DRAINAGE. 103 



necessary that every single tile be placed at the precise depth re- 

 quired to bring it into line with those above and below it (except 

 when the rate of fall is purposely changed). It has been well 

 said that " the worst laid tile is the measure of the goodness and 

 permanence of the whole drain^ just as the weakest link of a chain 

 is the measure of its strength."* No tile should be so placed as 

 to offer an impediment to the even flow and velocity oT the 

 current which reaches it from the tile above. The fall of a 

 drain should not decrease in velocity as we proceed toward the 

 outlet, lest particles of soil, (technically called silt^ which are car- 

 ried along by the rapid flow, be deposited by the slower current 

 and obstruct the drain, f 



Above all, should undulations and irregularities be avoided. 

 Draining is pre-eminently worth doing well, if worth doing at all. 

 The cost of tile, and the labor of digging and refilling the 

 ditches, constitute the chief expense of draining, and it is the most 

 improvident sort of "penny-wisdom" to economize in the item 

 of precision. One ill-laid tile in a main drain may render useless 



Fig. 31. 



five thousand whose outlet lies through it. Drains must not be 

 laid at a uniform depth from the surface, but on a straight line of 

 descent at the proper ^^w^rtf/ dV^//). Figure 31 shows a drain in 

 uniform depth, and the line, a r, passing through it shows how it 

 deviates from the proper inclination ; at the point d the tile would 

 be filled with " dead water," and might soon be obstructed with 

 silt. By aid of the levels taken at the grade stakes, the proper 

 depths to be given at these points may be readily computed and 



* Talpa, or the Chronicles of a Clay Farm. 



f Under the head of "Silt Basins," will be found directions for managing this 

 changeof grade when necessary to be made. 



