Timber bridges, which are in many cases advantageous from their cheapness, are fre quently preferred by modern engineers for their peculiar fitness in cases where an im perfect foundation, or some other circum stance, renders bridges of stone, brick, or iron unsuitable ; while the introduction of various methods for preserving wood from decay obviates one of the principal objections to them. By the due application of trussing, timber bridges of extraordinary span may be constructed with safety. Some such are called pendent or philosophical bridges, and many ex-. amples of this kind of structure are found in America. The Upper and Lower Schuylkill bridges, near Philadelphia, and that across the Delaware at Trenton, are among the most remarkable. The Upper Schuylkill bridge, which is also called the Colossus, consists of one very flat arch of 34.0 feet span. The Lower Schuylkill bridge has three arches, one of 195 and the others 150 feet span, resting, on stone piers. The Trenton bridge, erected in 1819, has five arches, one of 200, two of 180, and two of 160 feet span, resting upon light stone piers. Lattice bridges, so called from their structure resembling lattice-work, are much used in the United States, and have been introduced in English railway engi neering.
The securing of a good foundation for the piers is the point of primary importance in bridge-building. Where the ground is soft, this must be done by piling, or driving whole trunks of timber, pointed and shod with iron, into the earth ; it is in this kind of work that Nasmyth's steam pile-driver shows its great power; and still more, perhaps, Dr. Pott's mode of sinking hollow piles by pneumatic pressure. In order to exclude the water while laying the foundations and constructing the masonry of the piers, it is usual to form in the bed of the river a coffer-danz, or water tight enclosure of piling, from the interior of which the water is pumped out. To save the enormous expense of this plan caissons were invented ; they may be compared to great tubs or flat-bottomed boats, with very strong vertical sides. One of these being floated to the spot where a pier is to be founded, a few courses of masonry are built within it, and it is sunk into the required position. The ver tical sides are subsequently removed, leaving the flat bottom beneath the masonry, as a kind of floor or foundation.
An ingenious modern contrivance is the floating bridge contrived by Mr. Bendel in lieu of an ordinary steam ferry-boat, for which the current was found to be too strong, as a means of communication between Torpoint and the Cornwall shore, at the mouth of Plymouth harbour, and which has since been applied also to the harbours of Dartmouth, Ports mouth, and Southampton. It consists of a large flat-bottomed vessel, the deck of which is adapted to receive horses and carriages as well as foot passengers, and which is pro pelled by means of wheels turned by a steam engine mounted in the vessel. These wheels, however, instead of propelling the vessel by their action upon the water, do so by taking hold, by means of protuberances formed on their circumference, of chains which are ex tended from shore to shore, secured at each end, but allowed to hang under water in a festoon or curve of sufficient depth to allow ships to pass over them without danger. The
chains, which are not absolutely fixed at the ends, but are attached to very heavy balance weights, are lifted up by the vessel as it pro ceeds, and serve not only as an abutment to secure the progress of the vessel, but also to keep it in its right course.
The high-level bridge at Newcastle is one of the finest examples of modern bridge build ing. It accommodates the York and Berwick Railway, and also the ordinary road traffic. It is a double bridge, with the railway road 2:2 feet above the horse road. The total length is about 1,400 feet. The extreme height is 112i feet above high water mark. There are6 arches, each of 125 feet span, besides several land arches. The piers are of masonry; but the arches, pillars, braces, girders, and balus trades are of iron. The carriage roadway is 20 feet wide, with two foot pavements of 6 feet each. The bridge piers are no less than 131 feet high from the foundation; they are 48 feet by 164 in the square ; they are built on piles, some of which penetrate to a depth of 50 feet in the ground. The road bridge is suspended from the arches of the upper or railway bridge ; thus affording a very re markable example of the combination of two distinct principles in bridge building.
The border bridge over the Tweed at Ber wick is another fine example of railway work; constituting one of the finest stone bridges in existence. The still more extraordinary Bri tannia tubular bridge over the Menai will come for notice in a later article. [MENAI BRIDGES.] The hollow girder principle, so scientifically applied in the Britannia bridge, is now much used for small railway bridges over roads and streets.
Mr. Gladstone has recently suggested the employment of wrought iron bars as a mate rial for bridges. The astonishing strength of the angle irons and other parts of the Britan nia tubular bridge, is leading engineers to the consideration of further employment of simi lar materials. Mr. Gladstone proposes• the use of malleable iron bars, of the double T shape, to form almost the only material of the bridge. These bars are to be either rivetted at their flanges, or fastened together by nuts and screws. All the bars are to be placed horizontally ; and, at spots where additional strength is required, such as near the abut ments or piers, they might be rivetted side by side, so as to form a solid mass ; while near the centre of the water way or arch they might be so extended as to form an open iron-work of elegant character. The engineer who sug. gents this plan points out four advantages which he thinks would accompany its adop tion—it would enable a span of almost any length to be made ; it would render a nearly flat roadway practicable ; it would give a higher water-way than an arch springing from piers in the usual manner ; and it would render the use of expensive centering unnecessary.