built in cement, only the centres should be struck sooner; in large arches, however, built with quick-setting cement mortar, it is considered inadvisable to ease the centres at all. The cement in the portion of the work first built will have set before the completion of the arch, and any easing would injuriously affect the setting of the cement in the portion of the arch which was executed last. Disturbance of the work by vibration should be guarded against, as it destroys the setting power of all kinds of mortar, especially of cement. The centering for concrete arches should not be disturbed for at least fourteen days after finishing the concrete.

106. Centres for Arches of Bridge over the River Don and London Bridge. The centering shown in fig. 41 is Telford's design for a centre with intermediate supports, used for a bridge over the river Don, of which the span is 75 feet, while fig. 42 shows the pier centering of trussed girders adopted in the building of London Bridge.

FIG. 41.-Centre for arch of a bridge over the river Don.

The latter illustration shows that one-fourth of the span at either side was directly supported by piles and diagonal bracing. The rib across the middle half of the span was formed of a diagonally braced girder of great stiffness, the depth being one-fourth of its span. The striking plates and wedges by which the centre is lowered after the completion of the arch, are shown in the figure, a being the upper plate, b the lower plate, and c the compound wedge, which is kept in position by the cross wedges d.

107. Iron pipes 1 foot long, filled with dry sand, and provided with a cylindrical block of wood fitting into the top, placed under the ribs of the centering, are sometimes used as a substitute for wooden wedges in easing and striking centres. The process of lowering is accomplished by allowing the sand to run out of the cylinders as required.

108. Timber Bridges. Bridges erected with timber supports and superstructure are now an almost obsolete form of construction in this country, except for temporary purposes. Timber beams resting on abutments or piers of masonry are used to a limited extent when the span is

small and the traffic unimportant. In those countries where timber is abundant many large bridges have been erected with this class of material, and when properly protected and painted are said to last about twenty years. The simplest manner in which timber can be applied, when the

supports are of masonry, is in the form of beams or string pieces with cleading fixed on the top to carry the roadway. When the span exceeds Finished surface of Road

8 or 9 feet, struts are generally introduced, as shown in fig. 43. Another form of construction suitable for spans up to 30 feet is shown in fig. 44, and may be used for bridges carrying light traffic. Beyond these spans different forms are employed, such as trussed girders composed principally

of struts and ties, with parallel top and bottom beams, or in the form of an arch.

Timber bridges, owing to their perishable nature when subjected to the influence of alternate wet and drought, should not be made use of except when other suitable materials are not available. The annual cost of repairs upon timber bridges becomes a heavy item, and in the course of a few years will equal, if not greatly exceed, the expenditure which the building of a permanent structure of stone or brick work would entail.

109. Iron and Steel Superstructures.-The superstructure of bridges formed with iron or steel has to a great extent superseded the ordinary masonry arch, in bridges of long span, or where the line of the bridge is at a considerable angle to the object crossed. The employment of cast iron is limited to bridges having short spans, the road way being carried by jack arches built between and springing from the lower flanges of the beams; the arching being generally made 9 inches thick, and covered with a 4-inch layer of asphalte or other impervious material, The space between the beams and over the arches is filled with concrete, broken bricks, or other material of a like nature, up to the level on which the metalling is spread.

The thrust on the outside or face-girders, when jack-arches are em

ployed, is provided for by means of tie-rods 1 to 3 inches in diameter, placed at intervals across the bridge from 5 to 8 feet apart, which bind all the longitudinal girders together. This method of construction is illustrated in fig. 45.

The superstructure of most bridges is now constructed of wrought iron or or steel. The plate girder of uniform depth or hog-backed shape has been

greatly used, and was the form employed almost exclusively previous to the lattice type being introduced.

The plate girder is still made use of in spans up to 60 or 70 feet; beyond this it is more economical to employ girders of the lattice type, which may have parallel booms, or be constructed on the bowstring principle; sometimes, however, they are built in the form of an arch.

When the head-room or height to the under side of the girders in crossing an existing line of communication, railway, canal, or navigable river is limited, only outside main girders are employed to carry the roadway. When, on the other hand, no such restriction is involved, the roadway may be carried on the top of a series of plate or lattice girders having a uniform depth. In the former case cross girders, of the plate or lattice type, are required, and extend between the main girders to the lower flanges of which they are fixed. These transoms or cross girders are built with a camber similar to that of the rise of the transverse section of the road; they are spaced from 6 to 10 feet apart, on which bearers running parallel to the main girders are fixed, and on the bearers convex buckle or corrugated iron plates are riveted, which, extending the whole width across the bridge, completes the superstructure. These plates are generally covered with asphalte, but concrete, composed of cork and bitumen, is also made use of on account of its lightness, while fine cement concrete is at times employed to protect from moisture the ironwork on which rest the materials composing the road.

110. Steel Trough Decking.-Steel trough decking is now extensively used in forming the floor or deck between the main girders of bridges, and may be fixed on the top or to the web and lower flanges of these girders. This form of decking is gradually superseding that of cross girders and buckle plates in the construction of the steel superstructure of bridges. Trough decking may also be used, not in the form of a cross girder but longitudinally, in which case the outside or main girders are dispensed with, and their employment insures a considerable saving on the first cost of constructing bridges up to a span of 36 feet. They are designed to carry with safety live or moving loads, comprising the weight of road-rollers and of traction-engines, in addition to the ordinary loads to which road bridges are subjected. The cross-section of a road bridge with this type of superstructure is illustrated in fig. 46. The dimensions shown are suitable for spans of from 16 to 20 feet; the weight of the steel decking for these spans is from 25 to 30 lbs. per square foot respectively.

111. The gradients of the approaches to many existing bridges and culverts could be greatly improved by substituting trough decking for the rubble arches of which many of these are built when they fall into a state of disrepair and require renewing. The relative cost of building an arch of brickwork in cement, including haunches, spandrils, and centering, for a bridge having a span of 20 feet, would be about 3s. 2d. per square foot of area covered; and for trough decking, assuming that the abutments are

built to the same height in either case, 2s. 8d. per square foot. The saving effected in the construction of the superstructure of such a bridge 16 feet wide would be £15 in favour of steel trough decking.

112. Protecting Dangerous Places.-All roads at dangerous points should be protected to prevent accidents to pedestrian and vehicular traffic. This, as a rule, applies more forcibly to precipitous hillside and mountain roads. Road authorities are under an obligation to perform the work necessary in carrying out these protective works, while the roadsides on level ground are fenced by the proprietors through whose land the highway passes. Many methods are in use for accomplishing this.

Earthen mounds are the most economically formed, but are generally in time raised too high by the roadside accumulations being heaped upon them, whereby free circulation of air on the road surface is prevented. The outlets or cuts for discharging the surface drainage of the road require

TIron Standard

Kerb Stone

21'. o"--

Metalling
Cashion of
Sand
Concrete

--2--% Metal

FIG. 46. Cross-section showing arrangement of steel trough decking

as applied to bridge work.

special attention where this form of protection is adopted. Parapet walls of stone built dry are an efficient and economical means of protecting a road, where such material is available. In sidelong cuttings, where retaining walls are a necessity, the parapet walls are formed by continuing these to a proper height. They should have a thickness of at least 18 inches, and be 3 feet 6 inches to 4 feet high, finished with a rough cope set on edge, while the outside joints and cope should be pinned and pointed with lime mortar. The most common fence, however, is that of the quickset hedge, planted on the top of a raised mound made from the excavations in forming the ditches. It makes a good protection after the lapse of some time, but requires in the interval a considerable amount of attention. An open post and rail fence answers the purpose very well, while wooden and iron posts to which plain galvanized iron wires are attached is the method adopted in districts where building stones cannot be conveniently obtained. The great objection to stone fences is that, being liable to decay and damage,