DRAINAGE 89 



treads which provide a large area of supporting surface. 

 These machines can be used to the best advantage on long 

 lines of tile and where the soil is reasonably dry and free 

 from boulders. In no case should a machine be used which 

 does not permit of an inspection of the grade and of the tile 

 as it is laid. 



The Guess System of Laying Tile. At the present time 

 there is very little tile placed in the ground on grade lines 

 made simply by guess. The majority of such systems are 

 failures, and mistakes have been so evident where this 

 method was practiced that it is uncommon now to see a 

 system installed without a survey. 



The Water-Level Method. But little better than the 

 guess method of installing drainage systems, is the water- 

 level method, which is used to some extent today and is 

 responsible for a large number of failures. This method of 

 laying tile is used where there is some water in the ditch. 

 Where the fall is slight, water can not be depended upon to 

 give a proper grade. The ditch is sure to be dug below the 

 grade at certain places, giving a back fall. After the ditch 

 has been dug too deep, there is little chance of correcting the 

 mistake by filling in. The water-level method is so inaccu- 

 rate that, even where 

 the fall is great and 

 there is little danger 

 of creating back fall, 

 the grade line will be 

 so irregular that the 

 efficiency of the tile 

 will be much reduced. 



Method of Grad- 

 ing Ditches. Two 

 general methods of Fig ' 56 ' The line dl JS? od f srad ' ng Ule 



