columnar, being either straight or swelled, (Figs. 111, 112, 113, 114.) They should be built as high as the ordinary highest

E

Fig. 111-Represents in elevation starlings A, their hoods B, the voussoirs C, the spandrels D, and the combination of their courses and joints with each other in an oval arch of three centres.

E, parapet; F, cornice.

E

Fig. 112-Represents in elevation the combinations of the same elements as in Fig. 111 for a flat segmental arch.

Fig. 113-Represents in elevation the combinations of the same elements as in Fig. 112,

from the bridge of Neuilly, an oval of eleven centres.

om, curve of intrados.

on, arc of circle traced on the head of the bridge.

Fig. 114-Represents a cross section and elevation through the crown of Fig. 113, showing the arrangement also of the roadway, foot-paths, parapet, and cornice.

water-level. They are finished at top with a coping stone to preserve the masonry from the action of rain, &c.: this stone, termed the hood, may receive a conical, a spheroidal, or any other shape which will subserve the object in view, and produce a pleasing architectural effect, in keeping with the locality.

In streams subject to freshets and ice, the up stream starlings should receive a greater projection than those down streain, and, moreover, be built in the form of an inclined plane (Fig. 115)

to facilitate the breaking of the ice, and its passage through the arches.

567. Where the banks of a water-course spanned by a bridge

are so steep and difficult of access that the roadway must be raised to the same level with their crests, security for the founda tion, and economy in the construction demand that hollow, or open piers be used instead of a solid mass of masonry. A construction of this kind requires great precaution. The facing courses of the piers must be of heavy blocks dressed with extreme accuracy. The starlings must be built solid. The faces must be connected by one or more cross tie-walls of heavy, wellbonded blocks; the tie-walls being connected from distance to distance vertically by strong tie-blocks; or, if the width of the pier be considerable, by a tie-wall along its centre line.

568. The foundations, the dimensions, and the form of the abutments of a bridge will be regulated upon the same principles as the like parts of other arched structures; a judicious conformity to the character of strength demanded by the structure, and to the requirements of the locality being observed. The walls which at the extremities of the bridge form the continuation of the heads, and sustain the embankments of the approaches, and which, from their widening out from the general line of the heads, so as to form a gradual contraction of the avenue by which the bridge is approached, are termed the wingwalls,-serve as firm buttresses to the abutments. In some cases the back of the abutment is terminated by a cylindrical arch, (Fig. 116,) placed on end, or having its right-line elements ver

tical, which connects the two wing-walls. In others (Fig. 117)

a rectangular-shaped buttress is built back from the centre line of the abutment, and is connected with the wing-walls either by horizontal arches, or by a vertical cross tie-wall.

569. The wing-walls may be either plane surface walls (Fig. 118) arranged to make a given angle with the heads of the bridge,

N

M

Fig. 118-Represents an elevation M and plan N of a portion of a single arch bridge with straight wing-walls sustaining an embankment across the valley of the water-course.

a, a', face of wing-wall.

b, b', side slope of embankment.
c, c', top of wing-wall.

o, o, fender or guard stones.

or they may be curved surface-walls presenting their concavity, (Fig. 126,) or their convexity to the exterior; or of any other shape, whether presenting a continuous, or a broken surface, that the locality may demand. Their dimensions and form of profile will be regulated like those of any other sustaining wall; and they receive a suitable finish at top to connect them with the bridge, and make them conform to the outline of the embankments, or other approaches.

570. The arches of bridges demand great care in proportioning the dimensions of the voussoirs, and procuring accuracy in their forms, as the strength of the structure, and the permanence of its figure, will chiefly depend upon the attention bestowed on these points. Peculiar care should be given in arranging the masonry above the piers which lies between the two adjacent arches. In some of the more recent bridges, (Fig. 120,) this part is built up solid but a short distance above the imposts, generally not higher than a fourth of the rise, and is finished with a reversed

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