THE POTATO 35 



has been much longer than that. If tile are laid 

 more than two feet deep, it does not hurt them to 

 freeze if they are empty when they freeze. There 

 should be no sags in a grade line to prevent the 

 tile from emptying themselves. Silt also has a 

 tendency to settle in such sags. At that depth 

 the expansion of the water in the walls of the tile 

 has but Httle apparent effect upon the tile. Tile 

 exposed to the frequent and sudden freezing and 

 thawing occurring at an exposed outlet or on the 

 surface of the ground may crumble in a single 

 winter. 



"There is danger in sandy subsoils of the en- 

 trance of soil particles larger than can be removed 

 by the running water. Sand enters with ease, but 

 is carried away with difficulty. Tile laid in sandy 

 subsoil should be 'blinded' or entirely surrounded 

 by and packed with clay, muck, or old grass. 

 This being done, cracks an eighth of an inch wide 

 are permissible. 



"A single tile may break some years after being 

 laid, and the earth that enters is apt to render the 

 line useless above that point. Such a place can 

 usually be found with ease. The broken tile 

 should be replaced with new ones, after the earth 

 has been removed from the tile that have been 

 wholly or partly filled. 



"On springy areas having a slope sufficiently 

 great that water runs rapidly in shallow ditches 

 the flow of water will invariably indicate the 

 proper direction for main and laterals. On areas 

 less favored with a fall a few preliminary readings 

 with a level are of value to indicate how small the 

 fall is. If less than two feet in eighty rods, none 

 but a surveyor's instrument should be used for the 

 remainder of the work. If greater than tha't 



