BRIDGES.
The earliest efforts of the civil engineer were in all probability directed to the construction of roads, but it would be evident that unless they could be made to cross rivers and water-courses, they could continue but a short distance in any required direction. Bridges of some sort must therefore be constructed; small brooks were doubtless bridged over with timber laid across them, and “fords” or shallow parts of rivers taken advantage of when the stream was too wide to be spanned in this way. That these fords were once very numerous in England, is shown by the constant recurrence of such names as Stratford, Brentford, &c., and indeed fords are now in use in many places where the stream is shallow, and the traffic too little to pay for constructing a bridge. The next improvement in all probability consisted in placing stones in the fords to enable passengers to step from one to the next, and cross the stream dry-footed, which were called “stepping stones;” but the earliest bridges properly so called, were probably stones piled upon each other and united by beams of timber.
VIADUCT.
The Chinese, Romans, and several other nations were acquainted with the use and proper mode of constructing bridges, many centuries ago, when no such works had been attempted in England. The materials chiefly now in use for bridges are stone, brick, iron, and wood, and by far the greatest number, till within the last half century, were constructed of stone, but iron seems to be rapidly taking its place, especially since the mode of constructing suspension bridges has been more perfected. There are several ways of forming the foundation for the piers, or those parts on which the bridge rests. Old London bridge and many others of considerable size were based upon “piles,” these piles consist of great pieces of timber pointed at one end and driven deep down into the bed of the river close together, so as to form a solid mass on the top of which the stones for the piers were laid; these piles have been found scarcely altered after several centuries of immersion in the mud, but at the parts exposed to the action of the water they soon become decayed and eaten away by its inhabitants. For many bridges, when there is good foundation, the stonework is laid at once on the bed of the river, but in order that this may be got at, a double ring of piles is driven close together around the part where the pier is to be built; these piles are of a flat form, and the two rows inclose a space of about a foot between them, which space is filled with clay, well rammed in, and the water within the barrier pumped out, this forms what is called a “coffer-dam,” and is an expensive proceeding. The bottom of the river thus exposed can be built upon in the usual way. Another way of forming a foundation for piers consists in lowering large wooden frames or boxes filled with concrete which hardens and forms a solid basis, these are called “cassons.” Wooden bridges are now seldom made of any great size, but are very numerous in country places for crossing narrow streams, on account of their cheapness and the facility with which they can be constructed. Bridges of brick are not much used, but “viaducts” of this material often occur in the lines of railways, and very handsome arches of brick-work are used to span streets in towns where railways pass. Iron girder bridges also, are frequently employed for this purpose; they are made with girders of wrought or cast iron which generally pass in a straight line over the roadway from pier to pier. Some of them are made of iron-plate rivetted together, while others are strong girders of cast iron having small arches of brick-work built on them, running the length of the girders and resting upon ridges in them, so as to fill up the space between each girder.
THE BRITANNIA TUBULAR BRIDGE.
Iron bridges are of almost every form and construction conceivable; some are tubes of iron-plate, as the Britannia Bridge, rivetted together; some of solid cast iron work as Southwark Bridge, others called suspension bridges are hung by rods of wrought iron, from large chains suspended from pier to pier, as Hungerford Bridge. In many places bridges of boats are used; the boats are moored to the river’s-bed and have timber-work connecting them. Another kind, which can hardly lay claim to the name of bridge, called the “floating-bridge,” is used for the conveyance of carts, horses, passengers, or merchandise, from one side of a river to the other. It consists of a sort of platform guided by chains laid down for the purpose, which pass over wheels or drums on the side of the bridge. The motive power necessary for turning these wheels, is supplied by a steam-engine within the structure itself.