bridge, which has been proposed for crossing valleys, rivers, dtc, It consists of a strong wire chain, iron rod, or cord e e e, which passes over the supports a a, to which one of the ends is firmly fixed as at f, and adjustable by tightening screws as at g; c is an endless cord passing over the pulleys b b on the supports and round the pulley i, which i is attached to a light car, in which the passengers sit. The car is supported on e e e by the pulleys h h ; it is attached to the endless cord by the stem k 4 which is furnished with two holes to admit the cords at k and 1. These holes pass through the stem at right angles to each other, so that when the upper hole k is placed in the direction of the cord, it will pass freely along it, while the lower hole is placed across the cord, and thereby holds it fast, and the car is dragged along by it : but if the stem be turned (which may be done by the handle at 1,) the reverse operation will take place, and the vehicle will be dragged along by the upper cord. The endless cord may be put in motion either by persons in the car turning the pulley at i, or by persons turning the wheel at the station f, which is connected by an endless chain or band to the pulley b.