Size of Tile.--The size of the main depends upon the area that eventually may be drained, the amount of overflow from higher land, the nature of the soil, and the grade of the drain. It is a common mistake to make the main too small because the drainage immediately contemplated is less than that which will be desired when its value is known. In the determination of the size the judgment of an expert is needed, and if this cannot be had, the error should be on the side of safety. If the main will not be required to carry overflow from other land, and has a fall of 3 inches to 100 feet, one may assume that a 6-inch main will carry the surplus water from 12 to 20 acres of land, and an 8-inch main will carry the water of twice that area. Some drainage experts figure larger areas for such mains, but there is danger of loss of crop when the rainfall is very heavy. The laterals need not be larger than 3 inches in diameter when laid on a good bottom. Kind of Tile.--When clay tile is used, it should be well burned. Some manufacturers offer soft tile for sale, as the loss from warping and cracking is less in case of insufficient burning. The claim may be made that the efficiency of soft tile is greater than that of the hard tile whose porosity has been destroyed. This is an error, as the water enters the drain at the joints, and not through the walls of the tile. Underdrainage should be permanent in its character, and it is essential that every piece of tile be sound and well-burned. Vitrified clay tile is good for drainage, but no better than common clay hard-burned. Round or octagonal tile is the most desirable because it can be turned in laying to secure the best joints. Collars are not needed in ordinary drainage. Cement tile is coming into general use in regions having no good clay. Its durability has not been tested, but there is no apparent reason that it should not be a good substitute for clay. The Grade.--The outlet may fix the grade. If it does not, the main, branches, and laterals should have a fall of 3 inches, or more, to the 100 feet. This grade insures against deposits of silt and gives good capacity to the drains. If the outlet demands less fall in the system, the main may be laid on a grade of only a half inch to the 100 feet with satisfactory results. Such a small fall should be accepted only when a lower outlet cannot be secured, and great care should be used in grading the trench and laying the tile.