arch to give greater security against the effects of the pressure thrown upon it.

Fig. 120-Represents a longitudinal section of a portion of a pier and foundations, and of an arch and its centre of the new London bridge over the Thames.

A, finish of solid spandrel with reversed arch.

B, wedge of striking plates.

C, recess over the starlings for seats.

The backs of the arches should be covered with a water-tight capping of beton, and a coating of asphaltum.

571. The entire spandrel courses of the heads are usually not laid until the arches have been uncentred, and have settled, in order that the joints of these courses may not be subject to any other cause of displacement than what may arise from the effects of variations of temperature upon the arches. The thickness of

the head-walls will depend upon the method adopted for supporting the roadway. If this be by a filling of earth between the head-walls, then their thickness must be calculated not only to resist the pressure of the earth which they sustain, but allowance must also be made for the effects of the shocks of floating bodies in weakening the bond, and separating the blocks from their mortar-bed. The more approved methods of supporting the roadway, and which are now generally practised, except for very flat segment arches, are to lay the road materials either upon broad flagging stones, (Figs. 120, 121,) which rest upon thin brick walls built

parallel to the head-walls, and supported by the piers and arches; or by small arches, (Fig. 122,) for which these walls serve as piers; or by a system of small groined arches supported by pillars resting upon the piers and main arches. When either of these methods is used, the head-walls may receive a mean thickness of one fifth of their height above the solid spandrel.

572. Superstructure. The superstructure of a bridge consists of a cornice, the roadway and footpaths, &c., and a parapet.

The object of the cornice is to shelter the face of the headwalls from rain. To subserve this purpose, its projection beyond the surface to be sheltered should be the greater as the altitude of the sheltered part is the more considerable. This rule will require a cornice with supporting blocks, (Fig. 123,) termed modillions, below it, whenever the projecting part would be actually, or might seem insecure from its weight. The height of the cornice, including its supports, should generally be equal to its projections; this will often require more or less of detail in the profile of the cornice, in order that it may not appear heavy. The top surface of the cornice should be a little above that of the

footpath, or roadway, and be slightly sloped outward; the bottom should be arranged with a suitable larmier, or drip, to pre

Fig. 122-Represents a section through the axis of a pier of bridge built of stone with brick filling, showing the arrangement for supporting the roadway on small arches. vent the water from finding a passage along its under surface to the face of the wall.

573. The parapet surmounts the cornice, and should be high enough to secure vehicles and foot-passengers from accidents, without however intercepting the view from the bridge. The parapet is usually a plain low wall of cut stone, surmounted by a coping slightly rounded on its top surface. In bridges which

have a character of lightness, like those with flat segment arches, the parapet may consist of alternate panels of plain wall and balustrades, provided this arrangement be otherwise in keeping with the locality. The exterior face of the parapet should not project beyond that of the heads. The blocks of which it is formed, and particularly those of the coping, should be firmly secured with copper or iron cramps.

574. The width of the roadway and of the footpaths will be regulated by the locality; being greatest where the thoroughfares connected by the bridge are most frequented. They are made either of broken, or of paving stone. They should be so arranged that the surface-water from rain shall run quickly into the side channels left to receive it, and be conducted from thence by pipes which lead to vertical conduits (Fig. 121) in the piers that have their outlets in one of the faces of the piers, and below the lowest water-level.

be

575. Strong and durable stone, dressed with the chisel, or hammer, should alone be used for the masonry of bridges where the span of the arches exceeds fifty feet. The interior of the piers, and the backing of the abutments and head-walls may, for economy, be of good rubble, provided great attention be bestowed upon the bond and workmanship. For medium and small spans a mixed masonry of dressed stone and rubble, or brick, may be used; and, in some cases, brick alone. In all these cases (Figs. 122, 124) the starlings, the foundation courses, the impost stone, the ring courses, at least of the heads, and the keystone, should be of good dressed stone. The remainder may of coursed rubble, or of the best brick, for the facing, with good rubble or brick for the fillings and backings. In a mixed masonry of this character the courses of dressed stone may project slightly beyond the surfaces of the rest of the structure. The architectural effect of this arrangement is in some degree pleasing, particularly when the joints are chamfered; and the method is obviously useful in structures of this kind, as protection is afforded by it to the surfaces which, from the nature of the material, or the character of the work, offer the least resistance to the destructive action of floating bodies. Hydraulic mortar should alone be used in every part of the masonry of bridges.

576. Approaches. The arrangement of the approaches will depend upon the number and direction of the avenues leading to the bridge, the width of the avenues, and their position above or below the natural surface of the ground,—and the locality. The principal points to be kept in view in their arrangement are to procure an easy and safe access to the bridge for vehicles, and not to obstruct unnecessarily the channels, for purposes of navigation, which may be requisite under the extreme arches.

Fig. 124-Represents an elevation of a pier, a portion of two arches, and the centre of the bridge of which Fig. 122 is the section.

A, face of starling.

B, hood.

C, voussoirs with chamfered joints.

When the avenue to the bridge is, by an embankment, in the same line as its axis, and the roadway and bridge are of the same width, the head-walls of the bridge (Fig. 125) may be prolonged sufficiently far to allow the foot of the embankment slope to fall within a few feet of the crest of the slope of the water-course; this portion of the embankment slope being shaped into the form of a quarter of a cone, and reveted with dry stone or sods, to preserve its surface from the action of rain.

When several avenues meet at a bridge, or where the width of the roadway of a direct avenue is greater than that of the