DEAINAGE OF IRRIGATED 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 accessary 
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 will be 
necessary to provide 
some means of keeping 
the oozing material 
from entering the tile 
joints. Almost all of 
the water entering the 
tile lines makes its way 
through the joints, 
practically none en- 
tering through the 
walls of even the most porous tile, so the covering for the joints 
must provide for the ready passage of the water. Straw makes a 
very good filter 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 pebbles an inch in diameter, 
makes an excellent filter, but is not always available. Cinders also 
are satisfactory. Strips of burlap wrapped about the joints give 
good service. For genuine quicksand perhaps the best material is 
cheesecloth, which should be doubled once or twice and wrapped very 
carefully about the joint. This material soon disappears, but in the 
meantime the soil becomes compacted so that the purpose is served. 
Fig. '21. — Method of establishing grade by means of cord and gauge. 
