RAILWAY CONSTRUCTION. 131 



amount of centering. Every precaution should be taken to 

 prevent water percolating through any portion of the arching, 

 or haunching, and a thick layer of good asphalte should be 

 placed over the entire upper surface, and carried well up the 

 lower portion of the parapet walls, as shown on the sketch. 

 The cast-iron pipes with rose heads form a very efficient means 

 of taking away the rain-water which filters through the ballast 

 and filling. The pipes should be carried down in chases, or 

 recesses, built in the fronts of the piers, to protect them as much 

 as possible from injury in the yards below. Rose heads, pierced 

 with holes, and surrounded with small stones hand-laid, serve 

 well to conduct the water into the pipes. Where the arching is 

 of considerable length, recesses or refuges for the platelayers 

 may be obtained by substituting a short length of cast-iron- 

 plate parapet, instead of the stone or brick parapet, over some 

 of the piers, as indicated in the sketch. 



Fig. 127 shows a similar description of arching for spans 

 of 30 feet. The above two examples represent plain substantial 

 work, but if circumstances warrant more external finish, this 

 can readily be added without interfering with the general 

 arrangement. In a similar manner, if considered preferable, the 

 arches may be made semicircular or elliptical. 



In the sketches shown of the arched over-line and under- 

 line bridges, the arching and coping of parapets are in brick, 

 and the remainder of the work in stone. In very many cases 

 brick will be found cheaper and more expeditious for arching than 

 stone, unless the quarries turn out stone in blocks which can be 

 conveniently trimmed for arching. All bricks used for arch- 

 work should be hard and well burnt, and special care should 

 be taken in the selection of those to form the under-side course, 

 which will be exposed to the atmosphere. For moderate spans 

 arches have been successfully constructed of concrete. For this 

 description of work the materials should be carefully gauged 

 and mixed together, and the finished work should be allowed 

 to stand some time on the centres to allow the concrete to 

 become thoroughly set. 



In Fig. 102, the cutting being deep, almost up to the level 

 of the public road, the foundations of the wing walls are built 

 in steps, resulting in a minimum of masonry below the finished 

 ground line. Where the cutting is shallow, and the public 

 road has to be brought up to the bridge on an embanked 



