LAND DRAINAGE 641 



The vitrified tiles have walls so dense that they absorb 

 less than 3 or 4 per cent of moisture, and often less than 

 2 per cent, so that they are much less vulnerable to frost 

 action. Good tile should be well formed and should 

 give a clear ring when struck with a hammer. 



Concrete tile of good quality may be made, but the 

 quality is normally inferior to that of the best vitrified 

 tile. The porosity is likely to be 5 to 10 per cent. To 

 make good cement tile requires a rich proportion of cement, 

 good sand, and as wet a mixing and molding as is prac- 

 ticable. Several machines of both farm and factory size 

 are in the market for molding concrete tile. • 



Water enters tile through the joints, not through the 

 walls. Even the most porous tiles having a high absorp- 

 tion do not permit an appreciable amount of water to 

 pass through the walls. Therefore, soft tiles have no 

 higher efficiency than vitrified tiles, and, owing to the 

 risk of freezing, the effectiveness of a line of porous tiles 

 is much jeopardized. Since water enters at the joints of 

 the tile, short lengths are more efficient than long lengths. 

 The usual length of sections of tile under 12 inches in 

 diameter is 12 to 13 inches. In larger sizes, where the 

 carrying function predominates over the collecting func- 

 tion, lengths of 2 feet are employed. 



541. Shapes of tile. — Tile should have a round open- 

 ing and a round or a hexagonal exterior. A flat-bottomed 

 opening is objectionable because it reduces the flow and 

 promotes the accumulation of sediment. U-shaped tiles 

 with flat sides are called horseshoe, or single-sole tile. 

 This shape is unsatisfactory. Tiles are often warped 

 in the process of drying and burning, and the last-named 

 shape does not allow a close joint to be formed by turn- 

 ing the tile. Round and hexagonal shapes permit turn- 

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