34 THE POTATO 



of laterals may be increased by laying them shal- 

 low at the head and deep at the outlet, sometimes 

 only two feet deep at the head and three and a half 

 at the outlet. The gradient of a main cannot be 

 controlled in this way, because at the head it must 

 be deep enough to receive the discharge from the 

 laterals. However, the diameter of the main may 

 be increased to give it the required capacity. A 

 gradient of a tenth of a foot, or about one and a 

 quarter inches, to a hundred feet is generally con- 

 sidered a minimum gradient for laterals. The gra- 

 dient in large mains may be decreased to almost 

 nothing, because the diameter of the tile itself 

 may be considered as constituting a gradient. 



"With the cost of the main seldom more than 

 $4 an acre, and the cost of the labor and tile for 

 laterals fairly constant at about 75 cents a rod, 

 it is the frequency of the laterals that is the chief 

 factor in determining the cost of tile drainage. 

 The cost will range from $20 to $35 an acre ac- 

 cording as the laterals are placed four rods 

 apart or eight rods apart. This frequency should 

 be based upon principles which have been dis- 

 cussed in the preceding pages. It is safe to say, 

 however, that while eight rods is the interval 

 which in some soils may yield the greatest dividend 

 upon the money invested in the improvement, yet 

 there is no danger of eventually getting the lat- 

 erals too close together. 



" Where under-drainage is desired tile are cheaper 

 than open ditches of the same depth. Further- 

 more, they offer no obstacle to cultivation, take 

 up no surface space, and are more permanent than 

 the ditches. There are tile in Wisconsin that are 

 working as good or better than ever at the end of 

 thirty years, and in some states the life of tile 



