DRAINAGE. 



Ill 



Fig. 4J 



Silt-basin made of six- 

 inch tile. 



in proceeding cOward the outlet, there is danger that the silt, 

 which was carried by the more rapid stream above, will be 

 deposited by the slower current and cause a stop- 

 page of the drain. In the drainage of the Central 

 Park, this danger was guarded against by the use 

 of silt-basins at all points at which the fall of a 

 drain (which had not a very steep descent, say 

 two or three feet in one hundred feet) became 

 less rapid. The silt-basin is a vessel (larger 

 than the tile with which the drain is laid) extend- 

 ing some distance below the grade of the drain. 

 It has the effect of arresting the movement of the 

 water, thus allowing its silt to settle to the bottom, and has suf- 

 ficient depth to accumulate that which will probably enter it 

 before the drain commences to 

 run clear water. For a lateral 

 drain of small caliber, a very good 

 silt basin is made by placing a 

 single six-inch tile on end, sink- 

 ing it two-thirds of its length 

 below the floor of the ditch, and 

 admitting the tiles from above 

 and below at opposite sides. It 

 should be covered with a well- 

 fitting, flat stone, and should 

 stand on a stone or a board — not 

 on the earth, ( Fig. 43,) For 

 drains of somewhat larger size 

 a small chamber of brick-work 

 may be used, (Fig. 44,) and for 

 the collection of the mains of 

 several systems, it is satisfactory 

 to build a well two feet in 

 diameter, having a depth of two 



feet below the bottom of the out- ^'S- 44.-Square brick silt-basin. 



let drain, and reaching the surface, with a good cover which 



