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.
Illustration: 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.
Illustration: 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.