76 RAILWAY CONSTRUCTION. 



quickly, break, and become detached, leaving the foundation and 

 side walla exposed to the cutting inroads of the water. 



Where large flat bedded stones or flags of tough quality can 

 be obtained, they form good covers, or tops, for culverts up to 

 two feet in width. They should have not less than nine inches 

 bearing on the side walls, and their contact edges should be fairly 

 dressed, so as to fit sufficiently close to prevent the embankment 

 filling from falling through. 



Where the stream, or run of water, is very small, strong 

 earthenware pipes, 9 inches or 12 inches in diameter, well bedded, 

 may be sufficient to carry away all the water likely to arise. 

 For small springs in low swampy ground, dry stone drains may 

 in many cases be used with advantage. These are made by 

 cutting a trench, say two feet deep by twelve or eighteen inches 

 wide, in the seat of the embankment from side to side, and filling 

 it up with dry rubble stones, not boulders, hand-laid, the upper 

 layer placed on the flat to keep the earthwork as much as 

 possible from filling in between the stones. 



In soft boggy ground, where the depth to a hard bottom is 

 very considerable, wooden culverts are frequently adopted. 

 Although these cannot be classed as permanent structures, still, 

 when they are made of sound well-creosoted timber, and substan- 

 tially put together, they last for a number of years. Sometimes 

 they are made cylindrical in section a species of elongated cask 

 with strong iron hoops every few feet. Others are rectangular 

 in section, made with two strongly trussed side frames connected 

 and covered with cross-planking and longitudinal tie-planking 

 on the top and bottom. 



Wooden culverts are seldom made of very large size, rarely 

 exceeding an opening of 3 feet, and it is considered preferable to 

 use two of these culverts of moderate dimensions than one of 

 large size. Figs. 67 and 68 give sketches of wooden culverts 

 of cylindrical and rectangular section, and Fig. 69 of flag top 

 culverts of 12-inch, 18-inch, and 2-foot openings. In masonry 

 culverts the side walls are shown to be of rubble stonework, but 

 brickwork can be used instead, provided the bricks are well 

 burnt, hard, and capable of withstanding the action of the 

 water. 



In Figs. 70 and 71 are shown types of arch -top culverts ot 

 4 feet and 6 feet span respectively. The arch portion is shown to 

 be of brick, which, as a rule, is cheaper than stone rings, which 



