risen greatly at the haunches, so as to open the upper part of the joints, many of which gaped an inch, and this opening of the joints gradually extended from the haunches towards the crown, in the neighbourhood of which they opened on the under side. This evidently arose from a want of stiffness in the frame. But these joints closed again when the centres were struck.

this construction the angles of the alternate trusses | was not general; on the contrary, the frame had lie in lines pointing towards the centre of the curve. King posts were therefore placed in this direction between the adjoining beams of the trusses. These king posts consisted of two beams, one on each side of the truss, and embraced the truss beams between them, meeting in the middle of their thickness. The abutting beams were mortised half into each half of the post. The other beam, which formed the base of the triangle, passed through the post, and a strong bolt was driven through the joint and secured by a key or a nut. In this manner the whole was united, and it was expected that when the load was laid on the uppermost truss it would all butt together, forcing down the king posts, and therefore pressing them on the beams of all the inferior trusses, causing them also to abut on each other, and thus to bear a share of the load. This method of construction is the invention of Perrault. Its merits are fully discussed by Dr. Robison, in the first volume of his System of Mechanical Philosophy.

This construction, somewhat modified, was used by Perronet for the centering of the Bridge of Neuilly. The arch has 120 feet span and 30 feet rise, is 5 feet thick, and is remarkable for the flatness of its crown. The frames of the centering were 6 feet apart, and each carried an absolute load of 350 tons. The strut

Fig. 300 CENTERING OF THE PONT DE NEUILLY.

beams were 17 by 14 inches in scantling. The king posts were of 15 by 9 each half; and the horizontal bridles which bound the different frames together in five places were also 15 by 9 each half. There were 8 other horizontal binders of 9 inches square. As the stiffness of the framing depended on the transverse strength of the beams, care was taken not to weaken them by bolts. But notwithstanding this, the framing sunk upwards of 13 inches before the key-stones were laid; and during the progress of the work the crown rose and sunk by various steps as the loading was extended along it. When 20 courses were laid on each side, and about 16 tons laid on the crown of each frame, it sunk about an inch. When 46 courses were laid and the crown loaded with 50 tons, it sunk about half an inch more. It continued sinking as the work advanced, and when the key-stone was set it had sunk 13 inches. But this sinking

Dr. Robison remarks that the movements and twisting of this centre seem to indicate a deficiency not only of stiffness, but of abutment among the truss beams. The whole was too flexible because the angles were too obtuse. This arises from their multiplicity. Indeed, this centre should have consisted of fewer pieces, and their angles of meeting been proportionally more acute.

Dr. Robison gives by way of favourable contrast the details of Mr. Mylne's centre used for the arches of Blackfriars Bridge. The span of the arch is 100 feet and its height from the spring about 43. The leading maxim in this centre seems to be, that every part of the arch shall be supported by a simple truss of two legs resting one on each pier. The exterior joints are strengthened and the ring made as stiff as possible by apron pieces, from the ends of each of which proceed the two legs of the trusses. These legs are 12 inches square: they are not of an entire piece, but of several, meeting in firm abutment. Some of their meetings are secured by the double king posts, which grasp them firmly between them, and are held together by bolts. At other intersections the beams appear halved into each other; a practice which must weaken them and would endanger their breaking by cross strains, if it were possible for the frame to change its shape. But the great breadth of this frame is an effectual stop to any such change. No sinking or twisting was observed during the progress of the mason work. Three points in a straight line were marked on purpose for this observation, and were observed every day. The arch was more than 6 feet thick, and yet the sinking of the crown before setting the key-stones did not amount to one inch. This centre employs more timber than Perronet's, but is in every way stronger.

But with every care in the construction of centres there is always a sinking in the crown of the arch. Dr. Robison describes this in his peculiarly lucid manner. He says:-" By gradually withdrawing the centering, the joints close, the arch-stones begin to butt on each other and to force aside the lateral courses. This abutment gradually increasing, the pressure on the haunches of the centering is gradually diminished by the mutual abutment, and ceases entirely in that course which is the lowest that formerly pressed it: it then ceases in the course above, and then in the third, and so on. And in this manner not only the centering quits the arch gradually from the bottom to the top, by its own retiring from it, but the arch also quits the centering by changing its shape. If the centering were now pushed up again, it would touch the arch first at the crown; and it must

lift up that part gradually before it come again in | duce the deformity of a ragged soflit. They were contact with the haunches. It is evident therefore therefore not allowed to sink so much. In the places that an arch, built on a centre perfectly suited to of the blocks and bridgings which had been cut away, equilibration, will not be in equilibrio when the cen- small billets were set standing on their ends betwerd tering is removed. It is therefore necessary to form the centering and the arch-stones. These allowed the the centering in such a manner, (by raising the crown,) pendulous arch to push toward the crown without that it shall leave the arch of a proper form. This is sensibly descending; for the billets were pushed out a very delicate task, requiring a previous knowledge of the perpendicular, and some of them tumbled down of the ensuing change of form. But suppose this Proceeding in this way, he advanced to the very next attained, there is another difficulty. While the work course to the key-stone on each side, the joints closing advances, the centering is warped by the load laid all the way as he advanced. The detaching the three upon it, and continually increasing on each side. The uppermost courses from the centering was most first pressure on the centering forces down the troublesome; for the whole elasticity of the centre haunches and raises the crown. The arch is therefore was trying to unbend and pressing hard against them. less curved at the haunches than is intended: the They were found to be lifted up; for the joints beyond joints however accommodate themselves to this form, them, which had closed completely, now opened aga and are close and filled with mortar. When the below: but this was finished in one day, and the masons approach the middle of the arch, the frame centre sprung up 2 or 3 inches, and the whole arrá sinks there and rises up at the haunches. This opens sunk about 6 inches. This was an anxious time,” all the joints in that place on the upper side. By the says Dr. Robison, "for he dreaded the great motime that the key-stones are set, this warping has mentum of such a vast mass of matter: it was hard gone further; and the joints are open on the under to say where it would stop. It stopped, however, very side near the crown. It is true we are here speaking soon, settling slowly as the mortar was compressed, of an extreme case when the centering is very flexible, and, after one or two days, settling no more. This but this occurred to Perronet in the two great bridges settling was very considerable, both in the bridge at of Neuilly and of Mantz. In this last one, the crown Neuilly and in that at Mantz. In the former, the sunk above a foot before the key was set, and the sinking during the work amounted to 13 inches. It joints at the haunches opened more than an inch above, sunk 6 inches more when the blocks and bridgings while some nearer the crown opened nearly a quarter were taken out, and 14 when the small standaris of an inch below. In this condition of things it is a were destroyed, and 14 more next day; so that the delicate business to strike the centering. Were it whole sinking of the pendulous arch was 9 inches, removed in an instant, all would probably come down, besides what it had sunk by the bending and comfor the arch-stones are not yet abutting on each other, pression of the centering. In fact, the whole sinking and the joints in the middle are open below. "Per- of this arch was 23 inches. The sinking of the ronet's method was to begin to detach the centering arches at Mantz amounted in all to 20 inches, of at the very bottom on each side equally, where the which 12 inches was owing to the compression and pressure on the centering is very slight. He cut bending of the centering." away the blocks immediately under each arch-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 arch-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 forcing out the arch-stones at the haunches. He 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 arch-stones nearer the crown. He then proceeded more slowly, destroying the blocks and bridgings of those upper arch-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

After the centres of the arches 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 manœuvre more interesting, it was arranged that all the frames of each arch 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 34 minutes. The fall of such a vast quantity of timber, which weighed at least 729 milliers for each arch, caused the water to rise in foam upon the bridge. The arches were thus com

the arch-stones, the least sinking of the arch would | with these, and being connected with them by 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 as 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

Fig. 302.

the Orleans Bridge, and Fig. 303 that in Blackfriars Bridge. But, as these arches are easily deranged by any settlement of the main arch, a better plan is to build walls longitudinally, 2 or 3 feet apart, and from

Fig. 303.

18 inches to 3 feet in thickness: founded upon the solid rubble masonry, and increasing in length as they advance in height, they rest upon and abut against the backs of each row of arch-stones, and act as struts between them. These walls are kept steady by laying long stones occasionally across from one wall to another. The outside spandrel walls, running parallel

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. Smal 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 one 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 masoury, 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 are 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-wail 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 3 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. Gauthey remarks that most of the bridges erected before the eighteenth century were built with a view to economy, both with respect to the style of building and the degree of breadth allotted to them, since the most considerable scarcely allow room for two carriages to pass abreast. The bridges 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

Feet. Feet. Feet. Feet. Feet.

16 0 Circular... 183.25 70.25 94.87 5.25

pointed Circular... 140 87.5 2.5 (False el-) 127.83 38.25 134.6 6.25 lipse...f 42 0 Idem 100 41.5 56 6.58 20 48 0 Idem...... 127.83 31.83 260 5.25 76.67 6.25 121