Il8 ENGINEERING FOR LAND DRAINAGE. 



The connection of the box with the end of the line of 

 tile should be thoroughly and carefully made, other- 

 wise drainage-water will work its way around the box 

 and underwash it. The earth should be most thor- 

 oughly tamped about the box as the filling is thrown 

 in ; in no case should it be loosely filled, as is done 

 with ordinary ditches. 



The Silt-basin. 



Silt-basins are often injudiciously used in a drainage 

 system. They serve two purposes: one to collect 

 the water of several drains of a system so that it can be 

 combined and discharged through a common main, and 

 another to arrest the silt which a drain carries and 

 cause it to deposit itself in the bottom of the basin or 

 well from which it may be removed. 



The effect of the silt-basin is to retard the flow of the 

 water, and hence should never be used where this will 

 prove injurious to the drains. The breaking of the cur- 

 rent and the entrance friction of the outlet pipe, while 

 it causes the silt to be deposited, also diminishes the 

 velocity and discharge of the main drain, and is detri- 

 mental to the action of the drain on level grades. 

 Where there is abundant fall and the entering drains 

 can discharge into the basin not less than one foot 

 above the top of the outlet drain it may be used. 



The best plan for collecting the water of several 

 drains into one is by means of the Y junction and a 

 ''drop*' from branch into main as noted heretofore. 

 The current is preserved, and silt is carried to the outlet 

 before it is deposited. Where the drains are well laid 



