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 substantially 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. [Illustration: Fig. 67, 68, 69, 70, 71] In Figs. 70 and 71 are shown types of arch-top culverts of 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 must be cut and 78 dressed to suit the small radius of the arch. The side walls may be of brick of good quality. Occasionally they are built of concrete. The wing walls may either be carried out in the direction of the stream, as in the sketch of the 6-foot culvert, or they may be built transverse, as shown on the 4-foot culvert, whichever arrangement is found to work in the best for the case in question.