DBAINAGE OF IREIGATED LAND. 



27 



A pole gauge of this length may then be used to establish the grade 

 at each tile, as shown in the figure. To ascertain the height above 

 ground at which each grade plank must be set, it is only necessary 

 to subtract the calculated cut at that station from the length of the 

 gauge pole used, say 7 or 8 feet. For machine trenching, poles should 

 be erected at frequent stations and target arms set at a uniform 

 height above grade upon which sights may be taken by the operator 

 of the machine. Tile should be laid true to grade and in straight 

 lines. No attempt should be made to judge grade by the water in 

 the trench. It is easy to vary a foot from the proper grade in a short 

 distance in this man- 

 ner. Tile should be 

 laid within a half inch 

 of true grade under or- 

 dinary conditions, and 

 it is possible to do even 

 better than this. 



In laying tile the 

 joints should be placed 

 as close as possible. If 

 the soil be semifluid 

 and contains much 

 sand and silt, it wiU be 

 necessary to provide 

 some means of keeping 

 the oozing material 

 from enter mg the tile 

 joints. Almost all of 

 the water entering the 

 tile lines makes its way 

 through the joints, 

 practically none en- 

 tering through the 

 waUs of even the most porous tile, so the covering for the joints 

 must provide for the ready passage of the water. Straw makes a 

 \iicy good filt(5r when new, but it is likely to decompose and to form 

 a sort of cement over the joints. Brush and willows are not satisfac- 

 tory and render any subsequent removal of tile very difficult. Graded 

 gravel, ranging from coarse sand to pebbhis an inch in diameter, 

 makes an excellent filter, Imt is not always availabh^ Cinders also 

 are satisfactory. Strips of burhip wrapped about the joints give 

 good s(!rvico. For genuinis quicksand jx-rliaps the best material is 

 cheesecloth, which should be doubled once or twice and wrapped very 

 carefully about the joint. Tliis material soon disappears, but in the 

 meantime the soil becomes vumynwXvA so that the j)urp()suis served. 



Vrocfe SraAe, Cut 6/ ' 



Fig. 21.— Method of establisliiiig grade by means of cord and gauge. 



