SOILS. 53 



should be filled with them to the depth of eight or ten 

 inches. In using these, the drains require to be at least 

 six inches deeper than for tiles, in order that a sufficient 

 quantity of stones can be used without coming too near 

 the surface. Some brush, or turf, with the grassy side 

 downwards, should be laid on the stones before filling in 

 the earth, to keep :'t from filling up the crevices. 



