474 



STATE BOARD OF AGRICULTURE). 



make the changes as few as possible and keep the grade uniform in as 

 large sections as possible. 



There is no objection whatever to changing the grade from any fall 

 to a greater one, but care should be observed when a change is made from 

 any fall to a less. The water moving through the drain carries with 

 it more or less fine material which has worked its way through the joints 

 of the tile. This material is spoken of as silt. The particles of silt are 

 sometimes so large as to be moved but slightly by the running water in 

 the tile — so slightly, indeed, that if the rate of flow of the water should 

 be decreased ever so little the force of the water will then be insufficient 

 to continue to move them. If then, in a line of tile, the fall were lessened 

 at some point it might happen that considerable quantities of this silt 

 would accumulate at the point of change. Sometimes this does happen 

 to the extent that the tile is clogged, unless some means is provided to 

 prevent the clogging, 



sujT basins. 



Silt basins (see Figs. 6, 7 and S) are chambers or openings established 

 at intervals along a tile system for the purpose first, of gathering the 

 silt moved down the tile by the drain water, and second, as means 



Fig. 6. Silt basin built of brick. 



for examining the condition of the tile system. In extensive systems, 

 a silt basin should be placed at any point where the fall is reduced, where 

 a sub-main unites with a main, where a long lateral unites with the main 

 or submain, and at intervals along any considerable line of tile, whether 

 it be lateral, sub-main or main. 



The bottom of the silt basin should stand at least a foot below the lower 

 edge of the tile and the basin should be at least twelve inches in diameter, 



