ON BRIDGES. 135 



come into their proper places in the curve. If the stream can be 

 occupied during the construction of the bridge by piles driven across 

 it, the form of the centring will be simple enough, for it can be 

 supported along its whole length from below ; but if the water-way, 

 as is frequently the case, must be left clear all the time, considerable 

 ingenuity is sometimes required in designing a structure which, only 

 supported at its ends, shall sustain the varying loads during the pro- 

 gress of the work without change of form. 



After the centring is up, the construction of the arch is com- 

 menced by laying the first stones near the abutments on both sides, 

 and gradually building up towards the centre ; and as the weight is in 

 this way increased from the ends of the frame towards its middle, the 

 centring must be so trussed as to resist the change of figure which 

 this operation has a tendency to produce. 



After the arch-stones, which are cut previously of the proper size 

 and shape, depending upon the curve of the intrados, are all properly 

 laid up to the middle, the last stone, usually called the keystone, is 

 fitted tightly into its place at the crown of the arch, and after per- 

 mitting the structure to rest for a short time, the centering is slightly 

 lowered to permit the stones to commence to bear tightly upon each 

 other. To enable the centre to be thus lowered, or struck, as it is 

 termed, it is made at first to rest on wedges on the lower part of the 

 frame, and these wedges being slowly driven back the centre gradu- 

 ally sinks. 



In the bridges of the Alma, the Invalids, and of St. Michael's, all 

 in Paris, an arrangement, invented by M. Beaudemoulin, was substi-' 

 tuted for the wedges with success. Upon the fixed portion of the 

 frame a series of sheet-iron cylinders filled with fine dry sand, rested, 

 into which fitted wooden plungers which rested on the sand, and upon 

 these plungers rested the centring. When the centre was to be 

 struck, a small hole left at the bottom of the cylinder was opened, 

 and the sand running slowly out permitted the upper works to settle 

 down gradually. 



When the engineer considers the mortar sufficiently set, the centre 

 is lowered still further and taken away, leaving the arch to support 

 itself ; the rest of the masonry work is then carried up, the spandrels 

 filled in to the level with the crown of the arch, and the pavement, 

 sidewalks, and parapet walls added. 



Great care must be taken that the rain-water which falls upon the 

 surface of the bridge is not allowed to percolate through the struc- 

 ture ; as it not only carries portions of the lime with it and disfigures 

 the surface and the under side of the arches, but is frozen in winter, 

 and thus tends rapidly to destroy the bridge. Gutters must be laid 

 on the top, and the water from them conducted away in pipes through 

 the piers, while the use of asphalte or bitumen as a water-tight cov- 

 ering to the backs of the arches, is highly to be recommended. 



As a general rule, it seems well to avoid much architectural orna- 

 ment on a stone bridge, unless it has some obvious connexion with 

 the constructive features. Many of the European bridges are in- 



