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 Iron and Steel Superstructures.-The superstructure of bridges formed with iron or steel has to a great extent superseded the ordinary masonry arch, in bridges of long span, or where the line of the bridge is at a considerable angle to the object crossed.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 roadway 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

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

-20%-% 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,

they are expensive to maintain. So far as the durability of the road surface is concerned, this is best promoted by having open fences, as by this means. of protection the beneficial influence of the sun and wind in producing evaporation is not lessened; while with close fences, such as stone walls and hedges, especially if they be high, the beneficial effects mentioned are diminished to a considerable extent.

113. In districts where snowstorms are prevalent during the winter months, the existence of close fences alongside a highway, apart from those for protecting dangerous places, is a source of considerable expenditure in opening the roads for traffic after a snowdrift. Where walls and hedges form the boundary line of a road it is not an uncommon experience to encounter a snow cutting of 6 feet in depth the full width of the roadway, and extending in many instances for miles, while the adjoining fields are comparatively free from snow.

In the author's experience, the cost of clearing snowdrifts has amounted, on many occasions, to from £400 to £600 over a district of between 200 and 300 miles, and this expenditure in all probability would have been saved, or at all events greatly reduced, had open fences existed instead of walls and hedges. Besides the heavy cost thus entailed, considerable damage is done to the road surface by the melting snow, especially where there is heavy vehicular traffic. It is doubtful, even on consideration of the preceding facts, whether any improvement may be looked for in the direction indicated. The protection afforded to farm stock when grazing, and the capital expenditure already involved in providing these close fences, remove to a remote date any probability of a change, or it might be said reform, being brought about.

CHAPTER IV.

ROAD MATERIALS OR METAL.

114. The bearing of the science of geology in relation to the formation of new roads brings the engineer into close practical contact with this subject, and on the manner in which the prominent physical features and underlying strata composing the earth's crust are examined will the ultimate success of the undertaking greatly depend. A knowledge of geology is also of great service in selecting a site for a quarry from which suitable rock may be obtained, either for the construction of a new road, or for preparing metalling for the maintenance of existing highways. The hard igneous, plutonic, and metamorphic rocks generally used for road work constitute large masses in the crust of the earth, which are now exposed at the surface, or form veins, dykes, and intruded sheets in the strata overlying them.

115. A sufficient knowledge of the locality and general geological structure of the particular stratum desired may be readily obtained, in the first instance, by a study of the coloured maps, with sections of the strata of which the surface of the country is composed, issued by the Geological Survey of Great Britain.

116. Rock Materials for Roads.-The different varieties of rock from which macadam is prepared for the construction and maintenance of roads, and the making of setts for paved streets, vary to a considerable extent in their component parts, and also the degree of crystallization which has taken place. It is very seldom, indeed, that two samples of rock taken from adjoining quarries, at no great distance apart, are found to contain the same proportion of 'simple minerals,' while in cases of rock taken from the same quarry the primary forms of crystallization are quite distinct, forming different varieties of structure, although the chemical composition may be similar in each case.

The igneous and siliceous rocks form the greater bulk of the materials used in road making and repairs, but many other different kinds of stone have been, and still are, used for this purpose.