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lift up that part gradually before it come again in contact with the haunches. It is evident therefore that an areh, built on a centre perfectly suited to equilibration, will not be in cquilibrio when the centering is removed. It is therefore necessary to form the centering in such a manner, (by raising the crown,) that it shall leave the areh of a proper form. This is a very delicate task, requiring a previous knowledge of the ensuing change of form. But suppose this attained, there is another difficulty. While the work advances, the centering is warped by the load laid upon it, and continually increasing on each side. The first pressure on the centering forees down the haunches aud raises the crown. The areh is therefore less curved at the haunches than is intended: the joints however accommodate themselves to this form, and are close and filled with mortar. When the masons approach the middle of the arch, the frame sinks there and rises up at the haunches. This opens all the joints in that place on the upper side. By the time that the key-stones are set, this warping has gone further; and the joints are open on the under side near the crown. It is true we are here speaking of an extreme case when the centering is very flexible, but this occurred to Perronet in the two great bridges of Neuilly and of Mantz. In this last one, the crown sunk above a foot before the key was set, and the joints at the haunches opened more than an inch aboce, while some nearer the crown opened nearly a quarter of an inch below. In this condition of things it is a delicate business to strike the centering. Were it removed in an instant, all would probably come down, for the areh-stones are not yet abutting on each other, and the joints in the middle are open below. "Perronct's method was to begin to detach the centering at the very bottom on each side equally, where the pressure on the centering is very slight. He cut away the blocks immediately under each areh-stone, and proceeded gradually upwards in this way, till all was detached that had been put out of shape by the bending of the centering. This being no longer supported, sunk inward till it was stopped by the abutment which it found on the areh-stones near the crown, which were still resting on their blocks. During part of this process, the open joints opened still more, owing to the removal of the load from the haunches of the centering. This allowed the crown to sink still more, by foreing out the areh-stones at the haunches. Ho now paused some days, and during this time the two haunches, now hanging in the air, gradually pressed in toward the centering, their outer joints closing in the meanwhile. The haunches were now pressing pretty hard on the areh-stones nearer the crown. He then proceeded more slowly, destroying the blocks and bridgings of those upper areh-stones. As soon as the support of one was destroyed, it immediately yielded to the pressure of the haunch; and if the joint between it and the adjoining one toward the crown happened to be open, whether on the upper or the under side, it immediately closed on it. But in this way it was found that every stone sank a little while it closed on its neighbour, tending to pro

duce the deformity of a ragged soffit. They were therefore not allowed to sink so much. In the places of the blocks and bridgings which had been cut away, small billets were set standing on their ends between the centering and the areh-stones. These allowed the pendulous areh to push toward the crown without sensibly descending; for the billets were pushed out of the perpendicular, and some of them tumbled down. Proceeding in this way, he advanced to the very next course to the key-stone on each side, the joints closing all the way as he advanced. The detaching the three uppermost courses from the centering was most troublesome; for the whole elasticity of the centre was trying to unbend and pressing hard against them. They were found to be lifted up; for the joints beyond them, which had closed completely, now opened again below: but this was finished in one day, and the centre sprung up 2 or 3 inches, and the whole areh sunk about fi inches. This was an anxious time," says Dr. Robison, "for he dreaded the great momentum of such a vast mass of matter: it was hard to say where it would stop. It stopped, however, very soon, settling slowly as the mortar was compressed, and, after one or two days, settling no more. This settling was very considerable, both in the bridge at Neuilly and in that at Mantz. In the former, the sinking during the work amounted to 13 inches. It sunk 6 inches more when the blocks and bridgings were taken out, and \\ when the small standards were destroyed, and \\ more next day; so that the whole sinking of the pendulous areh was 9A inches, besides what it had sunk by the bending and compression of the centering. In fact, the whole sinking of this areh was 23J inches. The sinking of the arehes at Mantz amounted in all to 20J inches, of which 12 inches was owing to the compression and bending of the centering."

After the centres of the arehes of the Pont de Neuilly had been eased in the manner described, the principal timbers were suddenly removed, in the presence of the king (Louis XV.), the court, and a vast assemblage of spectators, collected together, as Perronet modestly observes, in consequence of the presence of royalty. The whole formed a sort of coup-de-spectacle of a very striking kind. Perronet describes minutely the arrangements made for the occasion, and gives an animated view of the scene as it appeared on the 22d September, 1771. To render the manoeuvre more interesting, it was arranged that all the frames of each areh should be made to fall in succession, within a few minutes. For this purpose, ropes were attached to the frames, and were drawn tight by capstans, placed at some distance on the land (see Fig. 301). Nine men were placed at each capstan, one of whom superintended the arrangement of the ropes, and another directed the motion of the capstan, each turn of which was regulated by the sound of drums. In this way the frames were all pulled down in 3-J- minutes. The fall of such a vast quantity of timber, which weighed at least 720 millicrs for each areh, caused the water to rise in foam upon the bridge. The arehes were thus completcly exposed to view, and their light appearance and bold construction were generally admired. The string-courses, parapets, and masonry above were yet to be added. In this unfmished state of the bridge, it occasioned great surprise to see all this timber framing suddenly fall down, which, an instant before, seemed necessary to the support of the arehes. The king, in returning to Marly, passed in a carriage over the new bridge. The spectacle came oft without any accident, and a medal was struck to commemorate the occasion.

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i. The arehes. When the centres have been placed, and properly secured, the setting of the areh-stones is the next step. The masonry of the piers and abutments near the springing is properly adjusted, and, in some cases, immediately below the commencement of the curvature, it is usual to lay a capping, string, or cordon, the slight projection of which covers any small inaccuracy in setting out or carrying up the abutments and piers. The form and dimensions of the areh-stones are of the greatest importance. These have been greatly varied in different works. The French have used very deep areh-stones, which, in conjunction with their wide mortar-joints, has contributed to the great sinking of their arehes. The depth of the areh-stones at the crown in the bridges of Orleans and Mantz is 6 feet. In Blackfriars and Westminster it is 5. Mr. Telford thinks from 2J to 4 feet a good length. "When they are longer, as the beds can scareely ever bo worked and set exactly true, they are apt to break when the weight comes upon them; and when shorter, there is not sufficient space to overlap or break the joints properly. Each course should be of equal thickness quite through between the headers. The thickness of each course should be from one-third to one-half their depth, and they should be chamfered or rusticated along the bed-joints, and also those of the outside heads. The beds should be worked as true as possible for the whole breadth of each stone; the neglect of which destroys every other precaution. Each stone should

be laid so as just to swim in the mortar, and be struck with a maul two or three good blows. The joints of the headers should be of equal thickness with those of the other stones in the same course. Inexperienced masons, by laying the headers with thinner joints, for show of fme-work, frequently create an unequal pressure, which bursts or splinters the headers before the interior areh-stones conic to an equal bearing." In setting the areh-stones, each course must point in the direction of the radius; and, that the workman may do this correctly, the thickness of each course should be marked upon the outer ribs, and its line of direction upon the lower part of the beams of the same ribs. The courses must also be carried equally on each side of the centre, and the masonry over the solid part of each pier in the spandrels must also be carried up. It is sometimes necessary to place a temporary weight upon the crown of the centre, until the load approaches the middle. This is done in Fig. 300. If the arehes are flat, one side of the pier must not be exposed until it have sufficient weight upon it, or is guarded by resistance on the opposite side. At the bridge of Mantz, a neglect of this precaution caused one of the piers to be pushed 4J inches out of the perpendicular. By loading the opposite side, it was made to return 2J inches.

The keystones should be driven with moderate foree, so as to fill their places firmly. When this is done, all the back and cud joints of the whole areh should be carefully examined, and all vacancies run full of mortar, and firmly wedged with slates. The whole should then be left for some time to dry and get hard. Meanwhile, the masonry should be brought up in the spandrels to the level of about one-fourth of the rise of the areh. This was formerly, and is now, in some cases, of rubble-work. The outside stones in the part over the pier are carried up to the same height, but close to the areh-stones they are stepped or racked back, and so left until the centre is removed, because, if finished close up to the back of the arch-stones, the least sinking of the arch would cause a fissure.

When the centres are removed, the soffit of the arch is carefully examined, and the joints pared, cleaned out, and pointed. The chamfered or rustic joints prevent the edges from chipping, and cover any trifling inequality, so that the cross joints only require paring.

5. The spandrels and wing-walls. The spandrels of a bridge are the spaces between the haunches of an arch and its vertex at the extrados of the roadway. When the arches are completed, the points of the piers are brought up and finished, at some distance above high-water mark, by sloping them back to the face of the spandrel, in a triangular or circular form, or otherwise disposing them to receive columns, pilasters, or turrets. The latter act ns buttresses, as well as ornaments, and contribute to the stability of the superstructure. The spandrels are finished in various ways: in some of the old bridges they were filled up with earth or gravel. In small bridges, the masonry should be brought up to the level of about one-fourth of the rise of the arch, and then be sloped up to the top of the back of the arch-stones, and the remaining space filled up with gravel or stone rubbish. In large French bridges they have been filled up entirely with rubble masonry; but Mr. Telford objects to this as throwing unnecessary weight upon the arches. To remedy this, openings have been made quite through, as in Fig. 294, and kept open or concealed, or vaults have been constructed, to lighten the piers which sunk, or those adjacent to them. Fig. 302 shows the arrangement of the spandrels in

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with these, and being connected with them by long stones, become a part of the general frame. These walls are carried up to near the level of the top of the arch-stones, where they are covered with two rows of flat stones, or the openings are arched over. Small openings are made in the walls upon the top of the rubble masonry, through which any water that may fall into the spandrels is conducted to ono point, where it issues through a pipe inserted in the arch-stones.

The outside walls are usually made thicker than the interior: they are faced with square masonry, and have a rubble backing; the whole thickness being about one-fifth of its height. When these walls are very high, a wall is also built along the middle of the piers and abutments which cross each other, and into which they are tied by bond-stones or pieces of timber laid at about every six feet in height. When these spandrels have been brought up to the level of the top of the arch-stones, they are dressed into the slope which it is proposed to make the roadway. Here there is usually laid a cordon and cornice, extending along the whole of the arches, spandrels, and wing-walls. Its upper course should be of sufficient breadth to allow for the projection, and to pass quite through under the parapet, which, by standing upon it, will keep it secure. The upper side of the projecting part should have a slope or weathering, to throw off the water.

The wing-walls behind the abutments are sometimes laid at the same depth as the abutments, and arc similarly secured by piles and platforms. If the ground be firm, they are founded by steps, rising up as they retreat. This saves much masonry. Their thickness is from about one-fourth to one-fifth their height. When very long and high, a cross-wall should be built, reaching between them, into which they should be tied. The wing-walls should terminate in newells or pilasters.

6. The parapets usually consist of a plinth, dado, and coping. They are made from Z\ to 6 feet in height above the footpaths or roadway; but 4 feet 4 inches is sufficient for protection and decoration, and is not too high to obstruct the view. The dado, or middle member, is about 10 or 12 inches thick, but sometimes more; the plinth so much more than this as to leave an offset of about an inch on each side. The coping is usually made to slope each way from the middle. Balustrades are sometimes introduced instead of the dado; and in some cases, as where a bridge is exposed to violent gusts of wind, there are only half-balusters on the outside, the inside being solid.

7. The roadway. Gaulhey remarks that most of the bridges erected before the eighteenth century were built with a view to economy, both with respect to the stylo of building and the degree of breadth allotted to them, since the most considerable scarcely allow room for two carriages to pass abreast. The bndges of Paris are for the most part very wide; but this extent was given to them solely with a view to erect two rows of small houses on their sides,—a circumstance which must naturally have produced a very narrow thoroughfare. In modern bridges, the convenience for traffic is very properly the first consideration; a proper space being allowed for foot passengers, and another and larger space for horses and wheel-carriages.

When the spandrels have been covered by arches or flat stones, the fonndations for the footpaths are built witn rubble-stone for the outside curbing. This should be of hard stone, set in lime-mortar. The space between the curbing and the parapets should be paved with hard flag-stones, laid in lime-mortar upon a bed of coarse sand or clean gravel. The breadth of the footpaths may vary from 3 to 6 feet, or more. If the carriage-way is to be paved, there should be laid upon the covering of the spandrels and over the top of the arches a bed of gravel, mixed with loam, from 12 to 18 inches in thickness, worked with water into the consistence of mortar. When this has become moderately firm, squared paving-stones arc set and well beaten, making a curve across the road of 6 inches in 24 in breadth, and that curve should be terminated by sinking 4 inches more in the distance of 2 feet from the inclined plane formed along the outer edge of the curbing-stones. But if the roadway be only of gravel, it must be laid 22 inches in depth in the middle, and 18 inches near the sides. The gravel should be mixed with a little loam, to consolidate it, and exclnde water. A gutter of small squared stones should be formed on each side of the roadway, to carry off the water. This in all cases

should be conducted beyond the extremity of the wing-walls, and got rid of in covered drains or sewers.

Respecting the decorations of a bridge, Mr. Telford sensibly remarks :—"The decorations should be varied according to the situation and accompaniments. In the country, the utmost simplicity consistent with distinguishing the essential parts, should be preserved; and even in the most splendid cities, or adjacent to palaces, all decorations should be kept perfectly subservient to, and in unison with, the essential parts. The neglect of this is a frequent error in designing bridges. Columns and entablatures, though proper in a Grecian temple, are ill suited to an edifice where forms unknown to the Greeks are the leading features. As columns can only be placed over the piers and abutments, the entablature, intended to represent beams of timber, cannot be supposed to be wholly upheld by supports placed at such great distances from each other. And the introduction of columns, in place of carrying up the piers, deprives the superstructure of powerful buttresses, in situations where they would prove very beneficial. The affectation of preserving the entablature upon a perfect level, has led to making the roadway along the bridge also level, which is nothing less than constructing, at a vast expense, a piece of road, more imperfect than what is formed by the common labourer in the open country; and besides, this mode of construction gives an appearance of feebleness to the outlines of a bridge."'

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thrust up to the abutments. The flattest brick arehes of large size arc those of the bridge that carries the Great Western Railway over the Thames, at Maidenhead. These arc semi-elliptical in form, and of 128 feet span, and 2-4J feet rise. The abutments are stepped on raking benches on the chalk stratum upon which they stand; and the resistance thus obtained appears to be sufficient. Mr. Law remarks, that, taking into consideration the materials of which it is composed, this bridge is certainly the boldest which has ever been constructed, the actual pressure at the crown of the areh being about one-third of that which would begin to injure the cohesive strength of the material of which it is composed. "And although the construction of this bridge has shown that it is practicable to approach much closer to the load which would cause failure than had before been considered safe, it is questionable how far prudence would warrant such an approach in ordinary cases; especially when we consider how many accidental cireumstances may deteriorate the stability of the areh, to guard against which, it seems desirable that a much wider margin should usually be given, and that the greatest load upon the keystone should not be greater than ^th of that which would begin to crash its material, in bridges exposed to only ordinary traffic; and in those which are continually exposed to the tremour and vibration occasioned by a continuous and very heavy traffie, not more that j^th."

Section IV.—On Timber Bridges.

The oldest wooden bridge on record is the bridge of Sublicins, which existed at Rome 500 years b.C. It is celebrated for the combat of Horatins Codes, a Koman knight, who saved the city by his noble defence of this bridge. It is stated to have been

put together without iron or nails. A wooden bridge was erected by Julins Cxsar for the passage of his army across the Rhine, The passage was effected ten days after they began to carry the timber for its erection. The bridge built by Trajan over the Danube appears to have been of timber, except the piers, which were of stone. The roadway seems to have been supported by three concentric curved ribs of timber, connected by radial pieces. There were 20 or 22 stone piers, and each wooden areh was above 100 feet span.

In the middle ages, when men began to establish bridges on the passages over the principal rivers, it was customary to erect piers 15 to 20 feet apart, consisting of one or more rows of piles. They were defended by a kind of jetty to break the ice, which also served to protect them from the shock of bodies borne down by the current; frequent repairs were, however, required, and the accumulation of matters against the piles blocked up the water-way, and the bridge became incapable of resisting the pressure of water in the time of high floods. In these early times of modern bridge-building, abundance of material was used, without much skill in its arrangement; but in places not subject to floods, and in situations where the piers could he kept light, more elegant structures were erected. A bridge built by Palladio over the Brenta, near Bassano, is a good example of this kind of bridge. This great arehitect has given several designs for bridges, which have been adopted in later timts. He appears to have been the first among the moderns who attempted that species of construction which renders numerous piers unnecessary, thereby avoiding the shock to the timberwork of bodies carried down by the current. His bridge over the torrent of Cismone, near Bassano,

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