Fig. 141-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,

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. 590. Strong and durable stone, dressed with the chisel, or hammer, should alone be used for the masonry of bridges where the span of the arch exceeds fifty feet. The interior of

Fig. 143-Represents an elevation of a pier, a portion of two arches, and the centre of the bridge of which Fig. 141 is the section.

A, face of starling.

B, hood.

C, voussoirs with chamfered joints.

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 me dium 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. 141, 143) the starlings, the foundation courses, the impost stone,--the ring courses, at least of the heads, and the key-stone, should be of good dressed stone. The remainder may be 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.

Fig. 144-Elevation M and plan N, showing the manner of arranging the embankments of

the approaches, when the head-walls of the bridge are simply prolonged,

a, a', side slope of embankment,

b, b', dry stone facing of the embankment where its end is rounded off, forming a quarter of a cone finish.

J., flight of steps for foot-passengers to ascend the embankment.

c, c, embankment arranged as above, but simply sodded.

d, d', facing of dry stone for the side slopes of the banks.

e, e', facing of the bottom of the stream.

591. Approaches. The approaches should be so made as to procure an easy and safe access to the bridge, and not obstruct unnecessarily other channels of communication.

When several avenues meet at a bridge, or where the width of the roadway of a direct avenue is greater than that of the bridge, the approaches are made by gradually widening the outlet from the bridge, until it attains the requisite width, by means of wing-walls of any of the usual forms that may

suit the locality. The form of wing-wall (Fig. 145) presenting a concave surface outward is usually preferred when suited to the locality, both for its architectural effect and its strength. When made of dressed stone it is of more difficult construction and more expensive than the plane surface wall.

592. Water-wings. To secure the natural banks near the bridge, and the foundations of the abutments from the action of the current, a facing of dry stone or of masonry should be laid upon the slope of the banks, which should be properly prepared to receive it, and the foot of the facing must be secured by a mass of loose stone blocks spread over the bed around it, in addition to which a line of square-jointed piles may be previously driven along the foot. When the face of the abutment projects beyond the natural banks, an embankment faced with stone should be formed, connecting the face with points on the natural banks above and below the bridge. By this arrangement, termed the water-wings, the natural water-way will be gradually contracted to conform to that left by the bridge.

593. Enlargement of Water-way. In the full centre and oval arches, when the springing lines are placed low, the spandrels present a considerable surface and obstruction to the current during the higher stages of the water. This not only endangers the safety of the bridge, by the accumulation of drift-wood and ice which it occasions, but, during these epochs, gives a heavy appearance to the structure. To remedy these defects the solid angle, formed by the heads and the soffit of the arch, may be truncated, the base of the cuneiform-shaped mass taken away being near the springing lines

The form of the

of the arch, and its apex near the crown. detached mass may be variously arranged. In the bridge of Neuilly, which is one of the first where this expedient was resorted to, the surface, marked F, (Figs. 133, 134) left by detaching the mass in question, is warped, and lies between two plane curves, the one an arc of a circle no, traced on the head of the bridge, the other an oval, m o o p, traced on the soffit of the arch. This affords a funnel-shaped water-way to each arch, and, during high water, gives a light appearance to the structure, as the voussoirs of the head ring-course have then the appearance of belonging to a flat segmental arch.

594. General Remarks. The architecture of stone bridges has, within a somewhat recent period, been carried to a very high degree of perfection, both in design and in mechanical execution. France, in this respect, has given an example to the world, and has found worthy rivals in the rest of Europe, and particularly in Great Britain. Her territory is dotted over with innumerable fine monuments of this character, which attest her solicitude as well for the public welfare as for the advancement of the industrial and liberal arts. For her progress in this branch of architecture, France is mainly indebted to her School and her Corps of Ponts et Chaussées; institutions which, from the time of her celebrated engineer Perronet, have supplied her with a long line of names, alike eminent in the sciences and arts which pertain to the profession of the engineer.

England, although on some points of mechanical skill pertaining to the engineer's art the superior of France, holds the second rank to her in the science of her engineers. Without establishments for professional training corresponding to those of France, the English engineers, as a body, have, until within a few years, labored under the disadvantage of having none of those institutions which, by creating a common, bond of union, serve not only to diffuse science throughout the whole body, but to raise merit to its proper level, and frown down alike, through an enlightened esprit de corps, the assumptions of ignorant pretension, and the malevolence of petty jealousies.

Among the works of this class, in this country, may be cited the railroad bridge, called the Thomas Viaduct, over the Patapsco, on the line of the Baltimore and Washington railroad, designed and built by Mr. B. H. Latrobe, the engineer of the road. This is one of the few existing bridge structures with a curved axis. The engineer has very happily met the double difficulty before him, of being obliged